Occurrence of N-formylkynurenine in extracts of Neurospora crassa: evidence for the activity of tryptophan pyrrolase.

N-formylkynurenine and kynurenine have been detected in extracts of tryptophan-grown Neurospora crassa. When the mycelia were grown in medium supplemented with l-[2-(14)C]tryptophan, the radioactivity was detected in N-formylkynurenine and N-formylanthranilic acid; with l-[beta-(14)C]tryptophan, radioactivity was detected in N-formylkynurenine, kynurenine, kynurenic acid, and xanthurenic acid. The occurrence of N-formylkynurenine in extracts of tryptophan-grown Neurospora is interpreted as direct evidence for the activity of tryptophan pyrrolase in this organism. The presence of this enzyme was expected on the basis of several earlier studies, but its activity in vitro has so far escaped detection. The in vivo evidence presented here suggests its presence and contributes importantly to our understanding of the tryptophan-anthranilic acid cycle.