Metabolism of 1,8-Cineole in Rat: Its Effects on Liver and Lung Microsomal Cytochrome P-450 Systems

The monoterpene cyclic ether, cineole (l,8-cineole, I) also known as eucalyptol, is a component of many essential oils and is widely distributed in nature. It is extensively used in pharmaceutical preparations for external application and also as a nasal spray. It was reported earlier that cineole when administered to sheep may be largely oxidized in the system (Scheline 1978). However the mode of metabolism of cineole is not known. Hence the present study was undertaken to investigate the metabolic fate of this ubiquitous terpenoid following its administration to rats by gastric intubation.

Metabolism of geraniol and linalool in the rat and effects on liver and lung microsomal enzymes

1. Metabolites isolated from the urine of rats after oral administration of geraniol (I) were: geranic acid (II), 3-hydroxy-citronellic acid (III), 8-hydroxy-geraniol (IV), 8-carboxy-geraniol (V) and Hildebrandt acid (VI). 2. Metabolites isolated from urine of rats after oral administration of linalool (VII) were 8-hydroxy-linalool (VIII) and 8-carboxy-linalool (IX). 3. After three days of feeding rats with either geraniol or linalool, liver-microsomal cytochrome P-450 was increased. Both NADH- and NADPH-cytochrome c reductase activities were not significantly changed during the six days of treatment. 4. Oral administration of these two terpenoids did not affect any of the lung-microsomal parameters measured.

Purification of clofibrate-induced carnitine acetyltransferase from rat liver mitochondria

The enzyme carnitine acetyltransferase (acetyl-CoA:carnitine O-acetyltransferase, EC 2.3.1.7) has been purified to homogeneity from hepatic mitochondria of clofibrate-fed rats. It is a protein of molecular weight 56 000 composed of two non-identical subunits of molecular weight 34 000 and 25 000. The enzyme is inhibited by palmityl-CoA as well as acetyl carnitine. The inhibition by fatty acyl-CoA is competitive with respect to both the substrates, carnitine and acetyl-CoA. The inhibition by acetylcarnitine is reversed by carnitine but not by acetyl-CoA.

The metabolism of phenolic acids in the rat

Some of the enzyme systems in the formation of p-hydroxybenzoate from tyrosine have been studied in the rat liver in vitro. The conversion of p-hydroxycinnamate into p-hydroxybenzoate, which was found in rat liver mitochondria showed a number of differences when compared with the b-oxidation of fatty acids. Studies with p-hydroxy[U-14C]cinnamate indicated that 14CO2 was released during the formation of p-hydroxybenzoate. The formation of p-hydroxycinnamate from tyrosine of p-hydroxyphenyl-lactate could not be demonstrated in vitro. The interconversion of p-hydroxycinnamate and p-hydroxyphenylpropionate was demonstrated in rat liver mitochondria.

Studies on the metabolism of β-carotene and apo-β-carotenoids in rats and chickens

1. (1) The relative abilities of the various cell fractions of rat and chicken liver to oxidize and reduce retinal and 8'- and 12'-apo-β-carotenal were investigated and it has been shown that, while retinal is exclusively oxidized by the soluble fraction, the apocarotenals are mostly oxidized by the particulate fractions of the homogenate. 2. (2) Addition of NAD+ or NADP+ markedly activated the oxidation of the apocarotenals, but not of retinal by the particulate fractions. 3. (3) Considerable amounts of retinal and 8'-, 10'- and 12'-apo-β-carotenal were isolated from the intestine of chickens fed β-carotene and these apocarotenoids were conclusively identified. 4. (4) Significant amounts of 8'-, 10'- and 12'-apo-β-carotenoic acids were isolated from the intestine of rats given 8'-apo-β-carotenal and these apocarotenoic acids were also conclusively identified. 5. (5) In the light of these observations it is suggested that during conversion to vitamin A, the β-carotene molecule is simultaneously attacked by the dioxygenase at several double bonds, the primary attack being at the central double bond and a tentative scheme for the mechanism of conversion is proposed.

Metabolism of ubiquinone in relation to vitamin a status in the rat

1. 1. Biosynthetic experiments in vitro with slices of livers from normal and vitamin A-deficient rats confirmed that synthesis of ubiquinone did not increase in vitamin A deficiency. 2. 2. During development of deficiency of vitamin A in the rat, there was a definite increase in the synthesis of ubiquinone at the 10-days stage but this reverted to low, initial level by 20 days and after. 3. 3. Vitamin A analogues, 3-dehydroretinal, 5,6-monoepoxyretinal and retinoic acid, which supported growth have restored ubiquinone concentration to the normal levels and relieved the lowering in its catabolism. The biologically inert 5,8-monoepoxyretinal was the least active of the analogues tested. 4. 4. The concentration and synthesis of ubiquinone in the liver decreased under conditions of hypervitaminosis A. 5. 5. The experimental evidence does not support the hypothesis of inverse relationship between vitamin A and ubiquinone synthesis.

