Induction of malate dehydrogenase isoenzymes in livers of young and old rats.

The activity of cytoplasmic malate dehydrogenase (EC 1.1.1.27) of livers of young (9-10 weeks) and old (60-70 weeks) rats decreases after adrenalectomy and increases after administration of cortisone to adrenalectomized rats. These changes are significantly lower in old rats. The induction by cortisone is inhibited by actinomycin D. Adrenalectomy decreases and cortisone increases the activity of mitochondrial malate dehydrogenase of young rats, but not of old rats. Cortisone, however, induces both mitochondrial and cytoplasmic malate dehydrogenase of the regenerating liver at both the ages. Thus, impairment of the inducibility of mitochondrial malate dehydrogenase that occurs in old age is repaired in dividing cells. Immunological studies on purified malate dehydrogenase from young and old rats show that the molecular species of the enzyme is apparently the same at both ages. The changes in activities of the enzymes seen in old age may be due to changes in template activity of the corresponding genes, which apparently do not change with age.