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Phosphonoacetaldehyde hydrolase (EC 3.11.1.1), the bacterial enzyme that catalyses the reaction HCO-CH2-PO(OH)2+H20 -O-HCO-CH3+P1, is inactivated by borohydride * Present address: 23 Brixton Street, Flemington, Vic. 303 1, Australia. t Present address:The present paper presents evidence that imine formation occurs between the enzyme and its substrate.

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SUMMARYAconitase and NAD-and NADP-linked isocitric dehydrogenases were examined in two strains of Aspergillus niger. The mutant strain 72-44 produced higher yields of citric acid on the culture medium used than did the parent strain 7 2 4 . After growth for 22 hr on a medium in which citric acid did not accumulate, the amounts of each enzyme in the parent strain were approximately the same as those in the mutant strain. The enzymes were also found, though in lower amounts, throughout the incubation period of 8 days in both strains grown on a medium in which citric acid accumulated. A 20-fold increase in the concentration of iron in this medium doubled the activity of aconitase in extracts from the mutant strain, though citric acid accumulation was only decreased by 25 %. The addition of monofluoroacetate to the culture medium was toxic to the mould and did not stimulate citric acid yields. It is suggested that during the incubation, some recycling of citric acid may take place. The difference in citric acid yields from the parent and mutant strains was not accounted for on the basis of the degrees of aconitase or isocitric dehydrogenase activity.

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