THYROXINE STIMULATION OF AMINO ACID INCORPORATION INTO MITOCHONDRIAL PROTEIN: DIFFERENCES BETWEEN IN VIVO AND IN VITRO EFFECTS

Abstract

ABSTRACT The in vivo and in vitro stimulation of rat hepatic mitochondrial protein synthesis by thyroxine (T 4 ) was compared. In confirmation of Buchanan & Tapley (1966). T 4 added to isolated mitochondria rapidly stimulated [ 14 C] leucine incorporation into mitochondrial protein. The in vitro stimulation was reversed after T 4 was removed by incubating the mitochondria with bovine serum albumin (BSA). The decrease in T 4 stimulation of protein synthesis appeared proportional to the T 4 removed by BSA. Thus, it appears probable that exchangeable T 4 controls the in vitro system. In contrast, the increase in mitochondrial protein synthesis which was observed 3 to 4 days after pretreatment of hypothyroid rats with labelled and non-radioactive T 4 was not reversed by BSA treatment. Moreover, mitochondrial radioactivity could not be extracted with albumin. The in vivo phenomenon does not, therefore, appear to be related to exchangeable hormone in the mitochondria. Furthermore, the estimated quantity of T 4 associated with mitochondria after in vivo stimulation was at least two orders of magnitude less than that required to produce comparable stimulation of mitochondrial protein synthesis in vitro . These findings strongly suggest that in vitro and in vivo stimulation of amino acid incorporation by T 4 may be mediated by different biochemical mechanisms.