What determines the half-life of proteins in, vivo? Some experiences with alanine aminotransferase of rat tissues

The half-life of alanine aminotransferase in rat muscle is about 20 days, compared with that of the same enzyme in liver of about three days. The thermal denaturation rate of the enzyme in , vitro gives a calculated half-life of about 400 days at body temperature. A liver lysosomal preparation effectively inactivated the enzyme under conditions in which it was otherwise quite stable. From our results we conclude that turnover of alanine aminotransferase in , vivo is a reflection of an active (enzymatic) process which does not depend on prior thermal denaturation.