The Multiple Active Enzyme Species of γ-Aminobutyric Acid Aminotransferase Are Not Isozymes

Purified γ-aminobutyric acid aminotransferase (GABA-AT) from pig brain under certain conditions gave a single band on 12% NaDodSO4–PAGE, whereas two or three distinct bands were observed on 7.5% native PAGE. These multiple active species were isolated by 5% preparative gel electrophoresis and characterized by N-terminal sequencing and MALDI-TOF mass spectrometry. The results indicate that these active enzyme species are not GABA-AT isozymes in pig brain, but are the products of proteolysis of the N-terminus of GABA-AT, differing by 3, 7, and 12 residues from the full sequence (as deduced from the cDNA), respectively. Conditions for obtaining the nontruncated GABA-AT were found, and the potential cause for the proteolysis was determined. It was found that Na2EDTA inhibits the N-terminal cleavage during GABA-AT preparation from pig brain. The presence of Triton X-100 in the homogenization step is partially responsible for this proteolysis, and Mn2+ strongly enhances the protease activity, suggesting the presence of a membrane-bound matrix metalloprotease that causes the N-terminal cleavage.