Molecular Pharmacology, Vol 12, 649-657, Copyright © 1976 by the American Society for Pharmacology and Experimental Therapeutics

Drug Metabolism in Liver Tumors

Resolution of Components and Reconstitution of Activity

¹ Department of Biochemistry and Molecular Biology, The University of Texas Medical School at Houston, Houston, Texas 77030

The microsomal drug metabolism system has been resolved from Hepatoma 5123 t.c.(H) into its component enzymes: cytochrome P-450 and NADPH-cytochrome P-450 reductase. Reconstitution of benzphetamine hydroxylation activity with these two hepatoma proteins requires the presence of phosphatidylcholine. The hepatoma cytochrome P-450 has been purified 12-fold and can be substituted for liver cytochrome P-450 in a reconstituted system containing phosphatidylcholine and liver reductase. Hepatoma cytochrome P-450 reductase has been purified 130-fold. The purified reductase has an apparent minimum molecular weight of about 80,000 and is free of cytochromes P-450 and P-420. The purified reductase catalyzes electron transfer to the artificial electron acceptors cytochrome c, dichlorophenolindophenol, and ferricyanide; the K_m concentrations required by the hepatoma reductase to generate half the maximal velocity for each acceptor are similar to those required by purified liver reductase. The K_m value of the hepatoma reductase for NADPH (76 µM), however, is an order of magnitude higher than that of the purified liver reductase (7.8 µM). Hepatoma reductase can substitute for liver reductase in a reconstituted benzphetamine hydroxylation system containing liver cytochrome P-450 and phosphatidylcholine. Reconstitution of benzphetamine hydroxylation activity from hepatoma components was shown to require, like the liver system, the presence of both cytochrome P-450 and NADPH-cytochrome P-450 reductase, as well as phosphatidylcholine, to achieve the maximal hydroxylation rate. In a benzphetamine hydroxylation system containing liver cytochrome P-450, hepatoma cytochrome P-450 reductase, and phosphatidylcholine, the K_m values for benzphetamine and NADPH were shown to be 390 and 15 µM, respectively. The value for benzphetamine is similar to that obtained in a reconstituted liver system, while the value for NADPH is an order of magnitude higher than that determined for the liven system.

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ACKNOWLEDGMENTS We wish to acknowledge the skilled technical assistance of Mr. Tracy Lanagan.