| Abstract
| - 1. The turnovers of NADH-cytochrome b5 reductase, cytochrome b5, and NADPH-cytochrome c reductase in rat liver were determined by two separate methods including a double-label method using 14C- and 3H-leucines. Although NADH-cytochrome b5 reductase and cytochrome b5 are functionally and spatially associated in the microsomal membrane, their turnover rates were definitely different, confirming the independent turnover of microsomal membrane proteins. 2. The NADH-cytochrome b5 reductase of mitochondrial outer membrane had the same turnover rate as the corresponding enzyme of microsomes. As the composition and functions of mitochondrial outer membrane are significantly different from those of microsomes, the identical turnover rates of the reductases associated with these two membranes suggest that the degradation of membrane-bound molecules of the reductase is catalyzed by a factor extrinsic to the membranes. 3. After a single intravenous injection of 3H-leucine into rats, the labeled NADH-cytochrome b5 reductase molecules appeared first in the membrane of rough microsomes and subsequently in smooth microsomes. The appearance of radioactive reductase molecules in the outer membrane of mitochondria lagged behind that in smooth microsomes. At about 3 h after the injection, the labeled reductase molecules were almost equally distributed among rough and smooth endoplasmic reticula and mitochondrial outer membrane. Possible mechanisms for the intracellular translocation of the reductase from the site of synthesis to different membrane systems are discussed.
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