The role of NADPH-cytochrome c reductase in microsomal hydroxylation reactions

The roles of NADH in the support of steroid aromatization by human placental microsomes

The ability of NADH to function as an alternative cofactor for the support of estrogen biosynthesis was validated. NADH supported rates of aromatization of up to 80% of those obtained with NADPH, with an apparent Km of 0.70 mM, and stimulated the NADPH-supported reaction only when supplies of the normal cofactor were limiting, both additive and synergistic effects being observed. NADH-supported aromatization was inhibited competitively by NADP+ and 2'-AMP with Ki values of 5 microM and 22 microM, respectively. Support by both cofactors was lost in parallel with the selective removal of NADPH-cytochrome c reductase from microsomes by graded subtilisin treatment. NADH-supported aromatization was differentiated from NADPH-supported aromatization by its sensitivity to inhibition by NAD+ and its response to changes in ionic strength. NADH appears to function, at high concentrations, as a surrogate for NADPH at the reduced nucleotide-binding site of NADPH-cytochrome c reductase but additional roles for NADH are also suggested both when acting alone and as a supplement to NADPH. A common oxidase (cytochrome P-450) appears to catalyze both NADH- and NADPH-supported aromatization.

Affinity labeling of rat liver microsomal NADH-5 alpha-reductase with a nucleoside analogue

A time dependent irreversible loss of rat liver microsomal NADH-5 alpha-reductase activity is caused by incubation of microsomes with the nucleoside 5'-p-fluorosulfonylbenzoyladenosine (FSA). The decrease of activity is dependent on FSA concentration and shows first order kinetics. Presence of NADH partially stabilizes the NADH-5 alpha-reductase. Thioglycerol present before incubation prevents loss of activity, and stops decrease of activity when added during incubation. NADPH-5 alpha-reductase (E.C. 1.3.1.4) and NADPH-cytochrome c reductase (E.C. 1.6.2.4) are not influenced while NADH-cytochrome c reductase (E.C. 1.6.99.3) is inhibited by FSA. Evidently FSA causes inactivation of the enzymes by binding to the NADH-binding site. Affinity labeling by FSA thus clearly distinguishes between NADH- and NADPH-dependent 5 alpha-reductases from rat liver microsomes.

Thermal instability in the endoplasmic reticulum of the rat hepatocyte

The rates of heat denaturation of a protein component of endoplasmic reticulum, NADPH cytochrome c reductase, measured in the temperature range of 37-47 degrees C, show that this protein is highly unstable in the range of temperatures used in clinical hyperthermia. The rate of enzyme inactivation increases some 20-fold as the temperature increases from 37 degrees C to 45 degrees C. The enzyme has an in vitro half-life of only 7 h due to thermal inactivation at 37 degrees C. This suggests a general scheme for the physiological degradation pathway of intracellular proteins in which the first step is rapid thermal denaturation of the molecule followed by a second and slower step of proteolytic degradation. The rapid physiological turnover of intracellular proteins may be an unavoidable cost of maintaining metabolic precision in the presence of thermal noise at normal body temperature.

Evoked effects of oestradiol on hepatic cholesterol 7 alpha-hydroxylase and drug oxidase in castrated rats

1. Oestradiol administration in castrated rats resulted in an increased activity of the cholesterol 7 alpha-hydroxylase and a decreased activity of the drug oxidase enzyme systems. 2. Aqueous solutions of oestradiol (up to 25.10(-6)M) incubated in vitro with microsomes, binds into the microsomal membrane framework reducing the activity of both enzyme systems. 3. The specific activity of cholesterol 7 alpha-hydroxylase, drops after 3 hr preincubation with oestradiol to at least 70% of its original value. 4. Actinomycin D and cycloheximide administration reduced the oestradiol-induced and control cholesterol 7 alpha-hydroxylase activity to the same level, 6 hr after the injections.

Liver microsomal cytochrome P-450 and the oxidative metabolism of arachidonic acid

Arachidonic acid is oxidatively metabolized by rat liver microsomes at a rate of approximately 5 nmol per min per mg of protein at 25 degrees C. This reaction is dependent on the presence of NADPH and oxygen. Studies with various inhibitors indicate a role for membrane-bound cytochrome P-450 in the transformation of arachidonic acid to a mixture of hydroxy acid derivatives. The stoichiometry of the reaction conforms to that of a monooxygenase reaction--i.e., one mole of NADPH is oxidized per mole of oxygen utilized--suggesting a reaction mechanism different from that proposed for lipid peroxidation reactions. No evidence for the formation of prostaglandin-like metabolites was obtained. The diene character of some of the metabolites formed suggests another role for cytochrome P-450--i.e., participation in hydrogen abstraction reactions for the activation of various substrates.

Effect of liposomes containing cholesterol on a hepatic cholesterol-7 a-hydroxylase and drug oxidation system

A statistically significant increase (p less than 0.01) in the activities of the microsomal enzymes cholesterol-7 a-hydroxylase (EC 1.14.13.17), biphenyl-4-hydroxylase, aniline hydroxylase, NADPH-cytochrome-c-reductase (EC 1.6.2.4) and p-nitroanisole-O-demethylase, was shown in cholesterol-loaded rat liver microsomes. The data indicate that profound functional changes are induced in membrane proteins by exogenous cholesterol.

NADH-dependent aryl hydrocarbon hydroxylase in rat liver mitochondrial outer membrane

NADH-dependent 3,4-benzpyrene hydroxylase activity was detected in the purified mitochondrial outer membrane fraction from the livers of rats treated with 3-methylcholanthrene. The specific activity in the outer membrane fraction is nearly equal to that of microsomes, a level too high to be accounted for only by the microsomal contamination. On the other hand, the NADPH-dependent 3,4-benzpyrene hydroxylase activity in the outer membrane fraction is about 50% of that of microsomes. The ratio of the specific activity of NADPH- to NADH-dependent 3,4-benzpyrene hydroxylase in microsomal fraction was about 3.5, while that of the outer membrane fraction was about 1.5. Moreover, it was found that NADH-dependent 3,4-benzpyrene hydroxylase activity in mitochondrial outer membrane from control rat liver was cyanide-insensitive, while that in microsomes was cyanide-sensitive. These results suggest the presence in the mitochondrial outer membrane fraction of aryl hydrocarbon hydroxylase activity which uses as electron donor NADH nearly to the same extent as NADPH. The hydroxylase system is composed of cyanide-insensitive cytochrome P-450 and is inducible markedly by 3-methylcholanthrene treatment. The probable electron transfer pathways in the mitochondrial outer membrane cytochrome P-450 oxidase system are discussed.