[Gluconic acid forming enzymes in Aspergillus niger (author's transl)]

At least three gluconic acid forming enzymes were identified in cell-free extracts of Aspergillus niger: glucose oxidase (EC 1.1.3.4), a glucose dehydrogenase (EC 1.1.99.10), and an enzyme or a mixture of enzymes catalyzing the cleavage of 6-phosphogluconate into gluconate and inorganic phosphate. 2,6-dichlorphenolindophenol was one of the hydrogen acceptors in vitro of the glucose dehydrogenase. Some properties of this enzyme (Km values, pH-dependence, substrate and hydrogen acceptor specificity), as determined in cell-free extracts, were found to be in good agreement with properties described in literature for a glucose dehydrogenase which has been purified from Aspergillus oryzae. The formation of Pi from 6-phosphogluconate and other phosphate esters was found to have an optimum between pH 7 and 8 , and another below pH 4. This suggests that it is catalyzed by an alkaline and an acid phosphomonoesterase (EC 3.1.3.1, 3.1.3.2), both enzymes exhibiting only low substrate specificity. The influence of extraction and assay buffers on the activity of gluconate forming enzymes was investigated. Loss of activity during preparation of cell-free extracts, as calculated from loss of activity storage of cell-free extracts at 4 degrees C, was found to be lower than 4%. Purified glucose oxidase added before homogenization was found in the extract almost quantitatively.