N-imidazolebenzyl-histidine substitution in somatostatin and in its octapeptide analogue modulates receptor selectivity and function

Despite 3 decades of focused chemical, biological, structural, and clinical developments, unusual properties of somatostatin (SRIF, 1) analogues are still being uncovered. Here we report the unexpected functional properties of 1 and the octapeptide cyclo(3-14)H-Cys-Phe-Phe-Trp(8)-Lys-Thr-Phe-Cys-OH (somatostatin numbering; OLT-8, 9) substituted by imBzl-l- or -d-His at position 8. These analogues were tested for their binding affinity to the five human somatostatin receptors (sst(1-5)), as well as for their functional properties (or functionalities) in an sst(3) internalization assay and in an sst(3) luciferase reporter gene assay. While substitution of Trp(8) in somatostatin by imBzl-l- or -d-His(8) results in sst(3) selectivity, substitution of Trp(8) in the octapeptide 9 by imBzl-l- or -d-His(8) results in loss of binding affinity for sst(1,2,4,5) and a radical functional switch from agonist to antagonist.

Orientation-selective functional magnetic resonance imaging adaptation in primary visual cortex revisited

The processing of orientations is at the core of our visual experience. Orientation selectivity in human visual cortex has been inferred from psychophysical experiments and more recently demonstrated with functional magnetic resonance imaging (fMRI). One method to identify orientation-selective responses is fMRI adaptation, in which two stimuli—either with the same or with different orientations—are presented successively. A region containing orientation-selective neurons should demonstrate an adapted response to the “same orientation” condition in contrast to the “different orientation” condition. So far, human primary visual cortex (V1) showed orientation-selective fMRI adaptation only in experimental designs using prolonged pre-adaptation periods (∼40 s) in combination with top-up stimuli that are thought to maintain the adapted level. This finding has led to the notion that orientation-selective short-term adaptation in V1 (but not V2 or V3) cannot be demonstrated using fMRI. The present study aimed at re-evaluating this question by testing three differently timed adaptation designs. With the use of a more sensitive analysis technique, we show robust orientation-selective fMRI adaptation in V1 evoked by a short-term adaptation design.

cDNA cloning and expression of a novel cell surface-expressed \b heparan sulfate/heparin \b interacting \b protein (HIP) from human cell lines and functional studies of a synthetic HIP peptide

Heparan sulfate proteoglycans and their corresponding binding sites have been suggested to play an important role during the initial attachment of blastocysts to uterine epithelium and human trophoblastic cell lines to uterine epithelial cell lines. Previous studies on RL95 cells, a human uterine epithelial cell line, characterized a single class of cell surface heparin/heparan sulfate (HP/HS)-binding sites. Three major HP/HS-binding peptide fragments were isolated from RL95 cell surfaces by tryptic digestion and partial amino-terminal amino acid sequence from each peptide fragment was obtained. In the current study, using the approaches of reverse transcription-polymerase chain reaction and cDNA library screening, a novel cell surface $\rm\underline{H}$P/HS $\rm\underline{i}$nteracting $\rm\underline{p}$rotein (HIP) has been isolated from RL95 cells. The full-length cDNA of HIP encodes a protein of 259 amino acids with a calculated molecular weight of 17,754 Da and pI of 11.75. Transfection of HIP cDNA into NIH-3T3 cells demonstrated cell surface expression and a size similar to that of HIP expressed by human cells. Predicted amino acid sequence indicates that HIP lacks a membrane spanning region and has no consensus sites for glycosylation. Northern blot analysis detected a single transcript of 1.3 kb in both total RNA and poly(A$\sp+$) RNA. Examination of human cell lines and normal tissues using both Northern blot and Western blot analysis revealed that HIP is differentially expressed in a variety of human cell lines and normal tissues, but absent in some cell lines examined. HIP has about 80% homology, at the level of both mRNA and protein, to a rodent protein, designated as ribosomal protein L29. Thus, members of the L29 family may be displayed on cell surfaces where they participate in HP/HS binding events. Studies on a synthetic peptide derived from HIP demonstrate that HIP peptide binds HS/HP with high selectivity and has high affinity (Kd = 10 nM) for a subset of polysaccharides found in commercial HIP preparations. Moreover, HIP peptide also binds certain forms of cell surface, but not secreted or intracellular. HS expressed by RL95 and JAR cells. This peptide supports the attachment of several human trophoblastic cell lines and a variety of mammalian adherent cell lines in a HS-dependent fashion. Furthermore, studies on the subset of HP specifically recognized by HIP peptide indicate that this high-affinity HP (HA-HP) has a larger median MW and a greater negative charge density than bulk HP. The minimum size of oligosaccharide required to bind to HIP peptide with high affinity is a septa- or octasaccharide. HA-HP also quantitatively binds to antithrombin-III (AT-III) with high affinity, indicating that HIP peptide and AT-III may recognize the same or similar oligosaccharide structure(s). Furthermore, HIP peptide antagonizes HP action and promotes blood coagulation in both factor Xa- and thrombin-dependent assays. Finally, HA-HP recognized by HP peptide is highly enriched with anticoagulant activity relative to bulk HP. Collectively, these results demonstrate that HIP may play a role in the HP/HS-involved cell-cell and cell-matrix interactions and recognizes a motif in HP similar or identical to that recognized by AT-III and therefore, may modulate blood coagulation. ^

