LAUSR

Here, we report the occurrence of the (2R,3S)-isomer of 2-amino-3,4-dihydroxybutanoic acid (d-ADHB) in the fruiting body of an edible mushroom, Hypsizygus marmoreus. This is an unusual example of the accumulation of a d-amino acid whose enantiomer is not a proteinogenic amino acid. We show that d-ADHB occurs specifically in the mushroom H. marmoreus. Other edible mushrooms examined, including Pholiota microspora, Pleurotus eryngii, Mycena chlorophos, Sparassis crispa, Grifola frondosa, Pleurotus ostreatus, and Flammulina velutipes, do not contain detectable levels of d-ADHB. The concentration of d-ADHB in the fruiting body of H. marmoreus is relatively high (approximately 1.3 mg/g of fruiting body) and is comparable to the concentration of some of the most abundant free proteinogenic amino acids. Quantitative analysis of d-ADHB during fruiting body development demonstrated that the amino acid is synthesized during the fruiting body formation period. The absence of the putative precursors of d-ADHB, the (2S,3S)-isomer of ADHB and 2-oxo-tetronate, and the enzyme activities of d-ADHB racemase (2-epimerase) and transaminase suggested that d-ADHB is synthesized by a unique mechanism in this organism. Our data also suggested that the lack of or low expression of a d-ADHB degradation enzyme is a key determinant of d-ADHB accumulation in H. marmoreus.