Identification of nitrogen mineralization enzymes, L-amino acid oxidases, from the ectomycorrhizal fungi Hebeloma spp. and Laccaria bicolor.

Amino acids are major nitrogen sources in soils and they harbour a central position in the nitrogen metabolism of cells. We determined whether Hebeloma spp. and Laccaria bicolor expressed the enzyme L-amino acid oxidase (LAO), which catalyses the oxidative deamination of the alpha-amino group of L-amino acids. We measured LAO activities from the mycelial extracts of seven laboratory-grown fungal strains with three methods, and we measured how LAO activities were expressed in one Hebeloma sp. strain grown on four nitrogen sources. Hebeloma spp. and L. bicolor converted L-phenylalanine, but not D-phenylalanine, to hydrogen peroxide, 2-oxoacid, and ammonia, suggesting that they expressed LAO enzymes. The enzymes utilized five out of seven tested L-amino acids as substrates. LAO activities were maximal at pH 8, where Michaelis constant (Km) values were 2-5mm. The LAO of Hebeloma sp. was expressed on every nitrogen source analysed, and the activities were the highest in mycelia grown in nitrogen-rich conditions. We suggest that LAO is a mechanism for cellular amino acid catabolism in Hebeloma spp. and L. bicolor. Many soil bacteria and fungi also express LAO enzymes that have broad substrate specificities. Therefore, LAO is a potential candidate for a mechanism that catalyses nitrogen mineralization from amino acids at the ecosystem level.