Rate-limiting steps in drug metabolism

Abstract

Comparative studies have been conducted of the activity of microsomal mixed-function oxidases from livers of normal, precirrhotic and cirrhotic rats linked with the metabolism of type-I (aminopyrine, hexobarbital), type-II (aniline, metyrapone) and “modified type-II” (corticosterone) substrates. The following factors were investigated : the possible role of cytochrome P-450 content, the state of the “substrate-binding protein” of this enzyme, the degree of affinity of this hemoprotein for both type-I and type-II substrates and finally, the activity of the enzymes of the microsomal electron-transport chain (both in the absence and in the presence of type-I substrate) — as rate-limiting reactions, “tight spots” in the biotransformation of drugs in experimental microsomes. It was found that the hydroxylation activity for type-II and “modified type-II” substrates during the entire period of liver cirrhosis development is determined by the cytochrome P-450 content and the amplitude of maximal spectral changes observed in the presence of excess substrate. Type-I substrate metabolism, however, is limited in the precirrhotic phase by the state of the“substrate-binding protein” contained in P-450 as well as by the NADPH-cytochrome P-450 reductase activity. On the other hand, the N-demethylating activity in CCl4-cirrhotic liver microsomes does not depend on either the concentration of P-450, on the amplitude of the maximal spectral changes or on the K8 value. The rate-limiting step in this case is the rate of reduction of the P-450-substrate complex by NADPH.