NADPH-CYTOCHROME P-450 REDUCTASE: EVIDENCE FOR TWO SEPARATE STRUCTURAL DOMAINS AND ISOLATION AND CHARACTERIZATION OF THE MEMBRANE ASSOCIATED SEGMENT

Homogenous detergent-solubilized NADPH-Cytochrome P-450 reductase was incorporated into microsomes and liposomes. This binding occurred spontaneously at temperatures between 4(DEGREES) and 37(DEGREES) and appeared to involve hydrophobic forces as the binding was not disrupted by 0.5 M sodium chloride. This exogenously-added reductase was active catalytically towards native cytochrome P-450, suggesting an association with the microsomal membrane similar to endogenous reductase. Homogeneous detergent-solubilized reductase was disaggregated by Renex-690 micelles, confirming the presence of a hydrophobic combining region on the enzyme. In contrast to these results, steapsin protease-solubilized reductase was incapable of microsomal attachment and did not interact with Renex-690 micelles. Detergent-solubilized reductase (76,500 daltons) was converted into a form with the electrophoretic mobility of steapsin protease-solubilized reductase (68,000 daltons) and a 12,500 dalton peptide (as determined by polyacrylamide-SDS gel electrophoresis) when the liposomal-incorporated enzyme was incubated with steapsin protease. The 68,000 dalton fragment thus obtained had properties identical with steapsin protease-solubilized reductase, i.e. it was catalytically active towards cytochrome c but inactive towards cytochrome P-450 and did not bind liposomes. The 12,500 dalton fragment remained associated with the liposomes when the digest was fractionated by gel filtration, suggesting that this is the segment of the enzyme which is embedded in the phospholipid bilayer. Thus, detergent-solubilized reductase appears to contain a soluble catalytic domain and a separate and separable membrane-binding domain. This latter domain is required for attaching the enzyme to the membrane and also to facilitate the catalytic interaction between the reductase and its native electron acceptor, cytochrome P-450. The membrane-binding segment of the reductase was isolated by preparative gel electrophoresis in SDS following its generation by proteolytic treatment of liposome-incorporated reductase. The peptide has a molecular weight of 6,400 as determined by gel filtration in 8 M guanidine hydrochloride and has an amino acid composition which is not especially hydrophobic. Following removal of SDS and dialysis out of 6 M urea, the membrane-binding peptide was unable to inhibit the activity of a reconstituted system containing purified reductase and cytochrome P-450. Moreover, when reductase and cytochrome P-450 were added to liposomes which contained the membrane-binding peptide, it was determined that mixed function oxidase activity was reconstituted as effectively as when vesicles without the membrane-binding peptide were used. Thus, the membrane-binding peptide was ineffective as an inhibitor of mixed function oxidase activity, suggesting perhaps that it facilitates catalysis by anchoring the catalytic domain of the reductase proximal to cytochrome P-450 (i.e. in the same mixed micelle) rather than through a specific interaction with cytochrome P-450. ^

Rifaximin-mediated changes to the epithelial cell proteome: 2-D gel analysis

Diarrhea remains a significant cause of worldwide morbidity and mortality. Over 4 million children die of diarrhea annually. Although antibiotics can be used as prophylaxis or for treatment of diarrhea, concern remains over antibiotic resistance. Rifaximin is a semi-synthetic rifamycin derivative that can be used to treat symptoms of infectious diarrhea, inflammatory bowel syndrome, bacterial overgrowth of the small bowel, pouchitis, and fulminant ulcerative colitis. Rifaximin is of particular interest because it is poorly adsorbed in the intestines, shows no indication of inducing bacterial resistance, and has minimal effect on intestinal flora. In order to better understand how rifaximin functions, we sought to compare the protein expression profile of cells pretreated with rifaximin, as compared to cells treated with acetone, rifamycin (control antibiotic), or media (untreated). 2-D gel electrophoresis identified 38 protein spots that were up- or down-regulated by over 2-fold in rifaximin treated cells compared to controls. 16 of these spots were down-regulated, including keratin, annexin A5, intestinal-type alkaline phosphatase, histone h4, and histone-binding protein RbbP4. 22 spots were up-regulated, including heat shock protein HSP 90 alpha, alkaline phosphatase, and fascin. Many of the identified proteins are associated with cell structure and cytoskeleton, transcription and translation, and cellular metabolism. A better understanding of the functionality of rifaximin will identify additional potential uses for rifaximin and determine for whom the drug is best suited. ^