The Degradative Effects of Porphyrins and Heme Compounds on Components of the Microsomal Mixed Function Oxidase

The effects of in vitro treatment of the hepatic microsomal fraction with various porphyrin compounds on the activity and the content of the heme-containing components of the mixed function oxidase system were studied. The compounds examined were hematin, methemalbumin (with heme to protein molar ratio of 13:l or l:l), mesohemalbumin, bilirubin, biliverdin, mesoporphyrin IX, and protoporphyrin IX. The activity of the system was monitored by measuring its oxidative activity for the type I and type II substrates, ethylmorphine and aniline, respectively; as well as the microsomal contents of cytochrome P-450 and b, and %-labeled heme. Mesoporphyrin IX was found to be most effective in inhibiting the oxidative activity of the mixed function oxidase system as well as in decreasing the microsomal contents of cytochromes P-450, b,, and heme. Biliverdin exerted no effect on these parameters. Hematin and the other compounds studied exerted variable inhibitory effects on the system. The degradative and inhibitory effects of protoporphyrin IX and mesoporphyrin IX could be blocked significantly by conducting the studies in the dark. The presence of biliverdin decreased the inhibitory effects of the porphyrins on the system; conversely the effects could be magnified in the presence of deuterium oxide. It is suggested that the mechanism by which porphyrins inhibit the mixed function oxidase system is through porphyrin-sensitized photo-oxidation of various constituents of the hepatic microsomal fraction and that the formation of singlet oxygen molecules is most likely involved in this process. Moreover the destructive effects of heme compounds on the microsomal components and activities of the drug-metabolizing mixed function oxidase system raise questions concerning the hypothesis that the components of this system, and specifically cytochrome P-450, are involved in the activity of the heme oxygenase system.