Detection and functional portrayal of a novel class of dihydrotestosterone derived selective progesterone receptor modulators (SPRM).

In early pregnancy, abortion can be induced by blocking the actions of progesterone receptors (PR). However, the PR antagonist, mifepristone (RU38486), is rather unselective in clinical use because it also cross-reacts with other nuclear receptors. Since the ligand-binding domain of human progesterone receptor (hPR) and androgen receptor (hAR) share 54% identity, we hypothesized that derivatives of dihydrotestosterone (DHT), the cognate ligand for hAR, might also regulate the hPR. Compounds designed and synthesized in our laboratory were investigated for their affinities for hPRB, hAR, glucocorticoid receptor (hGRα) and mineralocorticoid receptor (hMR), using whole cell receptor competitive binding assays. Agonistic and antagonistic activities were characterized by reporter assays. Nuclear translocation was monitored using cherry-hPRB and GFP-hAR chimeric receptors. Cytostatic properties and apoptosis were tested on breast cancer cells (MCF7, T-47D). One compound presented a favorable profile with an apparent neutral hPRB antagonistic function, a selective cherry-hPRB nuclear translocation and a cytostatic effect. 3D models of human PR and AR with this ligand were constructed to investigate the molecular basis of selectivity. Our data suggest that these novel DHT-derivatives provide suitable templates for the development of new selective steroidal hPR antagonists.

Reconstitution and functional comparison of purified GlpF and AqpZ, the glycerol and water channels from Escherichia coli

A large family of membrane channel proteins selective for transport of water (aquaporins) or water plus glycerol (aquaglyceroporins) has been found in diverse life forms. Escherichia coli has two members of this family—a water channel, AqpZ, and a glycerol facilitator, GlpF. Despite having similar primary amino acid sequences and predicted structures, the oligomeric state and solute selectivity of AqpZ and GlpF are disputed. Here we report biochemical and functional characterizations of affinity-purified GlpF and compare it to AqpZ. Histidine-tagged (His-GlpF) and hemagglutinin-tagged (HA-GlpF) polypeptides encoded by a bicistronic construct were expressed in bacteria. HA-GlpF and His-GlpF appear to form oligomers during Ni-nitrilotriacetate affinity purification. Sucrose gradient sedimentation analyses showed that the oligomeric state of octyl glucoside-solubilized GlpF varies: low ionic strength favors subunit dissociation, whereas Mg2+ stabilizes tetrameric assembly. Reconstitution of affinity-purified GlpF into proteoliposomes increases glycerol permeability more than 100-fold and water permeability up to 10-fold compared with control liposomes. Glycerol and water permeability of GlpF both occur with low Arrhenius activation energies and are reversibly inhibited by HgCl2. Our studies demonstrate that, unlike AqpZ, a water-selective stable tetramer, purified GlpF exists in multiple oligomeric forms under nondenaturing conditions and is highly permeable to glycerol but less well permeated by water.

Functional analysis of primary visual cortex (V1) in humans

Human area V1 offers an excellent opportunity to study, using functional MRI, a range of properties in a specific cortical visual area, whose borders are defined objectively and convergently by retinotopic criteria. The retinotopy in V1 (also known as primary visual cortex, striate cortex, or Brodmann’s area 17) was defined in each subject by using both stationary and phase-encoded polar coordinate stimuli. Data from V1 and neighboring retinotopic areas were displayed on flattened cortical maps. In additional tests we revealed the paired cortical representations of the monocular “blind spot.” We also activated area V1 preferentially (relative to other extrastriate areas) by presenting radial gratings alternating between 6% and 100% contrast. Finally, we showed evidence for orientation selectivity in V1 by measuring transient functional MRI increases produced at the change in response to gratings of differing orientations. By systematically varying the orientations presented, we were able to measure the bandwidth of the orientation “transients” (45°).

