Alterations in lactate dehydrogenase of the brain, heart, skeletal muscle, and liver of rats of various ages.

The activity of lactate dehydrogenase of the supernatant fraction of brain, heart, skeletal muscle, and liver of l-day and 4-, 12-, 30-, 74-, and 96-week old rats was determined to study the changes in this enzyme with increasing age. Units of enzyme per g, wet weight, of all the tissues increased during the growth phase, reaching a maximum at 30 weeks, and then decreased. NADH,:NADHn ratios, Km values, and percentage of inhibition of lactate dehydrogenase by oxalate showed that the ratio of H-lactate dehydrogenase to M-lactate dehydrogenase increased after adulthood. This was due to a decrease of M-lactate dehydrogenase and not due to any increase in H-lactate dehydrogenase. Lower levels of lactate dehydrogenase and of M-lactate dehydrogenase in old age may make the tissues more oxygen dependent and decrease the period of tolerance for anaerobic conditions.