Carnitine acetyltransferase in developing mammals.

Carnitine acetyltransferase (CAT) activity was determined in mitochondria and microsomes of liver and brown adipose tissue in fetal and postnatal rats, rabbits and guinea pigs. In rat liver and brown adipose tissue, mitochondrial CAT activity increased perinatally. Microsomal CAT activity also increased in brown adipose tissue. In liver, however, a rise was first noted after the 20th postnatal day. The ratio of mitochondrial to microsomal activity was higher in brown fat than liver throughout the period studied. Absolute values for both were always much higher in brown adipose tissue than in liver. Catalase activity (an enzyme localized in the peroxisomes) in rat liver increased after day 20 while in brown adipose tissue it attained a peak at 7 days after birth. At all times, hepatic activity exceeded activity in brown adipose tissue. The ratio on day 30 was 1 (brown adipose tissue) to 25 (liver). In both guinea pigs and rabbits, hepatic mitochondrial CAT activity was 10- to 20-fold higher than in the rat already prenatally. Microsomal activity, on the other hand, was approximately the same in all three species. It is concluded that probably only the mitochondrial CAT is directly related to fatty acid oxidation. The role of microsomal enzyme remains unclear.