Evidence for distinct kynureninase and hydroxykynureninase activities in Neurospora crassa

Previous studies have indicated that a single enzyme, "kynureninase," catalyzes the reactions of l-kynurenine to anthranilate and l-3-hydroxykynurenine to 3-hydroxyanthranilate in Neurospora crassa and in other organisms. The present report describes separate enzymes which catalyze these reactions in N. crassa. The first, a kynureninase, preferentially catalyzes kynurenine to anthranilate and is induced over 400-fold by tryptophan or a catabolite of tryptophan. The second, a hydroxykynureninase, is constitutive or noninducible by tryptophan and preferentially catalyzes l-3-hydroxykynurenine to 3-hydroxyanthranilate. The physiological significance of these enzymes may be inferred from the facts that (i) the noninducible enzyme hydroxykynureninase appears to be the main enzyme present in uninduced cells that is capable of catalyzing l-3-hydroxykynurenine to 3-hydroxyanthranilate for the indispensible synthesis of nicotinamide adenine dinucleotide, and (ii) the inducible enzyme kynureninase is induced by tryptophan to a concentration far in excess of that needed to meet the requirements of the cells for nicotinamide adenine dinucleotide, resulting in the excretion of anthranilate into the medium.