Leaves from a standard, insect-susceptible cauliflower variety and an insect-resistant strain were formulated at either 10 or 25% into semipurified diets for male and female weanling rats. After 3 weeks, relative liver weights, microsomal protein, cytochrome P-450, and activities of hepatic microsomal aminopyrine N-demethylase, aniline hydroxylase, p-nitroanisole O-demethylase, and N-methylaniline N-demethylase were determined. Growth, feed intake, and feed efficiency of male rats were not affected by the inclusion of the dried cauliflower leaf in the diet. However, female rats exhibited a depressed feed intake and increased feed efficiency with cauliflower leaf supplemental diets. Relative liver weights increased with increasing percentage of cauliflower leaves in the diet. Hepatic microsomal enzyme response to cauliflower leaf supplementation of the diet was greater in males than in females. Only aniline hydroxylase activity remained unchanged by the test diets. Male rats showed significant increases in N- and O-demethylation with both the 10 and 25% cauliflower diets, and increased values for microsomal protein and cytochrome P-450 at the 25% supplemental level. Female rats did not show significant hepatic microsomal induction from cauliflower leaf consumption at the 10% level. However, cytochrome P-450 and the metabolism of aminopyrine and p-nitroanisole were enhanced by consumption of cauliflower leaves at 25% of their diet. None of the parameters tested in this study evidenced a difference between the two cauliflower cultivars fed to either sex. (+info)
Species comparison of in vitro metabolism of aflatoxin B1.
The metabolism of [14C]aflatoxin B1 by 9000 X g supernatant fraction of livers of duck, rat, mouse, monkey, and humans was compared by incubating the compound and liver fractions in the presence of cofactors and air for 30 min. The incubation medium was extracted with chloroform, and the soluble metabolites were separated on thinlayer plates and quantified; radioactivity remaining in the aqueous phase was determined in order to quantify metabolism to water-soluble derivatives. Duck, monkey, and human livers were most active in total conversion, each metabolizing approximately 80% of available substrate in 30 min. Rat and mouse livers had lower activites, metabolizing 15 to 20%. Duck liver produced mainly (60%) chloroform-insoluble derivatives, but all other species produced larger quantities of chloroform-soluble than insoluble metabolites. Aflatoxin Q1 was the principle chloroform-soluble metabolite produced by monkey, human, and rat liver, whereas duck liver produced mainly aflatoxicol in that fraction. Aflatoxin P1 was produced by monkey, human, and mouse liver, but not by duck and rat. The chromatographic region containing M1 and B2alpha contained low levels of radioactivity in all species except human. No consistent pattern of metabolism emerged which could be correlated with species differences in response to aflatoxin B1 toxicity or carcinogenicity. (+info)
The involvement of cytochrome P-488 and P-450 in NADH-dependent O-demethylation of p-nitroanisole in rat liver microsomes.
These studies have shown that addition of p-nitroanisole to a reaction mixture containing rat liver microsomes resulted in an increase the reoxidation rate of NADH-reduced cytochrome b5. Fortification of rat liver microsomes with partially purified cytochrome b5 produces an increase in both NADPH-dependent and NADH-dependent p-nitroanisole O-demethylation activity. Antiserum to cytochrome P-450 isolated from phenobarbital-treated rat liver microsomes inhibited the NADH-dependent O-demethylation activity as well as the NADPH-dependent O-demethylation activity seen in rat liver microsomes. Addition of either purified cytochrome P-450 or cytochrome P-448 to an incubation mixture containing phenobarbital-treated rat liver microsomes enhanced the NADH-dependent p-nitroanisole O-demethylation activity. These results suggest that NADH-dependent and, in part, NADPH-dependent O-demethylations are catalyzed by cytochrome P-448 and cytochrome P-450 receiving electrons from cytochrome b5. (+info)
Regulation of p-nitroanisole O-demethylation in perfused rat liver. Adenine nucleotide inhibition of NADP+-dependent dehydrogenases and NADPH-cytochrome c reductase.
