Purification of rat-liver γ-hydroxyglutamate transminase and its probable identity with glutamate-aspartate transminase

1. 1. An enzyme which catalyzes the transamination of γ-hydroxyglutamate has been purified over 300-fold from rat-liver homogenates. 2. 2. The following observations strongly suggest that γ-hydroxyglutamate transminase is identical with glutamate-aspartate transaminase (L-aspartate: 2-oxo-glutarate aminotransferase, EC 2.6.1.1): a, at all stages of purification from either rat-liver or pig-heart extracts, the glutamate to γ-hydroxyglutamate transminase ratios are not significantly different; b, transaminase activity for both substrates declines at the same rate during controlled heat denaturation of the purified rat-liver enzyme; c, transamination of γ-hydroxyglutamate is strongly inhibited by either L-glutamate or L-aspartate; d, glutarate and maleate function as competitive inhibitors for either glutamate or γ-hydroxyglutamate and determined K₁ values for a given inhibitor are the same for either amino acid; and e, the purified rat-liver enzyme has the same pH-activity curve for both substrates. 3. 3. The erythro- and threo-isomers of γ-hydroxy-L-glutamate serve as substrates for both isozymes of the rat-liver enzyme as well as for the pig-heart enzyme, although the former isomer is a somewhat better substrate. The corresponding D-diastereo-isomers are enzymically inactive. 4. 4. With γ-hydroxyglutamate, only α-ketoglutarate and oxaloacetate serve as amino group acceptors; pyruvate, α-ketobutyrate, and β-phenylpyruvate are inactive. 5. 5. Other γ-substituted forms of glutamic acid, including γ-methyleneglutamic acid and γ-hydroxy-γ-methylglutamic acid, are also active with the purified enzymes.