Evidence for a predominantly NADH-dependent O-dealkylating system in rat hepatic microsomes

Cytochrome P450 of wood-rotting basidiomycetes and biotechnological applications

Wood-rotting basidiomycetes possess superior metabolic functions to degrade woody biomass, and these activities are indispensable for the carbon cycle of the biosphere. As well as basic studies of the biochemistry of basidiomycetes, many researchers have been focusing on utilizing basidiomycetes and/or their enzymes in the biotechnology sector; therefore, the unique activities of their extracellular and intracellular enzymes have been widely demonstrated. A rich history of applied study has established that basidiomycetes are capable of metabolizing a series of endogeneous and exogeneous compounds using cytochrome P450s (P450s). Recently, whole genome sequence analyses have revealed large-scale divergences in basidiomycetous P450s. The tremendous variation in P450s implies that basidiomycetes have vigorously diversified monooxygenase functions to acquire metabolic adaptations such as lignin degradation, secondary metabolite production, and xenobiotics detoxification. However, fungal P450s discovered from genome projects are often categorized into novel families and subfamilies, making it difficult to predict catalytic functions by sequence comparison. Experimental screening therefore remains essential to elucidate the catalytic potential of individual P450s, even in this postgenomic era. This paper archives the known metabolic capabilities of basidiomycetes, focusing on their P450s, outlines the molecular diversity of basidiomycetous P450s, and introduces new functions revealed by functionomic studies using a recently developed, rapid, functional screening system.

Heterologous expression and mechanistic investigation of a fungal cytochrome P450 (CYP5150A2): involvement of alternative redox partners

A fungal cytochrome P450 monooxygenase (CYP5150A2) from the white-rot basidiomycete Phanerochaete chrysosporium was heterologously expressed in Escherichia coli and purified as an active form. The purified CYP5150A2 was capable of hydroxylating 4-propylbenzoic acid (PBA) with NADPH-dependent cytochrome P450 oxidoreductase (CPR) as the single redox partner; the reaction efficiency was improved by the addition of electron transfer protein cytochrome b5 (Cyt-b5). Furthermore, CYP5150A2 exhibited substantial activity with redox partners Cyt-b5 and NADH-dependent Cyt-b5 reductase (CB5R) even in the absence of CPR. These results indicated that a combination of CB5R and Cyt-b5 may be capable of donating both the first and the second electrons required for the monooxygenation reaction. Under reaction conditions in which the redox system was associated with the CB5R-dependent Cyt-b5 reduction system, the exogenous addition of CPR and NADPH had no effect on the PBA hydroxylation rate or on coupling efficiency, indicating that the transfer of the second electron from Cyt-b5 was the rate-limiting step in the monooxygenase system. In addition, the rate of PBA hydroxylation was significantly dependent on Cyt-b5 concentration, exhibiting Michaelis-Menten kinetics. This study provides indubitable evidence that the combination of CB5R and Cyt-b5 is an alternative redox partner facilitating the monooxygenase reaction catalyzed by CYP5150A2.

Chlorzoxazone hydroxylation in microsomes and hepatocytes from cytochrome P450 oxidoreductase-null mice

Previous studies have demonstrated that the NADH-dependent cytochrome b(5) electron transfer pathway can support some cytochrome P450 monooxygenases in vitro in the absence of their normal redox partner, NADPH-cytochrome P450 oxidoreductase. However, the ability of this pathway to support P450 activity in whole cells and in vivo remains unresolved. To address this question, liver microsomes and hepatocytes were prepared from hepatic cytochrome P450 oxidoreductase-null mice and chlorzoxazone hydroxylation, a reaction catalyzed primarily by cytochrome P450 2E1, was evaluated. As expected, NADPH-supported chlorzoxazone hydroxylation was absent in liver microsomes from oxidoreductase-null mice, whereas NADH-supported activity was about twofold higher than that found in normal (wild-type) liver microsomes. This greater activity in oxidoreductase-null microsomes could be attributed to the fourfold higher level of CYP2E1 and 1.4-fold higher level of cytochrome b(5). Chlorzoxazone hydroxylation in hepatocytes from oxidoreductase-null mice was about 5% of that in hepatocytes from wild-type mice and matched the results obtained with wild-type microsomes, where activity obtained with NADH was about 5% of that obtained when both NADH and NADPH were included in the reaction mixture. These results argue that the cytochrome b(5) electron transfer pathway can support a low but measurable level of CYP2E1 activity under physiological conditions.

