Characterisation and biosynthesis of d-erythroascorbic acid in Phycomyces blakesleeanus

d-Erythroascorbate and d-erythroascorbate glucoside have been identified in the Zygomycete fungus Phycomyces blakesleeanus. Ascomycete and Basidiomycete fungi also synthesise d-erythroascorbate instead of l-ascorbate, suggesting that d-erythroascorbate synthesis evolved in the common ancestor of the fungi. Both compounds accumulate in P. blakesleeanus at higher levels than observed in other fungal species. d-Erythroascorbate glucoside reduced dichlorophenolindophenol as effectively as l-ascorbate, but was more stable to autoxidation. d-Erythroascorbate glucoside predominated in spores and stationary phase mycelium. Free d-erythroascorbate accumulated during the exponential phase of mycelial growth and decreased to very low levels in the stationary phase. This suggests an association between growth and free d-erythroascorbate. P. blakesleeanus converted exogenous d-arabinose to d-erythroascorbate and its glucoside. A monomeric NAD-dependent d-arabinose dehydrogenase of 41 kDa was purified to near homogeneity. The enzyme oxidised d-arabinose, l-galactose, and l-fucose. Correspondingly, mycelium converted exogenous l-galactose and l-fucose to l-ascorbate and 6-deoxyascorbate, respectively. The antioxidant role of d-erythroascorbate and its glucoside is discussed.