Perfusion of rat livers with 10 mM-fructose or pretreatment of the rat with 6-aminonicotinamide (70 mg/kg) 6 h before perfusion decreased intracellular ATP concentrations and increased the rate of p-nitroanisole O-demethylation. This increase was accompanied by a decrease in the free [NADP+]/[NADPH] ratio calculated from concentrations of substrates assumed to be in near-equilibrium with isocitrate dehydrogenase. After pretreatment with 6-aminonicotinamide the [NADP+]/[NADPH] ratio also declined. Reduction of NADP+ during mixed-function oxidation may be explained by inhibition of of one or more NADPH-generating enzymes. Glucose 6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, isocitrate dehydrogenase and "malic" enzyme, partially purified from livers of phenobarbital-treated rats, were inhibited by ATP and ADP. Inhibitor constants of ATP for the four dehydrogenases varied considerably, ranging from 9 micrometer for "malic" enzyme to 1.85 mM for glucose 6-phosphate dehydrogenase. NADPH-cytochrome c reductase was also inhibited by ATP (Ki 2.8 mM) and by ADP (Ki 0.9 mM), but not by AMP. Concentrations of ATP and ADP that inhibited glucose 6-phosphate dehydrogenase and the reductase were comparable with concentrations in the intact liver. Thus agents that lower intracellular ATP may accelerate rates of mixed-function oxidation by a concerted mechanism involving deinhibition of NADPH-cytochrome c reductase and one or more NADPH-generating enzymes. (+info)
Relationship between alveolar PO2 and the rate of p-nitroanisole O-demethylation by the cytochrome P-450 pathway in isolated rabbit lungs.
The relationship between alveolar PO2 and the rate of O-demethylation of p-nitroanisole, a model substrate for cytochrome P-450 -linked mixed-function oxidation, was evaluated in the isolated rabbit lung perfused with Krebs-Ringer bicarbonate buffer. The appearance of the product, p-nitrophenol, in the pulmonary perfusate was measured spectrophotometrically, The PO2 of the ventilating gas was varied with an accurate gas mixing pump and measured with an electrochemical O2 analyzer. In control lungs ventilated with 5% CO2 in air, the rate of p-nitrophenol production was approximately equal to 3.1 +/- 0.04 (mean +/- SE; n = 9) mumol/h per g dry wt. p-Nitrophenol production was unaltered when O2 in the ventilating gas was decreased to 1%, but it was depressed reversibly when alveolar O2 WAS 0.1% OR LESS AND WAS ABOLISHED DURING VENTILATION WITH 0.005% O2. The rate of the reaction was inhibited by 50% when alveolar PO2 was 0.3 mm Hg representing and intracellular [O2] OF approximately equal to muM. In the presence of metyrapone (0.1--1 mM), an inhibitor of cytochrome P-450-dependent reactions, p-nitrophenol production was 0.07--0.17 mumol/h per g dry wt. Ventilation of lungs with varying CO concentration in 20% O2 resulted in 50% inhibition of p-nitrophenol production when CO concentration was 10% (CO/O2 = 0.5). These results indicated that O-demethylation of p-nitroanisole by the lung is a cytochrome P-450-dependent reaction and that its rate is not affected until alveolar PO2 is less than 1 mm Hg. (+info)
Reduction of cytochrome P-450 LM2 by NADPH in reconstituted phospholipid vesicles is dependent on membrane charge.
The kinetics of the reduction of cytochrome P-450 LM2 mediated by NADPH-cytochrome P-450 reductase in reconstituted phospholipid vesicles was examined. An inefficient reduction of the hemoprotein in phosphatidylcholine vesicles was observed. However, by introducing negatively charged phospholipids into the membrane, the rate of reduction increased in a concomitant manner to the resulting net negative charge of the vesicles. In the presence of benzphetamine, the extent of cytochrome P-450 LM2 reduced 1 s after the addition of NADPH to the system was a linear function of the electrophoretic mobilities of the vesicles used. A similar relationship between the net negative charge of the vesicles, as measured electrophoretically, and the reduction rate was also attained in the absence of substrate. The enhanced reduction was mainly reflected in an altered phase distribution of the reduction; the extent of fast phase reduction in the absence or in the presence of added substrate was dependent upon the electrophoretic mobilities of the vesicles. A similar change in the distribution of the reduction phases was observed upon decreasing the phosphatidylcholine content of the vesicles; the fast phase reduction being more pronounced in membranes with higher relative amounts of the protein components. A decrease of the rate of O-demethylation of p-nitroanisole catalyzed by P-450 LM2 parallel to the extent of fast phase reduction was observed upon dilution of neutral phosphatidylcholine membranes with phospholipid. By contrast, no effect of lipid dilution was evident in negatively charged membranes. The results are consistent with the hypothesis that the extent of fast phase reduction is governed by the amount of complex formed between NADPH-cytochrome P-450 reductase and cytochrome P-450 in the membranes; negative membranes appear to favor the formation of such complexes, whereas similar complexes are less formed, or are not functional, in neutral membranes. (+info)
The effects of a marginally lipotrope-deficient diet on the hepatic levels of S-adenosylmethionine and on the urinary metabolites of 2-acetylaminofluorene in rats.
