Abstract
Quinolinic acid, a key intermediate in the de novo biosynthesis of niacin, is biosynthesized from L-tryptophan in mammals while it is biosynthesized from L-aspartic acid and dihydroxyacetone phosphate in Escherichia coil. Quinolinate synthetase of E. coli consists of two components A and B proteins. The B protein has been identified as L-aspartic acid oxidase that generates iminoaspartic acid. Iminoaspartic acid is then conjugated with dihydroxyacetone phosphate to generate quinolinic acid by the A protein. In the mammalian liver, unlike in E. coli, D-aspartic acid oxidase has been found, by which also iminoaspartic acid is generated. Significant amounts of D-aspartic acid are contained in tortillas (lime-treated corn) which has been served as anti-pellagra foodstuff. Therefore, we postulated and examined a possibility of the biosynthesis of niacin from D-aspartic acid in mammals. Administration of D-aspartic acid to rats and humans did not increase urinary excretion of niacin and its metabolites. In rats, administration of D-aspartic acid did not improve the growth of weaning rats fed a tryptophan-limiting and niacin free diet. Based on those results, the possibility of biosynthesis of niacin from D-aspartic acid in mammals was denied.