Effective NADH-dependent oxidation of 7beta-hydroxy-delta8-tetrahydrocannabinol to the corresponding ketone by Japanese monkey hepatic microsomes

The NADH-dependent activity by hepatic microsomes of Japanese monkeys for 7-oxo-Delta(8)-tetrahydrocannabinol (7-oxo-Delta(8)-THC) formation from 7beta-hydroxy-Delta(8)-THC exhibited about 70% of the NADPH-dependent activity (100%) at the substrate concentration of 72.7 microM, although NADPH was an obligatory cofactor for maximal activity. Both NADH- and NADPH-dependent activities were significantly inhibited by the typical P450 inhibitors, such as SKF525-A and metyrapone. Both activities were almost completely inhibited by the NADPH-P450 reductase inhibitor diphenyliodonium chloride. The ratio of NADH- and NADPH-dependent activities varied significantly according to the substrate concentration. Interestingly, the NADH-dependent activity was higher than that of NADPH at low substrate concentrations of 13-50 microM. The ratio was also affected by the cofactor concentration. In the reconstituted system of CYP3A8 purified from hepatic microsomes of Japanese monkeys as a major enzyme responsible for the NADPH-dependent oxidation, NADH as well as NADPH could sustain the oxidation of 7beta-hydroxy-Delta(8)-THC to the corresponding ketone. The NADH-dependent oxidation of 7beta-hydroxy-Delta(8)-THC by monkey livers is mainly catalyzed by CYP3A8 as well as the NADPH-dependent oxidation. These results indicate that NADH as a cofactor may be also useful for the oxidation of 7beta-hydroxy-Delta(8)-THC, and that the cofactor requirement for the reaction is varied by the concentrations of substrate and/or cofactor.

The roles of cytochrome b5 in cytochrome P450 reactions

Cytochrome b(5), a 17-kDa hemeprotein associated primarily with the endoplasmic reticulum of eukaryotic cells, has long been known to augment some cytochrome P450 monooxygenase reactions, but the mechanism of stimulation has remained controversial. Studies in recent years have clarified this issue by delineating three pathways by which cytochrome b(5) augments P450 reactions: direct electron transfer of both required electrons from NADH-cytochrome b(5) reductase to P450, in a pathway separate and independent of NADPH-cytochrome P450 reductase; transfer of the second electron to oxyferrous P450 from either cytochrome b(5) reductase or cytochrome P450 reductase; and allosteric stimulation of P450 without electron transfer. Evidence now indicates that each of these pathways is likely to operate in vivo.

Evidence for the presence of a microsomal NADH-dependent enzyme system that can bioactivate aromatic amines in the liver of rats and mice

Experimental evidence is presented for the presence in the liver of rats and mice of an Aroclor 1254-inducible, NADH-dependent enzyme system that can catalyse the bioactivation of aromatic and heterocyclic amines to genotoxic metabolites. It differs from the established microsomal cytochrome P450 and flavin monooxygenase systems in its response to treatment with cytochrome P450 inducing agents, optimum protein concentration and in vitro modulation by DMSO. The mutagenic metabolites generated by the NADH-supported system appear to be similar to those generated by the NADPH-mediated systems. Mutagenicity of the aminocompounds in the presence of either cosubstrate was less pronounced in an O-acetyltransferase-deficient bacterial strain, implying the presence of hydroxylamines. Moreover, glutathione potentiated the mutagenic response of both the NADH- and NADPH-generated metabolites. Cytochrome c suppressed markedly the NADPH-dependent mutagenicity of aromatic amines but had no such effect in the presence of NADH. Similarly, antibodies to cytochrome P450 reductase markedly inhibited the NADPH-, but not the NADH-dependent bioactivation of the aromatic amine 2-aminoanthracene. The cytochrome P450 suicide inhibitor, 1-aminobenzotriazole, decreased the mutagenicity of both, the NADH- and NADPH-mediated bioactivation of the aminocompounds. The above findings raise the possibility that a cytochrome P450-like protein, that can receive electrons from NADH, possibly through cytochrome b5 reductase, is present in the hepatic microsomes of rats and mice, and is capable of catalysing the bioactivation of aromatic amines through N-hydroxylation. Such a hypothesis is supported by the findings that NADH could support the O-dealkylation of 7-methoxy- and 7-ethoxy-resorufin, in the absence of NADPH. Finally the NADH-dependent bioactivation of aromatic amines was induced markedly by Aroclor 1254 and benzo(a)pyrene in Ah responsive, but not Ah nonresponsive, mice indicating that it is associated with the Ah locus.

