Purification and expression of a processing protease on β‐alanine‐oxoglutarate aminotransferase from rat liver mitochondria

GABA[arrow beta]AlaAT convertase is an endopeptidase that processes brain‐type 4‐aminobutyrate aminotransferase (GABA AT; EC 2.6.1.19) to liver‐type β‐alanine‐oxoglutarate aminotransferase (β‐AlaAT I) in rats. Its molecular mass was 180 kDa as determined by gel filtration. A subunit molecular mass of 97 652 Da was measured using MALDI‐TOF MS. The N‐terminal sequence of the purified GABA[arrow beta]AlaAT convertase was SRVEVSKVLILGSGGLSIGQAGEFDYSGSQAV‐ and was identical to residues 418–449 of carbamoyl‐phosphate synthetase I (CPS I; EC 1.2.1.27) purified from rat liver. The subunit molecular mass and the N‐terminal amino acid sequence suggested that GABA[arrow beta]AlaAT convertase was the 418–1305 peptide of CPS I. An expression vector containing the coding region of the 418–1305 peptide of rat CPS I was transfected into NIH3T3 cells and the extract of the cells showed GABA[arrow beta]AlaAT convertase activity.