Comparison of Tryptophan-Niacin Conversion in Rats Fed with a Nicotinic Acid-free Diet Containing Egg White, Egg White Proteolysate, or Mixtures of Amino Acids

Abstract

This investigation was carried out to learn whether or not tryptophan-niacin conversion is affected by the type of tryptophan, namely protein, small peptides, and mixtures of amino acids. Growing rats were fed with a nicotinic acid-free diet containing 20% egg white (EW; tryptophan content, 0.2085%), 25% egg white proteolysate-5 (EWP-5; tryptophan content, 0.2087%), or 18.13% mixtures of amino acids (AA; tryptophan content, 0.2085%) for 10 days. Urine samples were collected on the last day of the experiment. The body weigh gain and food intake during the experiment were almost the same among the animals in the three groups, and the content of total nicotinamide in the liver in the three groups was within the normal range. Under these conditions, the apparant tryptophan-niacin conversion [urinary excretion of nicotinamide + N1-methylnicotinamdide + N1-methyl-2-pyridone-5-carboxamide + N1-methyl-4-pyridone-3-carboxamide (μmol/day)/daily tryptophan intake for urine collection (μmol/day)] in the EW, EWP-5 and AA groups was 2.3%, 0.93% and 1.45%, respectively. Therefore, it was found that the tryptophan-niacin conversion varied according to the type of tryptophan, namely protein, small peptides, and mixtures of amino acids.