[Metabolism of γ-aminobutyrate in Agaricus bisporus Lge. : II: γ-aminobutyrate: γ-ketoglutarate aminotransferase].

Transamination between γ-aminobutyrate and α-ketoglutarate provides a pathway for the utilization of γ-aminobutyrate in fruit-bodies of Agaricus bisporus Lge. This reaction leads to the formation of succinic semialdehyde, a metabolic intermediate in the metabolism of γ-aminobutyrate to succinate in the cell. γ-aminobutyrate: α-ketoglutarate aminotransferase (E.C. 2.6.1.19) was sonically extracted from the mitochondrial fraction and partially purified by DEAE-cellulose column chromatography. Aminotransferase had a pH optimum between 8.1 and 8.5 and did not require pyridoxal-phosphate in vitro; however, the enzyme was inhibited by carbonyl-trapping reagents such as pyridoxal-phosphate activated enzymes. The Km values for γ-aminobutyrate and α-ketoglutarate calculated from Lineweaver-Burk plots were 2.2×10(-4) M and 2.5×10(-3) M, respectively. The transaminase was specific for α-ketoglutarate but not for γ-aminobutyrate; aspartate, α-alanine and δ-aminovalerianate also functioned as amino-group donors. Activity of the enzyme was not influenced by the addition of carboxylic acids of the Krebs cycle. The reversal of the transamination reaction showed optimal rates at pH 9.0-9.3. Some considerations on the physiological significance of these results are given.