A Comparative Study of Pig and Sheep‐Brain Glutamine Synthetases: Tryptic Peptides and Thiol Groups

A comparative study of the tryptic peptides of glutamine synthetase from pig and sheep brain has been made. Individual peptide maps of S ‐[ 14 C]carboxamidomethylated enzymes as well as mixed peptide maps of S ‐[ 14 C]carboxymethyl and S ‐[ 3 H]carboxymethyl enzymes reveal close similarities of the enzyme. The S ‐carboxyamidomethylated tryptic peptides have been separated by column chromatography. The amino acid composition of several of these peptides as well as their N‐terminal groups have been determined. Some of the tryptic peptides from both sources have very similar composition and properties. The major peptides obtained by cyanogen bromide cleavage of both the enzymes have identical N‐terminal groups. The various thiol groups of both the pig and sheep brain glutamine synthetase have similar reactivity towards iodoacetic acid and iodoacetamide. The [ 14 C]alkylated peptides from both sources have very similar properties. The pig brain enzyme is more sensitive to inhibition by N‐ethylmaleimide than by iodoacetamide. The inhibition can be prevented by ATP and Mg 2+ . Some thiol group or groups are located close to the active centre and are involved in the formation of active conformation rather than binding of ATP.