A comparative study of pig and sheep brain glutamine synthetases: tryptic peptides and thiol groups.

comparative study of the tryptic peptides of glutamine synthetase from pig and sheep brain
has been made. Individual peptide maps of S-[14C]carboxamidomethylatede nzymes as well as
mixed peptide maps of X-[14C]carboxymethyl and S-[3H]carboxymethyl enzymes reveal close
similarities of the enzyme. The 8-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 [14C]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 Mg2+.
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.