Purification, cDNA cloning, functional expression, and characterization of a 26-kDa endogenous mammalian carboxypeptidase inhibitor.

The recent demonstration of the occurrence in rat brain and other nonpancreatic tissues of carboxypeptidase A (CPA) gene transcripts without associated catalytic activity could be ascribed to the presence of a soluble endogenous protein inhibitor. This tissue carboxypeptidase inhibitor (TCI), detected by the inhibition of added bovine pancreatic CPA, was purified from rat brain. Peptides were obtained by partial proteolysis of purified TCI, a protein of approximately 30 kDa, and starting from their sequences, a full-length cDNA encoding a 223-amino acid protein containing three potential phosphorylation sites was cloned from a cDNA library. Its identity with TCI was shown by expression in Escherichia coli of a recombinant protein recognized by antibodies raised against native TCI and display characteristic CPA-inhibiting activity. TCI appears as a hardly reversible, non-competitive, and potent inhibitor of CPA1 and CPA2 (Ki approximately 3 nM) and mast-cell CPA (Ki = 16 nM) and inactive on various other proteases. This pattern of selectivity might be attributable to a limited homology of a 11-amino acid sequence with sequences within the activation segments of CPA and CPB known to interact with residues within their active sites. The widespread expression of TCI in a number of tissues (e.g., brain, lung, or digestive tract) and its apparently cytosolic localization point to a rather general functional role, e.g., in the control of cytosolic protein degradation.

Chemical and functional identification and characterization of novel sulfated alpha-conotoxins from the cone snail Conus anemone

An LC/MS analysis with diagnostic screening for the detection of peptides with posttranslational modifications revealed the presence of novel sulfated peptides within the -conotoxin molecular mass range in Conus anemone crude venom. A functional assay of the extract showed activity at several neuronal nicotinic acetylcholine receptors (nAChRs). Three sulfated alpha-conotoxins (AnIA, AnIB, and AnIC) were identified by LC/MS and assay-directed fractionation and sequenced after purification. The most active of these, alpha-AnIB, was further characterized and used to investigate the influence of posttranslational modifications on affinity. Synthetic AnIB exhibited subnanomolar potency at the rat alpha3/beta2 nAChR (IC50 0.3 nM) and was 200-fold less active on the rat alpha7 nAChR (IC50 76 nM). The unsulfated peptide [Tyr(16)]AnIB showed a 2-fold and 10-fold decrease in activities at alpha3beta2 (IC50 0.6 nM) and alpha7(IC50 836 nM) nAChR, respectively. Likewise, removal of the C-terminal amide had a greater influence on potency at the alpha7 (IC50 367 nM) than at the alpha3beta2 nAChR (IC50 0.5 nM). Stepwise removal of two N-terminal glycine residues revealed that these residues affect the binding kinetics of the peptide. Comparison with similar 4/7-alpha-conotoxin sequences suggests that residue 11 (alanine or glycine) and residue 14 (glutamine) constitute important determinants for alpha3beta2 selectivity, whereas the C-terminal amidation and sulfation at tyrosine-16 favor alpha7 affinity.

Metastable de-excitation spectroscopy and density functional theory study of the selective oxidation of crotyl alcohol over Pd(111)

The extremely surface sensitive technique of metastable de-excitation spectroscopy (MDS) has been utilized to probe the bonding and reactivity of crotyl alcohol over Pd(111) and provide insight into the selective oxidation pathway to crotonaldehyde. Auger de-excitation (AD) of metastable He (23S) atoms reveals distinct features associated with the molecular orbitals of the adsorbed alcohol, corresponding to emission from the hydrocarbon skeleton, the O n nonbonding, and C═C π states. The O n and C═C π states of the alcohol are reversed when compared to those of the aldehyde. Density functional theory (DFT) calculations of the alcohol show that an adsorption mode with both C═C and O bonds aligned somewhat parallel to the surface is energetically favored at a substrate temperature below 200 K. Density of states calculations for such configurations are in excellent agreement with experimental MDS measurements. MDS revealed oxidative dehydrogenation of crotyl alcohol to crotonaldehyde between 200 and 250 K, resulting in small peak shifts to higher binding energy. Intramolecular changes lead to the opposite assignment of the first two MOs in the alcohol versus the aldehyde, in accordance with DFT and UPS studies of the free molecules. Subsequent crotonaldehyde decarbonylation and associated propylidyne formation above 260 K could also be identified by MDS and complementary theoretical calculations as the origin of deactivation and selectivity loss. Combining MDS and DFT in this way represents a novel approach to elucidating surface catalyzed reaction pathways associated with a “real-world” practical chemical transformation, namely the selective oxidation of alcohols to aldehydes.

