Turnover of malate-dehydrogenase isozymes in rabbit liver and heart.

Abstract

1. The cytosolic and mitochondrial isozymes of malate dehydrogenase were purified from rabbit heart. Antisera against the two isoenzymes were obtained from sheep. The antiserum against mitochondrial malate dehydrogenase showed no cross‐reaction with the cytosolic isozyme and vice versa.2. Apparent turnover rates of the two isozymes were determined in rabbit liver and heart by means of single pulse labelling with [14C]leucine and isolation of the enzymes labelled in vivo by the immunochemical method. The apparent half‐lives of the cytosolic and mitochondrial isozyme are 8.6 and 21.9 h in liver and 21.6 and 33.9 h in heart. These values are shorter than the apparent half‐lives of total soluble protein of liver and heart.3. Turnover of mitochondrial malate dehydrogenase in rabbit liver was also calculated from the kinetics of enzyme accumulation and regression during a starvation‐refeeding cycle. The half‐life during starvation is 15.4 h and in the refeeding period 19.6 h. The approximately two‐fold increase in mitochondrial malate dehydrogenase content of liver during starvation appears thus to be brought about by an increased rate of synthesis.