Effect of cold exposure on the metabolism of ubiquinone in the rat

1. Accumulation of ubiquinone in the livers of rats exposed to a cold environment was shown to be due to both decreased catabolism during the entire experimental period and increased synthesis during an intermediate stage (10–20 days). 2. The increased endogenous synthesis in the cold-exposed rats was eliminated when ubiquinone accumulated in the liver after exposure for 40 days (coinciding with cclimatization), or by absorption of the exogenous dietary supply, possibly by the mechanism of end-product regulation.

The conversion of 3-hydroxyanthranilic acid to cinnabarinic acid by the nuclear fraction of rat liver

Although several authors have implicated 3-hydroxyanthranilic acid (3-OHA) as an intermediate in tryptophaniacin pathway in animals (Kaplan, 1961), alternative pathways of metabolism of this compound have not been fully explored. Madhusudanan Nair obtained an enzyme from spinach leaves which could convert 3-OHA to cinnabarinic acid (private communication). Viollier and Süllmann (1950) reported the conversion of 3-OHA to an unidentified red compound by rat liver homogenates. The present investigation describes the identification of this product as cinnabarinic acid (2-amino-3-H-isophenoxazine-3-one-1,9-dicarboxylic acid). Cinnabarinic acid is known to occur in nature along with cinnabarin is olated from the fungus Polystictus sanguineus (Gripenberg et al., 1957; Gripenberg, 1958).

Biotransformations of α-terpineol in the rat: Its effects on the liver microsomal cytochrome P-450 system

Alpha-Terpineol (I), a monocyclic monoterpene tertiary alcohol, is widely used in the manufacture of perfumes, cosmetics, soaps and antiseptic agents. It was reported earlier (Horning et al. 1976) that this monoterpene alcohol when administered to humans is hydroxylated to p-menth-l,2,8-triol (II). It is not known whether c~-terpineol also produces other metabolites during its metabolism in the mammalian system and if so, the nature of these metabolites.

Thyroidal control of hepatic release and metabolism of vitamin A

The inverse relationship that exists between thyroxine and the vitamin A level of plasma has been examined in chicken. Thyroxine treatment leads to a decrease in the level of vitamin A carrier proteins, retinol-binding protein and prealbumin-2 in plasma and liver. There is an accumulation of vitamin A in the liver, with a greater proportion of vitamin A alcohol being present compared to that of control birds. In thyroxine treatment there is enhanced plasma turnover of retinol-binding protein and prealbumin-2, while their rates of synthesis are marginally increased. Amino acid supplementation partially counteracts effects of thyroxine treatment. Amino acid supplementation of thyroxine-treated birds does not alter the plasma turnover rates of retinol-binding protein and prealbumin-2 but increases substentially their rates of synthesis. The release of vitamin A into circulation is interfered with in hyperthyroidism due to inadequate availability of retinol-binding protein being caused by enhanced plasma turnover rate not compensated for by synthesis.

Studies on the metabolism of l-menthol in rats

Metabolism of l-menthol in rats was investigated both in vivo and in vitro. Metabolites isolated and characterized from the urine of rats after oral administration (800 mg/kg of body weight/day) of l-menthol were the following: p-menthane-3,8-diol (II), p-menthane-3,9-diol (III), 3,8-oxy-p-menthane-7-carboxylic acid (IV), and 3,8-dihyroxy-p-menthane-7-carboxylic acid (V). In vivo, the major urinary metabolites were compounds II and V. Repeated oral administration (800 mg/kg of body weight/day) of l-menthol to rats for 3 days resulted in the increase of both liver microsomal cytochrome P-450 content and NADPH-cytochrome c reductase activity by nearly 80%. Further treatment (for 7 days total) reduced their levels considerably, although the levels were still higher than the control values. Both cytochrome b5 and NADH-cytochrome c reductase levels were not changed during the 7 days of treatment. Rat liver microsomes readily converted l-menthol to p-menthane-3,8-diol (II) in the presence of NADPH and O2. This activity was significantly higher in microsomes obtained from phenobarbital (PB)-induced rats than from control microsomal preparations, whereas 3-methylcholanthrene (3-MC)-induced microsomes failed to convert l-menthol to compound II in the presence of NADPH and O2. l-Menthol elicited a type I spectrum with control (Ks = 60.6 microM) and PB-induced (Ks = 32.3 microM) microsomes whereas with 3MC-induced microsomes it produced a reverse type I spectrum.

