Differential Regulation of Malate Dehydrogenase Isoenzymes by Hydrocortisone in the Liver and Brain of Aging Rats

The activities and induction patterns of the isoenzymes of malate dehydrogenase (MDH) of the liver and brain of male rats of various ages were studied. The activities of both the isoenzymes of MDH of the liver and brain show a gradual increase with increasing age of the rats. Adrenalectomy decreases and hydrocortisone treatment increases the activity of cytoplasmic MDH of the liver and brain of rats of all the ages except that of the brain isoenzyme of old rats. This hormone-mediated induction of the isoenzyme is actinomycin D-sensitive. Furthermore, adrenalectomy decreases and hydrocortisone treatment increases the activity of mitochondrial MDH of the liver of young and adult rats but not in old rats. However, these treatments do not show any significant effect on the activity of mitochondrial MDH of the brain of rats of all the ages.