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.