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 α-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 2–5 mm. 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.