Effect of hyperthyroidism on the in vitro metabolism and covalent binding of 1,1-dichloroethylene in rat liver microsomes

Hyperthyroidism potentiates the in vivo hepatotoxicity of 1,1-dicholoroethylene (DCE) in rats, with a concomitant increase in [14C]-DCE covalent binding. The enhanced injury produced in hyperthyroid livers by DCE could be due to alterations in either the bioactivation or detoxication phases of DCE metabolism. Previous in vitro studies suggested that hyperthyroidism did not potentiate DCE hepatotoxicity by increasing DCE oxidation to intermediates which were able to covalently bind. Several factors, however, that could contribute to the magnitude of DCE bioactivation or covalent binding were not examined. Our objectives were to characterize the effects of hyperthyroidism in male Sprague-Dawley rats on: (1) covalent binding of [14C]-DCE to microsomes and other subcellular fractions, (2) microsomal mixed-function oxidase (MFO) and glutathione S-transferase (GST) activities, and (3) inactivation of microsomal enzyme activities by presumptive DCE reactive intermediates. Hyperthyroid (HYPERT) and euthyroid (EUT) rats received 3 s.c. injections of thyroxine (100 micrograms/100 g) or vehicle, respectively, at 48-h intervals; microsomes and other subcellular fractions were isolated from HYPERT and EUT livers 24 h after the last injection. [14C]-DCE-derived covalent binding was consistently greater in EUT than HYPERT microsomes. The absence of NADH, and the addition of low concentrations (0.1 and 0.5 mM), but not higher concentrations (> 1 mM), of glutathione (GSH) diminished covalent binding to a greater extent in HYPERT than EUT microsomes. Covalent binding in mitochondrial, nuclear, and cytosolic fractions of EUT and HYPERT livers was equivalent. Regression analysis of covalent binding to liver cell fractions from both EUT and HYPERT rats showed a significant correlation with P-450 content. Hyperthyroidism decreased microsomal, but not mitochondrial, cytochrome P-450 content, and MFO activities for 7-ethoxycoumarin and benzphermine were similarly decreased. Hyperthyroidism also diminished microsomal GST activity, and altered GST kinetics for both GSH and 1-chloro-2,4-dinitrobenzene (CDNB). The magnitude of inactivation of MFO and GST activities in the presence of DCE (presumably by DCE reactive intermediates) was comparable between EUT and HYPERT microsomes. When covalent binding was standardized to cytochrome P-450 concentrations in microsomes and mitochondria, HYPERT fractions exhibited slightly greater covalent binding than EUT fractions, suggesting that hyperthyroidism does not reduce the capacity of P-450 hemoproteins to bioactive DCE. Thus, potentiation of DCE hepatotoxicity by hyperthyroidism may be predominantly due to diminished Phase II constituents, and major increases in reactive intermediate/conjugates that covalently bind to and impair critical cellular molecules.

Requirements for cytochrome b5 in the oxidation of 7-ethoxycoumarin, chlorzoxazone, aniline, and N-nitrosodimethylamine by recombinant cytochrome P450 2E1 and by human liver microsomes

NADH-dependent 7-ethoxycoumarin O-deethylation activities could be reconstituted in systems containing cytochrome b5 (b5), NADH-b5 reductase, and bacterial recombinant P450 2E1 in 100 mM potassium phosphate buffer (pH 7.4) containing a synthetic phospholipid mixture and cholate. Replacement of NADH-b5 reductase with NADPH-P450 reductase yielded a 4-fold increase in 7-ethoxycoumarin O-deethylation activity, and further stimulation (approximately 1.5-fold) could be obtained when NADPH was used as an electron donor. Removal of b5 from the NADH- and NADPH-supported systems caused a 90% loss of 7-ethoxycoumarin O-deethylation activities in the presence of NADPH-P450 reductase, but resulted in complete loss of the activities in the absence of NADPH-P450 reductase. Km values were increased and Vmax values were decreased for 7-ethoxycoumarin O-deethylation when b5 was omitted from the NADPH-supported P450 2E1-reconstituted systems. Requirements for b5 in P450 2E1 systems were also observed in chlorzoxazone 6-hydroxylation, aniline p-hydroxylation, and N-nitrosodimethylamine N-demethylation. In human liver microsomes, NADH-dependent 7-ethoxycoumarin O-deethylation, chlorzoxazone 6-hydroxylation, aniline p-hydroxylation, and N-nitrosodimethylamine N-demethylation activities were found to be about 55, 41, 33, and 50%, respectively, of those catalyzed by NADPH-supported systems. Anti-rat NADPH-P450 reductase immunoglobulin G inhibited 7-ethoxycoumarin O-deethylation activity catalyzed by human liver microsomes more strongly in NADPH- than NADH-supported reactions, while anti-human b5 immunoglobulin G inhibited microsomal activities in both NADH- and NADPH-supported systems to similar extents. These results suggest that b5 is an essential component in P450 2E1-catalyzed oxidations of several substrates used, that about 10% of the activities occur via P450 2E1 reduction by NADPH-P450 reductase in the absence of b5, and that the NADH-supported system contributes, in part, to some reactions catalyzed by P450 2E1 in human liver microsomes.

