Glutamine synthetase isolated from human brain:: Octameric structure and homology of partial primary structure with human liver glutamine synthetase

Glutamine synthetase (GS) has been purified from the cytosolic fraction of non-frozen human brain tissue. The purified GS migrated as a main band around 44 kD on reducing SDS-PAGE. Two-dimensional electrophoresis revealed heterogeneity within subunits of GS. The masses of eight different peptides from a tryptic digest of GS as measured by high resolution MALDI-MS matched with the respective masses from an in silico tryptic fingerprint of the Swiss-Prot database entry of human liver GS, proving that at least 24% of the primary sequences of GS from brain and liver are identical. Sedimentation equilibrium profiles obtained from analytical ultracentrifugation experiments at 10degreesC showed that human brain GS is mainly octameric. The quaternary structure of human brain GS at 10 muM (subunit concentration) was not significantly affected by cations, such as magnesium (5 and 20 mM) or manganese (0.2 and 1 mM) within the range of pH 7.1-7.8.