Mutants of Neurosgora Deficient in D-Amino Acid Oxidase*

Although many investigations have been carried out on the n-amino acid oxidase since its discovery in 1935 (2)) the metabolic role of this enzyme remains somewhat obscure. It is generally believed, on the basis of indirect evidence, that in animals the enzyme functions in the inversion of exogenous n-amino acids over the pathway n-amino acid --+ Lu-keto acid -+ L-amino acid (3). The first step is assumed to be catalyzed by the n-amino acid oxidase, the second by a transaminase. Other functions that have been proposed for the enzyme include the removal of n-amino acids of endogenous origin (4,5) and, in some organisms, the synthesis of n-amino acids (6). It has also been suggested that the enzyme has no physiological significance as a n-amino acid oxidase, but that it is a by-product of metabolism, or that it may have some function other than oxidation of n-amino acids (7,8). Neurospora crassa is known to possess a n-amino acid oxidase which resembles the mammalian enzyme in substrate specificity (5, 9). The evidence suggests that in this organism, too, the enzyme is required for the inversion of o-amino acids of exogenous origin (5). Operating on this assumption, it has been possible to obtain mutants of Neurospora showing little or no n-amino acid oxidase activity. A description of the mutants and their response to n-amino acids is reported below.