Glutamic-oxaloacetic transaminase of cauliflower. 2. Kinetics and mechanism of action

Glutamate and serine as competing donors for amination of glyoxylate in leaf peroxisomes

When provided with glycollate, peroxisomal extracts of leaves of spinach beet (Beta vulgaris L. cv.) converted L-serine and L-glutamate to hydroxypyruvate and 2-oxoglutarate respectively. When approximately saturating concentrations of each of these amino acids were incubated separately with glycollate, the utilization of serine was greater than that of glutamate. The utilization of glutamate was substantially reduced by the presence of relatively low concentrations of serine in the reaction mixture, whereas even high concentrations of glutamate caused only small reductions in serine utilization. Over the entire range of concentrations of amino acids examined, serine was invariably the preferred amino-group donor, but this preference was abolished at higher concentrations of glyoxylate. Serine not only competed favourably for glyoxylate but also inhibited L-glutamate: glyoxylate aminotransferase (GGAT), the degree of inhibition depending upon the glyoxylate concentration. Studies of L-serine: glyoxylate aminotransferase (SGAT) and GGAT in partially purified extracts from spinach-beet leaves confirmed that serine competitively inhibited GGAT but glutamate did not affect SGAT. Both enzymes were inhibited by high glyoxylate concentrations, the inhibition being relieved by suitably high concentrations of the appropriate amino acid. It is concluded that at the low glyoxylate concentrations likely to occur in vivo, the preferential utilization of serine would ensure flux through the glycollate pathway to glycerate, but at higher concentrations of glyoxylate, both enzymes could be fully active in glyoxylate amination.

Studies on glutamic-oxalacetic (GOT) and glutamic-pyruvic (GPT) transaminases of swine kidney worm Stephanurus dentatus (Diesing, 1839). I. Assay and general properties

Biochemical studies on the two transaminases GOT and GPT of swine kidney worm Stephanurus dentatus have been made. GOT has been found much more active than GPT. Enzyme activities are based on the formation of oxaloacetate (GOT) or pyruvate (GPT) from aspartic acid and alanine respectively with oxoglutarate. A linear relationship is observed between the enzyme concentration and activity. GOT shows a maximum activity at pH 8.0 and Michaelis constant 9 X 10(-3) M for male and 2.9 X 10(-3) M for female. GPT has an optimum pH of 7.5 and a Michaelis constant 19 X 10(-3) M for male and 8 X 10(-3) M for female. The optimum temperature for both GOT and GPT was 60 degrees C.