Hepatic levels of S-adenosylmethionine (AdoMet), of glutathione, and of the microsomal enzymes p-nitroanisole demethylase and benzo(a)pyrene hydroxylase were measured in male and female rats fed a diet marginally deficient in choline and methionine and void of folic acid (lipotrope deficient) or an adequate diet for 0 to 14 weeks with and without added 2-acetylaminofluorene (AAF). The urinary metabolites of AAF were determined throughout the experimental period. After 2 to 4 weeks of dietary administration, the hepatic AdoMet levels were 43% lower in male rats fed the lipotrope-deficient diet than in male rats fed the lipotrope-adequate diet; no differences were found in hepatic AdoMet of females fed the lipotrope-deficient or lipotrope-adequate diets for 2 to 14 weeks. Administration of AAF to lipotrope-deficient female rats for 2 weeks led to a transient decrease in hepatic levels of AdoMet. The administration of AAF for 2 to 14 weeks did not significantly affect hepatic AdoMet in female rats fed the lipotrope-adequate diet or in male rats fed either diet. Female rats fed the lipotrope-deficient diet and treated with AAF excreted decreased proportions of N-hydroxy-2-acetylaminofluorene and increased proportions of 5-hydroxy-2-acetylaminofluorene in their urine. However, the urine of lipotrope-deficient male rats treated with AAF contained increased proportions of N-hydroxy-2-acetylaminofluorene and decreased levels of 5-hydroxy-2-acetylaminofluorene. The urinary excretion of 7-hydroxy-2-acetylaminofluorene by male and female lipotrope-deficient rats treated with AAF was generally similar to that in lipotrope-adequate rats. The lipotrope-deficient diet did not appear to alter the hepatic levels of glutathione, p-nitroanisole demethylase, or benzo(a)pyrene hydroxylase activity was lower in the livers of lipotrope-deficient male rats treated with AAF for 8 to 14 weeks than in the livers of lipotrope-deficient rats not receiving the carcinogen. The altered metabolism of AAF correlated well with the previously reported effects of a marginal lipotrope deficiency on AAF carcinogenesis. (+info)
Interaction and electron transfer between cytochrome b5 and cytochrome P-450 in the reconstituted p-nitroanisole O-demethylase system.
The interaction and electron transfer between cytochrome b5 and cytochrome P-450B1 were investigated using the reconstituted p-nitroanisole O-demethylase system. Apocytochrome b5 was prepared from detergent-solubilized cytochrome b5 by the acid-butanone method. The apocytochrome b5 thus obtained has been substituted with several metalloporphyrin derivatives. The reconstituted system containing cytochrome b5 substituted with heme derivatives such as proto-, meso-, and deuteroheme exhibited demethylation activity at the maximum turnover rates of 94, 58, 30%, respectively, compared to that containing the native cytochrome b5, while neither apocytochrome b5 nor cobaltic protoporphyrin-cytochrome b5 displayed the activity. Kinetic analysis showed the formation of a 1:1 complex between cytochrome P-450B1 and each of these substituted cytochrome b5's, except for cobaltic protoporphyrin-cytochrome b5; the affinities differed with the cytochrome b5 species used. The synergistic effect with the addition of the NADH-linked electron transport system was more remarkable at the lower reduction levels of cytochrome b5 in the steady state. Interaction between the components involved in NADH- and NADPH-linked electron transport systems was modulated by the existence of Triton X-100. The optimal concentration in the reconstituted system for the demethylation was observed at around 0.03% of Triton X-100, where the reduction rates for cytochrome b5 and cytochrome P-450B1 by the respective reductases were maximal. These results indicate that the two electron transport systems are closely coupled and exhibit the demethylase activity. (+info)