Cytochrome P450-mediated metabolism in brain: functional roles and their implications

Introduction: Cytochromes P450 (P450) and associated monooxygenases are a family of heme proteins involved in metabolism of endogenous compounds (arachidonic acid, eicosanoids and prostaglandins) as also xenobiotics including drugs and environmental chemicals. Liver is the major organ involved in P450-mediated metabolism and hepatic enzymes have been characterized. Extrahepatic organs, such as lung, kidney and brain have the capability for biotransformation through P450 enzymes. Brain, including human brain, expresses P450 enzymes that metabolize xenobiotics and endogenous compounds. Areas covered: An overview of P450-mediated metabolism in brain is presented focusing on distinct differences seen in expression of P450 enzymes, generation of unique P450 enzymes in brain through alternate splicing and their consequences in terms of metabolism of psychoactive drugs and inflammatory prompts, such as leukotrienes, thus modulating inflammatory response. Expert opinion: The brain possesses unique P450s that metabolize drugs and endogenous compounds through pathways that are markedly different from that seen in liver indicating that extrapolation directly from liver to brain is not appropriate. It is therefore necessary to characterize the unique brain P450s and their ability to metabolize xenobiotics and endogenous compounds to better understand the functions of this important class of enzymes in brain, especially human brain.

Malaria Parasite-Synthesized Heme Is Essential in the Mosquito and Liver Stages and Complements Host Heme in the Blood Stages of Infection

Heme metabolism is central to malaria parasite biology. The parasite acquires heme from host hemoglobin in the intraerythrocytic stages and stores it as hemozoin to prevent free heme toxicity. The parasite can also synthesize heme de novo, and all the enzymes in the pathway are characterized. To study the role of the dual heme sources in malaria parasite growth and development, we knocked out the first enzyme, d-aminolevulinate synthase (ALAS), and the last enzyme, ferrochelatase (FC), in the heme-biosynthetic pathway of Plasmodium berghei (Pb). The wild-type and knockout (KO) parasites had similar intraerythrocytic growth patterns in mice. We carried out in vitro radiolabeling of heme in Pb-infected mouse reticulocytes and Plasmodium falciparum-infected human RBCs using 4-(14) C] aminolevulinic acid (ALA). We found that the parasites incorporated both host hemoglobin-heme and parasite-synthesized heme into hemozoin and mitochondrial cytochromes. The similar fates of the two heme sources suggest that they may serve as backup mechanisms to provide heme in the intraerythrocytic stages. Nevertheless, the de novo pathway is absolutely essential for parasite development in the mosquito and liver stages. PbKO parasites formed drastically reduced oocysts and did not form sporozoites in the salivary glands. Oocyst production in PbALASKO parasites recovered when mosquitoes received an ALA supplement. PbALASKO sporozoites could infect mice only when the mice received an ALA supplement. Our results indicate the potential for new therapeutic interventions targeting the heme-biosynthetic pathway in the parasite during the mosquito and liver stages.

UVB irradiation alters the activities and kinetic properties of the enzymes of energy metabolism in rat lens during aging

Ultraviolet (UV) radiation is one of the major risk factors of cataract (loss of eye-lens transparency). The influence of UVB radiation (300 nm, 100 mu W cm(-2)) on the activity and apparent kinetic constants (K-m and V-max) of rat lens hexokinase (HK;EC2.7.1.1), phosphofructokinase (PFK;EC2.7.1.11), isocitrate dehydrogenase (ICDH;EC1.1.1.41) and malate dehydrogenase (MDH;EC1.1.1.37) of energy metabolism has been investigated by irradiating the lens homogenate of three-and 12-month-old rats. In the three-month-old group specific activities of HK and PFK are reduced by 56 and 43 %, respectively, and there is no change in ICDH and MDH activities after a 24 h exposure. On the other hand, in the 12-month-old group the decreases are 72, 71, 24 and 16 % for HK, PFK. ICDH and MDH, respectively. UVB irradiation increases the apparent K-m of HK and PFK (in both age groups), whereas the K-m of ICDH and MDH is not altered. While the decrease in V-max of these enzymes due to UVB exposure is only marginal in three-month-old rats, it is more pronounced (significant) in 12-month-old rats. A similar decrease in enzyme activities of HK and PFK is also observe upon UVB exposure of the intact rat lens. The photoinduced changes in energy metabolism may in turn have a bearing on lens transparency, particularly at an older age.