Metabolism of tryptophan to niacin in Saccharomyces uvarum.

In Saccharomyces uvarum, the effect of metabolic intermediates of the tryptophan-NAD pathway on the niacin-production was investigated. Exogenously added kynurenine and 3-hydroxyanthranilic acid raised the content of total niacin of the cells 2-fold as compared to the control cells, although anthranilic acid and tryptophan were less effective. Tryptophan was taken up into the cells faster than kynurenine, and the intracellular pool of tryptophan was larger than that of kynurenine. Of kynurenine (0.05 mM) added to the medium, 55% went through the transaminase flux (2-H liberation), 20% through the kynureninase flux, but none through the acetyl-CoA flux. As for tryptophan, only 2% went through the kynureninase flux. The products through the transaminase flux were identified as kynurenic acid (85%) and xanthurenic acid. 3-Hydroxykynurenine, 3-hydroxyanthranilic acid, quinolinic acid and niacin were also detected. The metabolism of tryptophan via the kynureninase flux reached a plateau above 0.05 mM. The production of kynurenine and kynurenic acid gradually increased above 0.05 mM. Tryptophol was formed in parallel with the amount of tryptophan consumed, while the rate of niacin production increased after glucose and tryptophan were exhausted. Based on the data obtained, a possible regulatory mechanism of the tryptophan-NAD pathway was discussed.