Rate-limiting steps in drug metabolism by microsomes from CCl-4-cirrhotic rat liver.
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 CCl-4-cirrhotic liver microsomes does not depend on either the concentration of P-450, on the amplitude of the maximal spectral changes or on the Ks value. The rate-limiting step in this case is the rate of reduction of the P-450-substrate complex by NADPH.
D000633:Aminopyrine N-Demethylase / Q000378:metabolism; D000814:Aniline Compounds; D000818:Animals; D002251:Carbon Tetrachloride; D003345:Corticosterone; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D003579:Cytochrome Reductases / Q000378:metabolism; D004579:Electron Transport; D006420:Hemeproteins / Q000378:metabolism; D007700:Kinetics; D008099:Liver / Q000473:pathology; D008103:Liver Cirrhosis / Q000139:chemically induced / Q000378:metabolism* / Q000473:pathology; D008862:Microsomes, Liver / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism; D051381:Rats; D013997:Time Factors
D000814:Aniline Compounds; D006420:Hemeproteins; D003577:Cytochrome P-450 Enzyme System; D002251:Carbon Tetrachloride; D006899:Mixed Function Oxygenases; D000633:Aminopyrine N-Demethylase; D003579:Cytochrome Reductases; D003345:Corticosterone