Transamination of L-cystathionine and related compounds by a bovine liver enzyme. Possible identification with glutamine transaminase

Abstract

A transaminase which catalyses the monodeamination of L-cystathionine was purified 1100-fold with a yield of 15% from bovine liver. The monoketoderivative of cystathionine spontaneously produces the cyclic ketimine. Other sulfur-containing amino acids related to cystathionine such as cystine, lanthionine and aminoethylcysteine were also substrates for the enzyme. The relative molecular mass of the enzyme was determined to be 94 000 with a probable dimeric structure formed of identical subunits. The isoelectric point of the enzyme was at pH 5.0 and the maximal enzymatic activity was found at pH 9.0–9.2. Kinetic parameters for cystathionine and for the other sulfur amino acids as well as for some α-keto acids were also determined. Among the natural amino acids tested, glutamine, methionene and histidine were the best amino donors. The enzyme exhibited maximal activity toward phenylpyruvate and α-keto-γ-methiolbutyrate as amino acceptors. The broad specificity of the enzyme leads us to infer that the cystathionine transaminase is very similar or identical to glutamine transaminase.