Selectivity control in Pt-catalyzed cinnamaldehyde hydrogenation

Chemoselectivity is a cornerstone of catalysis, permitting the targeted modification of specific functional groups within complex starting materials. Here we elucidate key structural and electronic factors controlling the liquid phase hydrogenation of cinnamaldehyde and related benzylic aldehydes over Pt nanoparticles. Mechanistic insight from kinetic mapping reveals cinnamaldehyde hydrogenation is structure-insensitive over metallic platinum, proceeding with a common Turnover Frequency independent of precursor, particle size or support architecture. In contrast, selectivity to the desired cinnamyl alcohol product is highly structure sensitive, with large nanoparticles and high hydrogen pressures favoring C=O over C=C hydrogenation, attributed to molecular surface crowding and suppression of sterically-demanding adsorption modes. In situ vibrational spectroscopies highlight the role of support polarity in enhancing C=O hydrogenation (through cinnamaldehyde reorientation), a general phenomenon extending to alkyl-substituted benzaldehydes. Tuning nanoparticle size and support polarity affords a flexible means to control the chemoselective hydrogenation of aromatic aldehydes.

Tailoring heme -thiolate proteins into efficient biocatalysts with high specificity and selectivity

Cytochrome P450 monooxygenases, one of the most important classes of heme-thiolate proteins, have attracted considerable interest in the biochemical community because of its catalytic versatility, substrate diversity and great number in the superfamily. Although P450s are capable of catalyzing numerous difficult oxidation reactions, the relatively low stability, low turnover rates and the need of electron-donating cofactors have limited their practical biotechnological and pharmaceutical applications as isolated enzymes. The goal of this study is to tailor such heme-thiolate proteins into efficient biocatalysts with high specificity and selectivity by protein engineering and to better understand the structure-function relationship in cytochromes P450. In the effort to engineer P450cam, the prototype member of the P450 superfamily, into an efficient peroxygenase that utilizes hydrogen peroxide via the “peroxide-shunt” pathway, site-directed mutagenesis has been used to elucidate the critical roles of hydrophobic residues in the active site. Various biophysical, biochemical and spectroscopic techniques have been utilized to investigate the wild-type and mutant proteins. Three important P450cam variants were obtained showing distinct structural and functional features. In P450camV247H mutant, which exhibited almost identical spectral properties with the wild-type, it is demonstrated that a single amino acid switch turned the monooxygenase into an efficient preoxidase by increasing the peroxidase activity nearly one thousand folds. In order to tune the distal pocket of P450cam with polar residues, Leu 246 was replaced with a basic residue, lysine, resulting in a mutant with spectral features identical to P420, the inactive species of P450. But this inactive-species-like mutant showed catalytic activities without the facilitation of any cofactors. By substituting Gly 248 with a histidine, a novel Cys-Fe-His ligation set was obtained in P450cam which represented the very rare case of His ligation in heme-thiolate proteins. In addition to serving as a convenient model for hemoprotein structural studies, the G248H mutant also provided evidence about the nature of the axial ligand in cytochrome P420 and other engineered hemoproteins with thiolate ligations. Furthermore, attempts have been made to replace the proximal ligand in sperm whale myoglobin to construct a heme-thiolate protein model by mimicking the protein environment of cytochrome P450cam and chloroperoxidase.

Tailoring Heme-Thiolate Proteins into Efficient Biocatalysts with High Specificity and Selectivity

