Control of Ornithine δ-Transaminase in Rat Liver and Kidney

Abstract

Abstract Ornithine δ-transaminase activity in the livers of rats, on a commercial diet from 21 days after birth, increased to a maximum at about 35 days of age and decreased thereafter, reaching the 21-day level by about 120 days after birth. Under the same conditions the activity in the kidneys also reached a maximum at about 35 days but did not decline thereafter. One reason for the difference in the shapes of the developmental curves for ornithine δ-transaminase activity for the two organs appeared to depend on different responses to dietary components. The activity in the livers of younger animals was increased by a high nitrogen intake and was decreased by restricting the nitrogen intake. In older animals, high nitrogen intake did not increase ornithine δ-transaminase activity. The activity in kidneys of either young or older rats did not decrease on a restricted nitrogen intake, and the changes in total and specific activities brought about by dietary protein depended on the total food intake. Fasting decreased total ornithine δ-transaminase activity in both liver and kidney, but did not change the specific activities. With carbohydrate as the sole food source, liver activity decreased but kidney activity increased. In experiments with amino acids added to the diet, liver ornithine δ-transaminase activity was decreased by added arginine and increased by arginine deficiency, or increased ingestion of glycine or serine. The activity in kidney was increased by dietary leucine, valine, and isoleucine, all of which are known to inhibit the ornithine δ-transaminase reaction per se and decreased by extra glutamate in the diet. The only response observed in regard to ornithine δ-transaminase which is common to liver and kidney was a significant increase of activity in response to dietary methionine. This methionine-induced increase of activity was apparently not decreased by arginine in the case of liver or by glutamate in the case of kidney. It is speculated that ornithine δ-transaminase may exist in liver and kidney in two different forms, the former functioning in the direction of arginine synthesis and the latter in the direction of arginine metabolism to glutamate.