Aldrin Epoxidation Catalyzed by Purified Rat‐Liver Cytochromes P‐450 and P‐448 High Selectivity for Cytochrome P‐450

Abstract

Aldrin epoxidation was studied in monooxygenase systems reconstituted from purified rat liver microsomal cytochrome P-450 or P-448, NADPH-cytochrome c reductase, dilauroylphosphatidylcholine and sodium cholate. Cytochrome P-450, purified from hepatic microsomes of phenobarbital-treated rats, exhibited a high rate of dieldrin formation. The low enzyme activity observed in the absence of the lipid and sodium cholate was increased threefold by addition of dilauroylphosphatidylcholine and was further stimulated twofold by addition of sodium cholate. The apparent K , for aldrin in the complete system was 7 f 2 pM. SKF 525-A, at a concentration of 250 pM, inhibited aldrin epoxidation by 65 %, whereas 7,8-benzoflavone had no inhibitory effect at concentrations up to 250 pM. Addition of ethanol markedly increased epoxidase activity. The increase was threefold in the presence of 5 % ethanol.When cytochrome P-448 purified from hepatic microsomes of 3-methylcholanthrene-treated rats was used, a very low rate of epoxidation was observed which was less than 3 % of the activity mediated by cytochrome P-450 under similar assay conditions. Enzyme activity was independent of the lipid factor dilauroylphosphatidylcholine. The apparent K, for aldrin was 27 k 7 yM. The modifiers of monooxygenase reactions, 7,8-benzoflavone, SKF 525-A and ethanol, inhibited the activity mediated by cytochrome P-448. The Is0 was 0.05, 0.2 and 800 mM, respectively.These results indicate that aldrin is a highly selective substrate for cytochrome P-450 species present in microsomes of phenobarbital-treated animals and is a poor substrate for cytochrome P-448. The two forms of aldrin epoxidase can be characterised by their turnover number, their apparent K , and their sensitivity to modifiers, like 7,8-benzoflavone and ethanol.A variety of compounds of widely different chemical structure are oxidized by the monooxygenase system in the endoplasmic reticulum which contains cytochrome P-450 as the terminal oxidase. The broad substrate specificity of this enzyme system has been partially attributed to the existence of multiple cytochrome P-450 species of different but overlapping substrate specificities [l -51. This concept of the multiplicity of cytochrome P-450 was first supported by findings suggesting the occurrence of two major groups of microsomal hemoproteins typified by their response to inducing agents like phenobarbital and 3-methylcholanthrene, by spectral properties and by differences in substrate specificity [6 -81. Compelling

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