Studies on the Induction and Repression of Enzymes in Rat Liver. 3. Induction of Ornithine Delta-transaminase and Threonine Dehydrase by Oral Intubation of Free Amino Acids.

It is well known that the feeding ad libitum of high protein diets to rats over a period varying from several days to several weeks produces an increase in the activities of enzymes involved in amino acid catabolism (3-8). Studies in our laboratory have shown that at least two amino -acid-catabolizing enzymes, threonine dehydrase and ornithine transaminase, can be elevated to high levels within a few hours in the livers of protein-depleted rats fed casein hydrolysate by stomach tube (9-11). In the long term adaptation studies cited above, as well as in our rapid induction experiments, relatively nonspecific dietary inducing agents were used, and no information was obtained on the possible existence of specific dietary requirements for the induction of individual amino acid catabolizing enzymes. To date, tryptophan pyrrolase is the only mammalian enzyme that has received thorough study with respect to its induction requirements (1217). Other very limited studies on threonine dehydrase induction have yielded contradictory results (18, 19). The present investigation was designed to determine whether or not a degree of specificity exists in the dietary amino acid requirements for the induction of threonine dehydrase and ornithine transaminase. In these studies the rapid induction of threonine dehydrase and ornithine transaminase was measured as a function of the qualitative and quantitative composition of a mixture of free amino acids administ.ered to rats by stomach tube.