Abstract
Urinary excretion of N^1-methylnicotinamide (MNA) accounted for only a few per cent of nicotinamide and its catabolic metabolites, but, its pyridones [N^1-methyl-2-pyridone-5-carboxamide(2-Pyr) and N^1-methyl-4-pyridone-3-carboxamide (4-Pyr)] accounted for around 90% in rats fed an adequate amino acid diet. However, the proportion of the pyridones decreased and the proportion of MNA increased concomitantly, when rats were fed an inadequate amino acid diet. Therefore, urinary ratio of (2-Pyr+4-Pyr)/MNA increased greatly when rats fed an adequate amino acid diet were compared with those fed an inadequate amino and free-tryptophan intake, namely niacin-equivalent intake. These phenomena were mainly attributed to the facts that the activity of liver 4-Pyr-forming MNA oxidase increased with improvement of amino acid intake and that this enzyme activity decreased with increasing niacin-equivalent intake because of inactivation during the catalysis. From the above results, it was pointed out that the increased urinary ratio of (2-Pyr+4-Pyr)/MNA can serve as a biological marker for adequate amino acid intake, but not for adequate niacin intake.
