Fractional flux rates of Nt-methylhistidine in skin and gastrointestine: the contribution of these tissues to urinary excretion of Nt-methylhistidine in the rat.

1. Fractional flux rates of Ntau-methylhistidine (3-methylhistidine; Me-His) of skin and gastrointestine were measured by administering [methyl-3H]methionine to rats. 2. The results showed that the contribution of these tissues to urinary excretion of Me-His was at least 16-6%. This means that when fractional catabolic rates of myosin and actin were estimated from urinary excretion of Me-His, the part of Me-His derived from skin and gastrointestine should not be neglected.

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.

Quantitative aspect of the myofibrillar protein turnover in transient state on dietary protein depletion and repletion revealed by urinary excretion of N7-methylhistid;ne.

The fractional rates of synthesis and breakdown of myosin and actin in skeletal muscle of young adult male rats were measured during 2 weeks of ad libitum feeding of a protein-free diet, and 8 days of refeeding with an adequate protein diet. Daily urinary excretion of NT-methylhistidine (3-methylhistidine) by the NT-methylhistidine pool of the body gave the fractional breakdown rate of the myosin-actin pool. The fractional synthesis rate of the myosin-actin pool was calculated from the fractional breakdown rate and the size of NT-methylhistidine pool in the body. The feeding of the protein-free diet resulted in a decrease in body weight and a decrease in daily urinary excretion of NT-methylhistidine. Refeeding caused an increase in body weight and a progressive increase in daily urinary excretion of NT-methylhistidine. At the start of the experiment, the fractional breakdown rate of the myosin-actin pool was 4% per day and with prolonged protein depletion, the rate decreased to 1.25% per day. The fractional synthesis rate also decreased more rapidly than the breakdown rate. On refeeding for one day with an adequate protein diet, the fractional synthesis rate increased from 0.75 to 5.75% per day. Accumulation of skeletal muscle protein by refeeding was accompanied by a difference between the faster rate of synthesis and slower rate of breakdown even though the fractional breakdown rate increased during the rehabilitation period.