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{gamma}-Amiobutyric Acid Metabolism in Plants: Part 1. Metabolism in Yeasts
Authors:PIETRUSZKO  R; FOWDEN  L
Institution:Department of Botany, University College London Gower Street, W.C. I
Abstract:The metabolism of {gamma}-aminobutyric acid ({gamma}AB) by two yeasts, Saccharomycescerevisiae and Torulopsis utilis, was investigated. Both yeastsgrew well upon {gamma}AB as a sole source of nitrogen (N), and thelag phase for Torulopsis was shorter than when provided the N-source. The metabolism of {gamma}AB by Torulopsis, whichwas associated with an increased O2 uptake, was adaptive incharacter. The enzyme whose formation was induced by the supplyof {gamma}AB was a transaminase, which was apparently specific for{gamma}AB as the amino donor. Small amounts of transaminase were presentin unadapted, -grown cells. The optimum pH, equilibrium constant, Michaelis' constant, and coenzyme requirementwere investigated for the transamination reaction involving{alpha}-ketoglutaric acid ({alpha}KG) as amino group acceptor. Succinic semi-aldehyde(SSA) was a product of this transamination reaction.The possibility;that some {gamma}AB was converted into SSA by a direct oxidative deaminationremained unconfirmed. The further conversion of SSA into succinic acid was establishedusing intact. cells for both yeasts. This oxidation processwas shown to be linked to the reduction of pyridine nucleotidesvising extracts of Saccharomyces as a source of SSA dehydrogenase.Dehydrogenase activity could be ascribed to two separate enzymes,one linked to DPN, and the other utilizing TPN and requiringMg++ as an activator. The properties of the former enzyme, whichwas more important quantitatively, were investigated and comparedwith those described in the literature for an aldehyde dehydrogenaseof baker's yeast and for SSA dehydro-genases of Pseudomonas.Torulopsis extracts could catalyse the reduction of SSA to {gamma}-hydroxybutyricacid ({gamma}OHB); the {gamma}OHB dehydrogenase involved required TPNH asa coenzyme. Certain other properties of this enzyme are recorded. The possibility is discussed that {gamma}AB and SSA act as intermediatesin a metabolic pathway that may form a by-pass of the {alpha}KG-succinatestage of the tricarboxylic acid cycle.
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