Substrate induction of threonine dehydrase in vivo and in perfused rat livers.

The activity of the enzyme threonine dehydrase in the livers of mice and rats is increased by the intraperitoneal injection of threonine, while that of serine dehydrase shows no change. The injection of a comparable amount of L-serine produces no increase in the activity of either L-serine dehydrase or L-threonine dehydrase. This is substantiating evidence that these two dehydrases are separate enzymes (1). The inducible character of L-threonine dehydrase was previously observed in Escherichia coli (2). This increase in activity is analogous to the increases in tryptophan peroxidase-oxidace activity observed by Knox (3) upon intraperitoneal injection of tryptophan. The phenomenon of substrate-induced enzym.e formation has been frequently found in the case of microorganisms (2, 4), but in animals the only other reported cases which have been clearly demonstrated to represent substrate induction are those of tryptophan peroxidase-oxidase, ferritin (5), and chick embryo adenosine deaminase (6). An excellent review of the subject of metabolic adaptation in animals has recently appeared (7). Formation of tryptophan peroxidase-oxidase has been shown to be induced in at least two ways, by substrate (3) and by hormonal induction (8-11). Threonine dehydrase induction has been shown to take place in the absence of external hormonal influences in a perfused liver system.