Metabolism of nicotinamide and nicotinic acid in Neurospora crassa

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Abstract
The mould neither methylates the vitamin nor conjugates it with glycine and the predominant end-product of the vitamin metabolism is free nicotinic acid, even nicotinamide being deamidated. The deamidation of nicotinamide occurs also with N. crassa mycelium under resting conditions. The presence of an enzyme capable of releasing free nicotinic acid and ammonia from nicotinamide in stoichiometric proportions was demonstrated in cell-free extracts of N. crassa. Studies on the fractionation of N. crassa into its subcellular components revealed that the nicotinamide-deamidating activity is localized in the soluble portion of the cytoplasm. The enzyme is constitutive in nature; it is present in the wild as well as mutant strains of the mould, and its formation is independent of the presence of nicotinamide or nicotinic acid in the culture medium. Some properties of the enzyme are investigated. The comparative metabolism of the vitamin in different orders of living systems and the significance of the deamidation of nicotinamide in the context have been discussed.