Cytochrome P450 monooxygenases, one of the most important classes of heme-thiolate proteins, have attracted considerable interest in the biochemical community because of its catalytic versatility, substrate diversity and great number in the superfamily. Although P450s are capable of catalyzing numerous difficult oxidation reactions, the relatively low stability, low turnover rates and the need of electron-donating cofactors have limited their practical biotechnological and pharmaceutical applications as isolated enzymes. The goal of this study is to tailor such heme-thiolate proteins into efficient biocatalysts with high specificity and selectivity by protein engineering and to better understand the structure-function relationship in cytochromes P450. In the effort to engineer P450cam, the prototype member of the P450 superfamily, into an efficient peroxygenase that utilizes hydrogen peroxide via the “peroxide-shunt” pathway, site-directed mutagenesis has been used to elucidate the critical roles of hydrophobic residues in the active site. Various biophysical, biochemical and spectroscopic techniques have been utilized to investigate the wild-type and mutant proteins. Three important P450cam variants were obtained showing distinct structural and functional features. In P450camV247H mutant, which exhibited almost identical spectral properties with the wild-type, it is demonstrated that a single amino acid switch turned the monooxygenase into an efficient preoxidase by increasing the peroxidase activity nearly one thousand folds. In order to tune the distal pocket of P450cam with polar residues, Leu 246 was replaced with a basic residue, lysine, resulting in a mutant with spectral features identical to P420, the inactive species of P450. But this inactive-species-like mutant showed catalytic activities without the facilitation of any cofactors. By substituting Gly 248 with a histidine, a novel Cys-Fe-His ligation set was obtained in P450cam which represented the very rare case of His ligation in heme-thiolate proteins. In addition to serving as a convenient model for hemoprotein structural studies, the G248H mutant also provided evidence about the nature of the axial ligand in cytochrome P420 and other engineered hemoproteins with thiolate ligations. Furthermore, attempts have been made to replace the proximal ligand in sperm whale myoglobin to construct a heme-thiolate protein model by mimicking the protein environment of cytochrome P450cam and chloroperoxidase.

Functional surface chemistry of carbon-based nanostructures

The recently discovered abilities to synthesize single-walled carbon nanotubes and prepare single layer graphene have spurred interest in these sp2-bonded carbon nanostructures. In particular, studies of their potential use in electronic devices are many as silicon integrated circuits are encountering processing limitations, quantum effects, and thermal management issues due to rapid device scaling. Nanotube and graphene implementation in devices does come with significant hurdles itself. Among these issues are the ability to dope these materials and understanding what influences defects have on expected properties. Because these nanostructures are entirely all-surface, with every atom exposed to ambient, introduction of defects and doping by chemical means is expected to be an effective route for addressing these issues. Raman spectroscopy has been a proven characterization method for understanding vibrational and even electronic structure of graphene, nanotubes, and graphite, especially when combined with electrical measurements, due to a wealth of information contained in each spectrum. In Chapter 1, a discussion of the electronic structure of graphene is presented. This outlines the foundation for all sp2-bonded carbon electronic properties and is easily extended to carbon nanotubes. Motivation for why these materials are of interest is readily gained. Chapter 2 presents various synthesis/preparation methods for both nanotubes and graphene, discusses fabrication techniques for making devices, and describes characterization methods such as electrical measurements as well as static and time-resolved Raman spectroscopy. Chapter 3 outlines changes in the Raman spectra of individual metallic single-walled carbon nantoubes (SWNTs) upon sidewall covalent bond formation. It is observed that the initial degree of disorder has a strong influence on covalent sidewall functionalization which has implications on developing electronically selective covalent chemistries and assessing their selectivity in separating metallic and semiconducting SWNTs. Chapter 4 describes how optical phonon population extinction lifetime is affected by covalent functionalization and doping and includes discussions on static Raman linewidths. Increasing defect concentration is shown to decrease G-band phonon population lifetime and increase G-band linewidth. Doping only increases G-band linewidth, leaving non-equilibrium population decay rate unaffected. Phonon mediated electron scattering is especially strong in nanotubes making optical phonon decay of interest for device applications. Optical phonon decay also has implications on device thermal management. Chapter 5 treats doping of graphene showing ambient air can lead to inadvertent Fermi level shifts which exemplifies the sensitivity that sp2-bonded carbon nanostructures have to chemical doping through sidewall adsorption. Removal of this doping allows for an investigation of electron-phonon coupling dependence on temperature, also of interest for devices operating above room temperature. Finally, in Chapter 6, utilizing the information obtained in previous chapters, single carbon nanotube diodes are fabricated and characterized. Electrical performance shows these diodes are nearly ideal and photovoltaic response yields 1.4 nA and 205 mV of short circuit current and open circuit voltage from a single nanotube device. A summary and discussion of future directions in Chapter 7 concludes my work.

