Effect of Protein and Riboflavin on Plasma Amino Acids and Hepatic Riboflavin-coenzymes in the Rat

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Abstract
To study the effect of protein quality and riboflavin on the concentrations of free amino acids in plasma and riboflavin-coenzymes in liver, weanling rats were fed diets containing casein, gluten or gluten plus lysine as sources of protein, supplemented with two levels of riboflavin. When compared with data obtained for animals fed 18% casein, the feeding of 18% gluten with an oral administration of 100 µg riboflavin resulted in a depression of plasma lysine, which was markedly greater than that of other essential amino acids, and in a significant increase in threonine. When the gluten diet was supplemented with 10 µg riboflavin, an interaction between riboflavin and protein resulted in a less pronounced change in the concentration of plasma lysine with no apparent change in threonine. A reduction in the essential to nonessential (EN) amino acid ratio in plasma of animals fed gluten was attributed to a decrease of most essential amino acids and to an increase in serine and glycine. Addition of lysine to the gluten diet did not significantly alter the EN ratio. Values for flavin adenine dinucleotide and total riboflavin in liver were significantly less in animals fed gluten than in animals fed casein. Comparison of data obtained for animals fed 18% casein with those of animals fed 18% gluten plus lysine showed that lysine supplementation had no significant effect on riboflavin fractions or total riboflavin of liver. The value observed for free riboflavin plus flavin mononucleotide in liver of animals fed gluten with 10 µg riboflavin was greater than that of animals fed casein or gluten plus lysine. The interaction between dietary riboflavin and protein was significant at the 5% level. The data indicated that although plasma amino acids may have value in evaluating protein status, other factors may be involved.