A Comparative study of the utilization of glucose, acetate, glutamine and GABA as precursors of amino acids by retinae of the rat, frog, rabbit and pigeon

Abstract

The distribution of 14C among the amino acids aspartate, glutamate, glutamine and GABA, both in the tissues and incubation media, was studied using retinae from the rat, frog, rabbit and pigeon which had been incubated with radioactive glucose, acetate, glutamine or GABA. Tissue free amino acid contents were also measured and the specific activities of the amino acids calculated. All four species metabolized acetate and GABA within a small glutamate pool, since the specific activities of glutamine relative to glutamate were always greater than 1·0. However, in each case glucose carbon entered a large glutamate pool as the RSA of glutamine was invariably less than unity. The ability of [ 14C]glutamine to pass out of retinae and to accumulate in the extracellular fluid in large quantities was not shared by the other amino acids. The implications of this finding are discussed with reference to its significance as a detoxication mechanism for ammonia, and also as a means for glutamine to participate in the synthesis of GABA. Various differences existed between species in the patterns of metabolism of the various precursors, but none of these was so prominent as to permit one to ascribe separate functions to the routes of breakdown of GABA by complex (frog, rabbit and pigeon) and simple (rat) retinae.