Fractional catabolic rates of myosin and actin estimated by urinary excretion of Ntau-methylhistidine: the effect of dietary protein level on catabolic rates under conditions of restricted food intake.

1. Critical studies on the distribution of Ntau-methylhistidine (3-methylhistidine; Me-His) among organs and tissues in adult rats are reported. Adult rats contained 46-5+/-3-6 mg Me-His/kg body-weight. Almost 90% of the Me-His in the body was recovered from skeletal muscle. These results support the hypothesis that fractional catabolic rates of myosin and actin in skeletal muscle can be estimated by measuring urinary excretion of Me-His. 2. Dietary protein level did not affect the total amount of Me-His in the body. However, urinary excretion of Me-His increased as dietary protein intake was increased. 3. From these results it was concluded that fractional catabolic rates of myosin and actin increase as dietary protein intake increases.