Azo dyes are reduced to primary amines by the microsomal enzymes NADPH-cytochrome P450 reductase and cytochrome P450. Amaranth, a highly polar dye, is reduced almost exclusively by rat liver microsomal cytochrome P450 and the reaction is inhibited almost totally by oxygen or CO. Activity is induced by pretreatment with phenobarbital or 3-methylcholanthrene. In contrast, microsomal reduction of the hepatocarcinogen dimethylaminoazobenzene (DAB), a lipid soluble, weakly polar compound, is insensitive to both oxygen and CO. However, reconstitution of activity with purified NADPH-cytochrome P450 reductase and a partially purified cytochrome P450 preparation indicates that activity is catalyzed almost exclusively by cytochrome P450. Activity is induced by clofibrate but not phenobarbital, β-naphthoflavone, 3-methylcholanthrene, isosafrol, or pregnenolone-16α-carbonitrile. These observations suggest the existence of at least two classes of azoreductase activity catalyzed by cytochrome P450. To investigate this possibility, the reduction of a number of azo dyes was investigated using microsomal and partially purified systems and the characteristics of the reactions were observed. Microsomal reduction of azo dyes structurally related to DAB required a polar electron-donating substituent on one ring. Activity was insensitive to oxygen and CO if the substrates had no additional substituents on either ring or contained only electron-donating substituents. Introduction of an electron-withdrawing group into the prime ring conferred oxygen and CO sensitivity on the reaction. Substrates in the former group are referred to as insensitive and substrates in the latter group as sensitive. Inhibitors of cytochrome P450 activity depressed reduction of both insensitive and sensitive substrates. In a fully reconstituted system containing lipid, highly purified NADPH-cytochrome P450 reductase and a partially purified cytochrome P450 preparation, rates of reduction of various insensitive substrates varied several-fold, whereas rates of reduction of sensitive substrates varied by three orders of magnitude. Using purified enzymes, each of the insensitive substrates was shown to be reduced by reductase alone, but only at a fraction of the rate seen in the fully reconstituted system, implying that reducing electrons were transferred to the dyes mainly from cytochrome P450. Conversely, there was substantial, in some cases almost exclusive, reduction of sensitive substrates by purified reductase alone and almost no inhibition by CO. Their reduction, however, was inhibited by CO in microsomal systems. This apparent discrepancy may be attributed to less effective coupling between reductase and cytochrome P450 in the reconstituted system compared to that in microsomes, permitting “leakage” of electrons from flavoprotein directly to substrate. Microsomal reduction of sensitive substrates was induced by treatment with phenobarbital, isosafrole, and pregnenolone-16α-carbonitrile or clofibrate, whereas reduction of insensitive substrates was induced only by clofibrate, suggesting catalysis by a more restrictive group of cytochromes P450. It is concluded that both sensitive and insensitive substrates are reduced by cytochrome P450. Observations on differential induction and sensitivity to oxygen and CO imply that at least two distinct species of cytochrome P450 catalyze the reduction of insensitive and sensitive substrates. Speculation is also made of the significance of the differences of oxygen and CO sensitivity.
Read full abstract
Apr 1, 2000
Anna K Hull + 1
Open Access