Applications of functional nucleic acids in imaging, sensing and drug delivery and investigations of DNAzyme-metal interactions

Functional nucleic acids (FNA), including nucleic acids catalysts (ribozymes and DNAzymes) and ligands (aptamers), have been discovered in nature or isolated in a laboratory through a process called in vitro selection. They are nucleic acids with functions similar to protein enzymes or antibodies. They have been developed into sensors with high sensitivity and selectivity; it is realized by converting the reaction catalyzed by a DNAzyme/ribozyme or the binding event of an aptamer to a fluorescent, colorimetric or electrochemical signal. While a number of studies have been reported for in vitro sensing using DNAzymes or aptamers, there are few reports on in vivo sensing or imaging. MRI is a non-invasive imaging technique; smart MRI contrast agents were synthesized for molecular imaging purposes. However, their rational design remains a challenge due to the difficulty to predict molecular interactions. Chapter 2 focuses on rational design of smart T1-weighted MRI contrast agents with high specificity based on DNAzymes and aptamers. It was realized by changing the molecular weight of the gadolinium conjugated DNA strand with the analytes, which lead to analyte-specific water proton relaxation responses and contrast changes on an MRI image. The designs are general; the high selectivity of FNA was retained. Most FNA-based fluorescent sensors require covalent labeling of fluorophore/quencher to FNAs, which incurrs extra expenses and could interfere the function of FNAs. Chapter 3 describes a new sensor design avoiding the covalent labeling of fluorophore and quencher. The fluorescence of malachite green (MG) was regulated by the presence of adenosine. Conjugate of aptamers of MG and adenosine and a bridge strand were annealed in a solution containing MG. The MG aptamer did not bind MG because of its hybridization to the bridge strand, resulting in low fluorescence signal of MG. The hybridization was weakened in the presence of adenosine, leading to the binding of MG to its aptamer and a fluorescence increase. The sensor has comparable detection limit (20 micromolar) and specificity to its labeled derivatives. Enzymatic activity of most DNAzymes requires metal cations. The research on the metal-DNAzyme interaction is of interest and challenge to scientists because of the lack of structural information. Chapters 4 presents the research on the characterization of the interaction between a Cu2+-dependent DNAzyme and Cu2+. Electron paramagnetic resonance (EPR) and UV-Vis spectroscopy were used to probe the binding of Cu2+ to the DNAzyme; circular dichroism was used to probe the conformational change of the DNAzyme induced by Cu2+. It was proposed that the conformational change by the Cu2+ binding is important for the activity of the DNAzyme. Chapter 5 reports the dependence of the activity of 8-17 DNAzyme on the presence of both Pb2+ and other metal cations including Zn2+, Cd2+ and Mg2+. It was discovered that presence of those metal cations can be cooperative or inhibitive to 8-17 activity. It is hypothesized that the 8-17 DNAzyme had multiple binding sites for metal cations based on the results. Cisplatin is effective killing tumor cells, but with significant side effects, which can be minimized by its targeted delivery. Chapter 6 focuses on the effort to functionalize liposomes encapsulating cisplatin by an aptamer that selectively bind nucleolin, an overexpressed protein by breast cancer cells. The study proved the selective cytotoxicity to breast cancer cells of the aptamer-functionalized liposome.

An alternative methodology for fitting selectivity curves to pre-defined distributions

A non-linear least-squares methodology for simultaneously estimating parameters of selectivity curves with a pre-defined functional form, across size classes and mesh sizes, using catch size frequency distributions, was developed based on the model of Kirkwood and Walker [Kirkwood, G.P., Walker, T.L, 1986. Gill net selectivities for gummy shark, Mustelus antarcticus Gunther, taken in south-eastern Australian waters. Aust. J. Mar. Freshw. Res. 37, 689-697] and [Wulff, A., 1986. Mathematical model for selectivity of gill nets. Arch. Fish Wiss. 37, 101-106]. Observed catches of fish of size class I in mesh m are modeled as a function of the estimated numbers of fish of that size class in the population and the corresponding selectivities. A comparison was made with the maximum likelihood methodology of [Kirkwood, G.P., Walker, T.I., 1986. Gill net selectivities for gummy shark, Mustelus antarcticus Gunther, taken in south-eastern Australian waters. Aust. J. Mar. Freshw. Res. 37, 689-697] and [Wulff, A., 1986. Mathematical model for selectivity of gill nets. Arch. Fish Wiss; 37, 101-106], using simulated catch data with known selectivity curve parameters, and two published data sets. The estimated parameters and selectivity curves were generally consistent for both methods, with smaller standard errors for parameters estimated by non-linear least-squares. The proposed methodology is a useful and accessible alternative which can be used to model selectivity in situations where the parameters of a pre-defined model can be assumed to be functions of gear size; facilitating statistical evaluation of different models and of goodness of fit. (C) 1998 Elsevier Science B.V.