The roles of synthesis and degradation in determining tissue concentrations of lactate dehydrogenase-5.

C(14)-labeled amino acids were incorporated in vivo into LDH-5 of rat tissues. Different amounts of LDH-5 in liver, heart muscle, and skeletal muscle resulted from differences in rates of both intracellular degradation and synthesis. Rate constants for LDH-5 synthesis were 65, 2, and 5 picomoles per day per gram in rat liver, heart muscle, and skeletal muscle, respectively; rate constants for degradation were 0.041, 0.399, and 0.018 reciprocal days, respectively. The corresponding half-lives for LDH-5 in these tissues were 16, 1.6, and 31 days. Markedly divergent rates of LDH-5 catabolism in various tissues suggest the possibility that one isozyme with a slow rate of destruction may serve to maintain critical enzymatic activity in a tissue where rapid degradation of another isozyme occurs.