Bovine retinal glutamine synthetase 1. Purification, characterization and immunological properties

Glutamine synthetase (GS) from bovine retina was purified to apparent homogeneity by ammonium sulfate fractionation followed by Sephacryl S-200, hydroxylapatite, and Sephadex G-150 chromatography. The purified enzyme showed a single band on polyacrylamide gel electrophoresis. Based on the purification data, retinal GS was shown to be approximately 2% of the total soluble retinal protein. By gel filtration, sedimentation velocity centrifugation, and gel electrophoresis, it was shown that the enzyme has a subunit molecular weight of 45 000 daltons and a native molecular weight of 360 000 daltons, which is consistent with an octameric structure. Throughout the various stages of purification, it was found that GS and glutamyl transferase (GT) activities were maintained at a constant ratio. Thus, the GS and GT reactions are catalyzed by the same enzyme. Immunodiffusion of antiretinal GS antibodies gave a single line of precipitation with both crude retinal and brain enzymes as well as purified enzyme preparations. Precipitation lines of retinal and brain enzymes completely fused with each other without any spur formation. The immunochemical titration of brain enzyme activity with antiretinal GS antibodies also revealed an immunological homology between retinal and brain enzymes.