The Early Stage of Labeling of Microsomal Membrane Proteins in Rat Liver by Radioactive Amino Acids

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Abstract
The early stage of labeling of microsomal membrane proteins in rat liver by radio-active amino acids was examined by using 1-14C-L, 1-14C-D-, and 1-14C-DL-leucines and guanidino-14C-L-arginine. Radioactivity was incorporated into microsomal proteins from the radioactive D-leucine as well as from the corresponding L-isomer, but the rate of incorporation in the former case was much more slower than in the latter. The time course of radioactivity incorporation into microsomal proteins from 1-14C-DL-leucine could be explained by assuming additive contributions of the L- and D-isomers in the labeling. Almost all radioactivity incorporated from 1-14C-D-leucine into protein was recovered in the leucine fraction when the labeled protein was hydrolyzed and examined. The experiments with 1-14C-L-leucine and guanidino-14C-L-arginine showed that more than half of the newly incorporated radioactivity was lost from microsomal proteins during the initial few hours after the labeling. Since this initial loss of the once-incorporated radioactivity from microsomal proteins was equally or even more pronounced for purified NADPH-cytochrome c reductase than for total microsomal protein, we regarded this observation as signifying the actual behavior of newly synthesized microsomal membrane proteins in their early stage of binding to the membrane. Thus, we concluded that the- turnover of microsomal NADPH-cyto-chrome c reductase in rat liver was biphasic consisting of a rapid reaction and a slow one, and the rapid phase with a half life of about lhr has been overlooked in previous turnover studies. The injection of phenobarbital, an inducer of microsomal NADPH-cytochrome c reductase, to the rats caused the disappearance of this rapid turnover phase of the reductase.

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