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Effect of Supplementing Low Protein Diets with the Limiting Amino Acids on the Excretion of N¹-Methylnicotinamide and Its Pyridones in Rats

Department of Food Science and Nutrition, Teikoku Women's University, Moriguchi, Osaka 570, Japan

We have hypothesized that the ratio of the excreted by-products of niacin metabolism, N¹-methyl-2-pyridone-5-carboxamide (2-pyr) + N¹-methyl-4-pyridone-3-carboxamide (4-pyr)/N¹-methylnicotinamide (MNA), might be useful as an index to assess the adequacy of amino acid intake in rats. The experiment reported herein was performed to test this hypothesis. When a 10, 20 or 40% casein diet supplemented with 0.1, 0.2 or 0.4% L-methionine, respectively, was fed to rats, the urinary excretion of MNA decreased, and that of 4-pyr increased, as the level of dietary casein and methionine increased. Therefore, the ratio of (2-pyr + 4-pyr)/MNA increased with increasing dietary casein and methionine levels. When the limiting amino acids of casein or soy protein isolate were added to a low casein or low soy protein isolate diet, the urinary ratio of (2-pyr + 4-pyr)/MNA also increased. These results indicate that the increased urinary ratio of (2-pyr + 4-pyr)/MNA can serve as a biological marker for adequate amino acid intake.

KEY WORDS: • protein adequacy • N¹-methyl-4-pyridone-3-carboxamide • N¹-methylnicotinamide • methionine • cystine • cysteine • threonine • glycine • nicotinamide methyltransferase • MNA oxidase • rat

Manuscript received 31 October 1988. Revision accepted 6 March 1989.

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