Nutrition of Threonine

Among the threonine dehydratase activities in liver of rat fed on basal diet (B-group), lysine added diet (L-group) and threonine added diet (T-group), a following relationship was found: A_B?A_LT, where A_B, A_L and A_T were the activities of B-, L-and T-group, respectively. The types of the activity increase in L- or T-group were investigated by examining the possibility of the cause—the existence of activators in L- or T-group (I), the existence of inhibitors in B-group (II), the activation of the latent enzyme in L- or T-group (III), and the induction of the dehydratase by biosynthesis in L- or T-group (IV). Addition of various amounts of liver homogenate of L- or T-group to a given amount of that of B-group gave a result as would be obtained in cases where neither activators nor inhibitors existed in the liver (I and II). No correlationship was found between the activity and the preincubation time, which denied the presence of the latent enzyme which would easily change into the active form by preincubation (III). Actinomycin D administered to rats inhibited the increase of the dehydratase activity of L- or T-group by about 50% (IV). On the other hand, preliminary experiments using hypophysectomized or adrenal-ectomizecl rats showed the results of A_в?A_L?A_т. Both results may suggest the possibility that the increase of the dehydratase activity is ascribed to the induction of this enzyme through biosynthesis, and perhaps through endocrine systems.