Regulation of monooxygenation during development of chick embryo liver: role of cytochrome P-450 isozymes and NADPH.
The endoplasmic reticulum of the liver cell, supplied with molecular oxygen and NADPH, can monooxygenate a variety of lipid soluble compounds including steroids and many drugs. The essential enzymatic steps are carried out by cytochrome P-450 and NADPH cytochrome P-450 reductase. There are several molecular forms (isozymes) of cytochrome P-450 in liver, and it has been shown that their relative concentrations change during development and in response to phenobarbital. Because substrate specificities of the isozymes differ to some extent, changes in the isozyme pattern should alter the pattern of substrate utilization if the isozymes are controlling the rates of monooxygenation. We found that the rates of testosterone hydroxylation and the oxidative N-demethylations of benzphetamine, codeine, and ethylmorphine were equal in endoplasmic reticulum isolated from chick embryo liver on day 11 of egg incubation, and this equality was unaffected by age or phenobarbital. Thus, monooxygenation rates do not appear to be controlled by the isozymes of cytochrome P-450. A factor that might affect the pattern of steroid and drug utilization by endoplasmic reticulum may be the availability of reducing equivalents. Lowering the concentration of NADPH to one half-optimum values in our reaction mixtures (0.5 to 0.25 mM) diminished the monooxygenation rate of benzphetamine by 8%, of testosterone by 40%, and of codeine and ethylmorphine by 60%. These differential effects were independent of age and phenobarbital. Thus, the pattern of substrate utilization by the monooxygenase system in vivo is probably altered by changes in the tissue concentration of NADPH.
D000818:Animals; D001589:Benzphetamine / Q000378:metabolism; D002642:Chick Embryo; D003061:Codeine / Q000378:metabolism; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000201:enzymology / Q000378:metabolism*; D005036:Ethylmorphine / Q000378:metabolism; D006861:Hydrogen Peroxide / Q000378:metabolism; D007527:Isoenzymes / Q000378:metabolism*; D008099:Liver / Q000196:embryology / Q000201:enzymology / Q000378:metabolism* / Q000648:ultrastructure; D009249:NADP / Q000494:pharmacology; D010634:Phenobarbital / Q000494:pharmacology; D013739:Testosterone / Q000378:metabolism
D007527:Isoenzymes; D001589:Benzphetamine; D013739:Testosterone; D009249:NADP; D003577:Cytochrome P-450 Enzyme System; D006861:Hydrogen Peroxide; D005036:Ethylmorphine; D003061:Codeine; D010634:Phenobarbital