Synthesis of glutamic acid in animal tissues

Abstract

A search was made for reactions, other than urea synthesis, causing a disappearance of added ammonia in isolated liver prepns. Homogeniza-tion of liver was found to abolish, under certain conditions, the conversion of ammonia into urea, while ammonia was still removed when pyruvate, oxaloacetate, alpha-ketoglutarate, or citrate were added. The main reaction responsible for the removal of ammonia in homogenates, on addition of the above substrates, was found to be the synthesis of glutamic acid. The synthesis of glutamic acid in the presence of alpha-ketoglutarate and ammonia occurred both anaerobically and aerobically, the anaerobic rates being somewhat higher than the aerobic rates. The reductive amination of alpha-ketoglutarate by liver can be coupled with the following 4 oxidative reactions: alpha-ketoglutarate[long dash]succinate + CO2; isocitrate[long dash]*alpha-ketoglutarate + CO2; beta-hydroxybutyrate[long dash]acetoacetate; L-malate[long dash]oxalo-acetate. L-Lactate and hexosediphosphate, and to a less extent acetate and butyrate, inhibited the formation of glutamic acid from alpha-ketoglutarate and ammonia. Adenosine triphosphate delayed the fall in the rate of glutamic acid synthesis which occurred in homogenates when the exptl. period exceeded 10 min. The rate of glutamic acid synthesis in liver homogenates was 3-15 times greater than in liver slices. In kidney cortex the differences in the rates observed in homogenized and sliced material were small. The highest rates of glutamic acid synthesis, in terms of Qglutamic acid, were 20 for rat liver, 27 for pigeon liver, 8 for kidney cortex (sheep, rat), 5 for brain cortex, 3 for heart muscle, 2 for pigeon breast muscle, less than 1 for spleen and lung. Glutamic acid was removed by liver slices without forming ammonia or urea, an indication of the occurrence of unknown reactions in which glutamic acid is utilized. The synthesis of glutamic acid is responsible for a part, but probably not for the whole, of the ammonia which is removed by liver slices and not accounted for by urea formation. The physiological significance of the synthesis of glutamic acid is discussed. One of the reactions for which glutamic acid is required in liver tissue is presumably the conversion of citruline into arginine in the course of the synthesis of urea.