Binding of dietary cobalt to sarcoplasmic reticulum proteins

It has previously been shown that cobalt accumulates in the myocardium of rats, mainly the sarcoplasmic reticulum (SR) and the mitochondrial inner membrane. In order to investigate the mode of accumulation of cobalt in the SR, rats were given a dietary cobalt supplementation of 40 mg of CoSO4 x 7H2O kg-1 body wt, after which the rats were sacrificed and the sarcoplasmic reticulum was isolated. The SR proteins were subjected to analysis by polyacrylamide gel electrophoresis followed by protein staining and determination of the content of cobalt in each protein band. The major cobalt-binding protein was found to have a molecular weight of about 100,000; a 200,000 molecular weight protein was also found to bind cobalt, although less extensively. These results suggest that cobalt is bound to the monomeric and dimeric forms of Ca(2+)-ATPase in the SR of the myocardium.

The intracellular distribution of cobalt in exposed and unexposed rat myocardium

The intracellular distribution of cobalt was analysed in the myocardium of exposed and unexposed rats. The exposed rats were given a dietary cobalt supplementation of 40 mg CoSO4.7 H2O/kg body weight for 8 weeks. The mitochondrial fraction showed the greatest relative increase in cobalt: 0.09 ng/mg protein in the unexposed rats to 8.43 ng/mg protein in the exposed rats. In the exposed rats the submitochondrial particles had the highest levels of cobalt: 19.43 ng/mg protein, followed by the sarcoplasmatic reticulum: 12.3 ng/mg protein. The microsomal 44,000 g supernatant also showed an increase, although the levels remained low (0.51 ng/mg protein in the exposed animals). Apparently the calcium-storing organelles had the highest levels of cobalt. This could affect calcium flux in myocardial cells and, secondarily, tension development in cardiac muscle.

The effect of cobalt on mitochondrial ATP-production in the rat myocardium and skeletal muscle

Cobalt has been shown to accumulate in the myocardium of uraemic patients and has been suggested as a myocardial toxin inhibiting mitochondrial respiration. In order to study the cellular effects of cobalt exposure three groups of rats (n = 12 per group) were fed a diet containing 12% protein without supplementation or with 20 mg and 40 mg CoSO4 7 H2O/kg body weight/day respectively. After 8 weeks the hearts and soleus muscles were removed. Cobalt in tissues and in four cell fractions were analysed with neutron-activation analysis (ng/g wet weight and ng/mg protein respectively). Mitochondrial respiration was analysed as ATP-production rate using pyruvate + malate and palmitoyl-carnitine + malate as substrate. The ATP-production from pyruvate + malate was unchanged in both heart and skeletal muscle in the exposed animals. With palmitate as substrate, the heart muscle showed a slightly lower ATP-production rate (p less than 0.05) after the 20 mg cobalt dose, but the rate was unchanged in the group with higher cobalt intake. No changes in ATP-production rate from palmitate was observed in soleus muscle. The microsomal (100,000 g) fraction in the myocardial cells contained significantly higher cobalt concentrations compared to the mitochondrial fraction in both the unexposed (1.4 ng/mg protein vs 0.19, p less than 0.05) and exposed rats (53.4 ng/mg protein vs 13.2, p less than 0.005). In conclusion, cobalt showed a large accumulation in myocardial cells, without significant effects on mitochondrial ATP-formation rate from oxidation of pyruvate or palmitate and with the highest cobalt content contained in the microsomal (100,000 g) fraction.

BCNU-induced quantitative and qualitative changes in hepatic cytochrome P-450 can be correlated with cholestasis

Male Sprague-Dawley rats were given single i.p. injections of 1,3-bis(2-chloroethyl)-1-Nitrosourea (BCNU) to investigate changes in hepatic microsomal cytochrome P-450 content and metabolic activity. On day 14 after treatment (20 mg/kg), cytochrome P-450 content had decreased by approximately 25% and ethylmorphine N-demethylase activity (nmol product/nmol P-450/min) had decreased by 36%. In contrast, ethylmorphine O-deethylase and 7-ethoxycoumarin O-deethylase activities were not significantly decreased by BCNU treatment. Hepatic delta-aminolevulinic acid synthetase activity was only 60% of control values, and microsomal heme oxygenase activity was slightly but not statistically elevated. Cytochrome P-450 content in control and BCNU-treated rats increased in a similar manner after phenobarbital or beta-naphthoflavone induction. Electrophoretic analysis of cytochrome P-450 proteins isolated from hepatic endoplasmic reticular membranes of treated and control rats suggested that alterations in these proteins occurred in BCNU-treated rats. These changes in cytochrome P-450 content and activity are very similar to those reported in isolated systems exposed to bile acids or in rats with experimentally produced cholestasis. BCNU has been shown to produce cholestasis, which precedes its effects on microsomal mixed-function oxygenase activity. Thus, the delayed effects of BCNU on microsomal drug metabolism are probably secondary to its interference with bile formation.