Bovine retinal glutamine synthetase 2. Regulation and properties on the basis of glutamine synthetase and glutamyl transferase reactions.

Glutamine, the end product formed by the glutamine synthetase (GS) reaction, inhibits retinal GS activity in the presence of Mn2+, but not in the presence of Mg2+. In the presence of Mg2+, Mn2+ itself inhibits retinal GS activity. Other compounds which inhibit retinal GS activity significantly are methionine sulfoximine, D-alanine and carbamyl phosphate. Amino acids, such as L-alanine, L-serine and glycine, do not affect the enzyme activity. These amino acids, however, significantly inhibit the enzyme activity when measured on the basis of the glutamyl transferase (GT) reaction. GS isolated from neuronal tissues is regulated differently from that previously reported by others for non-neuronal tissues. The enzyme activity, as measured by GS activity, shows three-fold higher activity with Mg2+ over Mn2+ or Co2+ and on the basis of GT activity, shows about three-fold higher activity with Mn2+ over Mg2+ or Co2+. The optimum pH for the GS reaction lies in the range of 7.2-7.8 and for the GT reaction is 6.4-7.0. Both the GS and GT activities of the enzyme show similar heat stabilities.