The effects of incorporation into microsomes of purified NADPH-cytochrome c (P-450) reductase on drug oxidations

Significance of cytochrome P-450 (P-450 HFLa) of human fetal livers in the steroid and drug oxidations

The purpose of this study was to clarify the pharmacological and physiological significance of P-450 HFLa. Thus, correlations between cytochrome P-450 (P-450 HFLa) level and different monooxygenase activities were investigated in liver homogenates from human fetuses. Poor correlation was seen between P-450 HFLa level and the activity of benzphetamine N-demethylation or aniline hydroxylation. In contrast, the content of P-450 HFLa was highly correlated with the activity of benzo(a)pyrene hydroxylation, 7-ethoxycoumarin O-deethylation or testosterone 6 beta-hydroxylation. In microsomes from human adult livers, a moderate relationship was also observed between testosterone 6 beta-hydroxylation and P-450 HFLa level. Furthermore, antibodies to P-450 HFLa inhibited testosterone 6 beta-hydroxylase activity in fetal and adult livers to similar extents. We conclude that P-450 HFLa is a form of cytochrome P-450 which catalyzes testosterone 6 beta-hydroxylation and limited drug oxidations in human fetal and adult livers.

NADPH: cytochrome P-450 reductase in olfactory epithelium. Relevance to cytochrome P-450-dependent reactions

The presence of a very active cytochrome P-450-dependent drug-metabolizing system in the olfactory epithelium has been confirmed by using 7-ethoxycoumarin, 7-ethoxyresorufin, hexobarbitone and aniline as substrates, and the reasons for the marked activity of the cytochrome P-450 in this tissue have been investigated. The spectral interaction of hexobarbitone and aniline with hepatic and olfactory microsomes has been examined. By this criterion there was no evidence for marked differences in the spin state of the cytochromes of the two tissues, or for the olfactory epithelium containing a greater amount of cytochrome capable of binding hexobarbitone, a very actively metabolized substrate. Rates of NADPH and NADH: cytochrome c reductase activity were found to be higher in the olfactory epithelium than in the liver, and direct evidence was obtained for a greater amount of the NADPH-dependent flavoprotein in the olfactory microsomes. Investigation of male rats and male and female mice, as well as male hamsters, demonstrated that, in all cases, the cytochrome P-450 levels of the olfactory epithelium were lower than those of the liver, while the 7-ethoxycoumarin de-ethylase and NADPH:cytochrome c reductase activities were higher. A correlation was found between 7-ethoxycoumarin de-ethylase and NADPH:cytochrome c reductase activities for both tissues in all species examined. The ratio of reductase to cytochrome P-450 was found to be considerably higher in the olfactory epithelium (1:2-1:3) than in the liver (1:11-1:15), regardless of the species examined, suggesting that facilitated electron flow may contribute significantly to the cytochrome P-450 catalytic turnover in the olfactory tissue.

Induction and catalytic activities of cytochromes P-450b/e and P-450c in the liver microsomes of neonatal rats

The activities of cytochrome P-450-dependent monooxygenases has been investigated in the liver microsomes of newborn rats (3-16 days after birth) induced with PB or 3-MC. It has been shown that the induction by PB and 3-MC results in the increase of both the total amount of cytochrome P-450 as determined by the CO-reduced spectrum and the amount of induced forms P-450b/e and P-450c respectively. In the course of induction of the specific forms of cytochrome P-450 BP-hydroxylase and 7-ER-O-deethylase activities increased at 3-MC-induction, while BPh-N-demethylase and BP-hydroxylase increased at PB-induction. Analysis of inhibition of monooxygenase reactions with antibodies has showed that only P-450c was involved in metabolism of BP and 7-ER. Participation of P-450b/e in BPh N-demethylation was notably lower in the neonates in comparison to the adult rats. In the one-week-old rats induced with 3-MC a considerable rate of BP hydroxylation and 7-ER O-deethylation (2-4.5 nmol of product min-1 mg-1) has been observed despite a small amount of P-450 (0.02-0.1 nmol/mg of protein). This fact shows the higher catalytic activity of this cytochrome P-450 in the neonates compared to similar characteristics of P-450c in the 3-MC-induced microsomes. Metabolism of BP in the PB-microsomes of the neonatal rats was inhibited neither by anti-P-450b/e nor anti-P-450c in contrast to the adults, where this reaction was inhibited by antibodies against P-450b/e.

Membrane bound cytochrome P-450 determines the optimal temperatures of NADPH-cytochrome P-450 reductase and cytochrome P-450-linked monooxygenase reactions in rat and trout hepatic microsomes

The hepatic monooxygenase systems largely responsible for the biotransformation of drugs and other xenobiotics are comprised of NADPH-cytochrome P-450 reductase and multiple forms of cytochrome P-450. Optimal temperatures for these systems in the trout and rat are 26 degrees and 37 degrees, respectively. Purified trout and rat reductases are optimally functional at 26 degrees and 37 degrees, respectively, when added to trout and rat microsomes. However, rat reductase was shown to function optimally at 26 degrees when added to trout microsomes and trout reductase functioned optimally at 37 degrees when added to rat microsomes. Corresponding shifts in optimal temperatures of cytochrome P-450-linked 0-deethylation of 7-ethoxycoumarin occurred when these reductases were added to rat or trout microsomes. It is proposed that the phospholipid annulus surrounding the active site of membrane-bound cytochrome P-450 determines the optimal temperature of cytochrome P-450 systems.

Effect of successive administration of zopiclone, a new class of minor tranquillizer, on mouse liver microsomal mono-oxygenase system

Oral administration of zopiclone to mice (10 mg/kg daily) for 14 days had no significant effect on body weight, liver weight, hepatic microsomal protein or cytochrome P-450 contents but produced significant increases in the activities of NADPH-cytochrome c reductase, NADH-ferricyanide reductase, aniline hydroxylation and aminopyrine N-demethylation.

Time-dependent kinetics VII: effect of diurnal oscillations on the time course of carbamazepine autoinduction in the rhesus monkey

Extensive blood sampling and repeated long-term carbamazepine infusions were carried out in four rhesus monkeys to examine the time course of carbamazepine autoinduction in detail and assess the intraanimal variability in the rate constant of induction. Diurnal oscillations in carbamazepine blood levels were observed during all infusions and these prevented a good data fit for the biochemical model previously proposed for describing the decline in drug blood levels during induction by carbamazepine. An attempt at fitting only selected blood samples to the model resulted in variable (and perhaps questionable) induction rate constants, even in the same animal. Previous variability in calculated induction rate constants may be due to the presence of diurnal oscillations superimposed on the autoinduction phenomenon. It is proposed that the simultaneous expression of diurnal oscillations and autoinduction are the result of effects on drug metabolism at two independent levels.

Cytochrome P-450 as a determinant of sex difference of drug metabolism in the rat

1. Recent advances in the studies of sex-related differences of drug metabolism in the rat are described. Experiments with novel substrates have provided new insights into the sex differences of drug metabolism. 2. One of each form of cytochrome P-450, P-450-male and P-450-female, from liver microsomes of male and female rats, respectively, were purified to electrophoretic homogeneity. 3. These forms of cytochrome P-450 are distinguishable from each other in their catalytic, physical and immunochemical properties. 4. The use of antibodies to the two forms of cytochrome P-450 shows that P-450-male and P-450-female, present in rat-liver microsomes, are dependent on sex hormones. 5. In addition, evidence for the involvement of multiple forms of cytochrome P-450 in the occurrence of the sex difference has been presented.