Effect of dietary protein on urinary excretion of 3-methylhistidine in rat.

The excretion of the amino acid 3-methylhistidine in urine has been shown to be correlated with protein catabolism in skeletal muscle. In rats, 3-methylhistidine is partly acetylated (N-acetyl-3-methylhistidine) and it has been proposed that the relative amounts of 3-methylhistidine and N-acetyl-3-methylhistidine in urine is age dependent. In this experiment the effect of dietary protein quality on urine excretion of 3-methylhistidine and N-acetyl-3-methylhistidine was studied. Six groups of rats (mean weight 80 g) were fed diets containing 10% protein of different quality, the net protein utilization ranging from 76.7 for egg albumin to 28.9 for wheat gluten. The excretion of non-acetylated 3-methylhistidine was not dependent on the diet. There was, however, a good correlation between protein quality and total urine 3-methylhistidine (3-methylhistidine plus N-acetyl-3-methylhistidine), the higher the protein quality, the greater being the excretion of total 3-methylhistidine. The relative amounts of 3-methylhistidine and N-acetyl-3-methylhistidine correlated with the mean body weight, but not the age, of the animals. This study therefore demonstrates that a relationship exists between the nutritive quality of the dietary protein and the urinary excretion of 3-methylhistidine in rats.