METABOLISM OF [C-14] GLUTAMATE AND [C-14] GLUTAMINE BY THE ECTOMYCORRHIZAL FUNGUS PAXILLUS-INVOLUTUS

To examine pathways of glutamate and glutamine metabolism in the ectom ycorrhizal fungus Paxillus involutus, tracer kinetic experiments were performed using L-[U-C-14]glutamate and L-[U-C-14]glutamine and the en zyme inhibitors methionine sulfoximine (MSX), azaserine (AZA) and amin ooxyacetate (AOA). When [C-14]glutamate was supplied to fungal culture s, 25% of the radioactivity of the amino acid fraction was incorporate d into glutamine after 5 min feeding, but MSX inhibited incorporation of C-14 into glutamine by 85%, suggesting the rapid operation of gluta mine synthetase. Conversely, when P. involutus was fed with [C-14]glut amine, 46% of the label was found in glutamate within 30 min of feedin g and AZA inhibited glutamate formation by 90%. Taken together, these data indicate that glutamate synthase (GOGAT) is the major enzyme of g lutamine degradation. In addition, the strong inhibition of glutamine utilization by AOA indicates that glutamine catabolism in P. involutus might involve a transamination process as an alternative pathway to G OCAT for glutamine degradation. The high (CO2)-C-14 evolution shows th at glutamate and glutamine are further actively consumed as respirator y substrates, being channelled through the tricarboxylic acid (TCA) cy cle and oxidized as CO2. It appears that synthesis of amino acid precu rsors during TCA cycle operation is an essential step far aspartate an d alanine synthesis through aminotransferase activities in P. involutu s.