Degradation of some sugars and sugar acids by the nonphosphorylated D-gluconate pathway in Aspergillus ustus

The degradation of fifteen sugars, sugar acids and related substrates were examined using cellfree extracts of Aspergillus ustus growing on D-glucose, D-mannose, D-galactose or D-gluconate as the only carbon source. D-gluconate was superior for the induction of the enzymes capable for the degradation of some of these substrates. The addition of 0.5% malt extract with D-gluconate to the growth medium or the presence of shaking conditions resulted to an increase in the degradation of those substrates, whileas the incorporation of 0.5% malt extract alone to the medium has no effect. Extracts of D-gluconate-grown mycelia of A. ustus degraded D-gluconate > D-galactonate > 1 : 5 gluconolactone and > L-arabonate nonphosphorolytically more effectively. Optimum pH and temperature for the degradation of D-gluconate were found to be 8.0 and 40°C, respectively. Thermal stability studies on the behaviour of D-gluconate dehydratase showed that this enzyme was stable at 50°C and 60°C for 30 and 5 minutes, respectively. Specific activity of this enzyme was increased three times when cell-free extracts were incubated at 60°C for 5 minutes. MgCl₂ and CoSO₄ were good activators, while CaCl₂ p-mercurychlorobenzoate (PMCB), sodium arsenite, ZnSO₄, CuSO₄, iodoacetic acid, MnCl₂ and FeSO₄ were potent inhibitors for D-gluconate dehydratase activity. K_m was calculated for D-gluconate and found to be 2.5 × 10^?2 M.