Metabolism of [N-15]alanine in the ectomycorrhizal fungus Paxillus involutus

Alanine metabolism in the ectomycorrhizal fungus Paxillus involutus was investigated using [N-15]alanine. Short-term exposure of mycelial discs to [N-15]alanine showed that the greatest flow of N-15 was to glutamate and to aspartate. Levels of enrichment were as high as 15-20% for glutamate and 13-18% for aspartate, whereas that of alanine reached 30%. Label was also detected in the amino-N of glutamine and in serine and glycine, although at lower levels. Preincubation of mycelia with aminooxyacetate, an inhibitor of transamination reactions, resulted in complete inhibition of the flow of the label to glutamate, aspartate, and amino-N of glutamine, whereas [N-15]alanine rapidly accumulated. This evidence indicates the direct involvement of alanine aminotransferase for translocation of N-15 from alanine to glutamate. Alanine may be a convenient reservoir of both nitrogen and carbon. (C) 1995 Academic Press, Inc.