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.
Nuutinen
Jaro T
JT
University of Helsinki, Department of Applied Biology, P.O. Box 27, FI-00014 University of Helsinki, Finland. jaro.nuutinen@iki.fi
Timonen
Sari
S
D001483:Base Sequence; D004271:DNA, Fungal / Q000737:chemistry / Q000235:genetics; D021901:DNA, Intergenic / Q000737:chemistry / Q000235:genetics; D055364:Hebeloma / Q000201:enzymology* / Q000235:genetics; D006861:Hydrogen Peroxide / Q000378:metabolism; D007651:Keto Acids / Q000378:metabolism; D050830:L-Amino Acid Oxidase / Q000235:genetics / Q000302:isolation & purification* / Q000378:metabolism; D055399:Laccaria / Q000201:enzymology* / Q000235:genetics; D008969:Molecular Sequence Data; D025282:Mycelium / Q000201:enzymology / Q000235:genetics; D038821:Mycorrhizae / Q000201:enzymology* / Q000235:genetics; D010802:Phylogeny; D000644:Quaternary Ammonium Compounds / Q000378:metabolism
D004271:DNA, Fungal; D021901:DNA, Intergenic; D007651:Keto Acids; D000644:Quaternary Ammonium Compounds; D006861:Hydrogen Peroxide; D050830:L-Amino Acid Oxidase