Turnover of membrane proteins: Kinetics of induction and degradation of seven forms of rat liver microsomal cytochrome P-450, NADPH-cytochrome P-450 reductase, and epoxide hydrolase

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Abstract

The in vivo turnover of several rat liver microsomal proteins was studied using techniques designed to maximize antibody recognition specificity and minimize reutilization of radioactive labels. The kinetics of degradation of seven cytochrome P-450 isozymes, NADPH-cytochrome P-450 reductase, and epoxide hydrolase were determined in untreated rats and rats treated with phenobarbital or β-naphthoflavone. In the cases where induction of these enzymes occurred with the above chemicals, rates of synthesis of the proteins were also estimated. In general, the degradation rates of the different proteins were rather similar to each other, and the effects of phenobarbital and β-naphthoflavone on these rates were not very great. However, in the case of cytochromes P-450, a general trend was observed in which the heme moiety was degraded more rapidly than the apoprotein. Changes in the rates of synthesis of the individual proteins appear to contribute more to the altered steady-state levels which are expressed than do the rates of degradation, and profiles of steady-state enzyme concentrations predicted by the kinetic constants approximate those observed in vivo.

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    This research was supported in part by Grants ES 01590 and ES 00267 from the National Institutes of Health.

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    On leave from the Institute for Protein Research, Osaka University, 3-2 Yamadoka, Suita, Osaka 565, Japan.

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    F.P.G. was the recipient of United States Public Health Service Research Career Development Award ES 00041 (1978–1983).

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