MUSCLE PROTEIN BREAKDOWN RATES IN HUMANS BASED ON NTAU-METHYLHISTIDINE (3-METHYLHISTIDINE) CONTENT OF MIXED PROTEINS IN SKELETAL-MUSCLE AND URINARY OUTPUT OF NTAU-METHYLHISTIDINE

Samples of psoas muscle from nine infants (aged 1 day to 14 mo) and of several skeletal muscles from seven adult males (age 19-74 yr) were analyzed for content of protein-bound Nτ-methylhistidine (3-methylhistidine; 3-Mehis). The mean content of 3-Mehis (expressed as μmoles/g mixed protein) was 3.2 (range 2.4-3.7) in infants and 4.2 (range 3.7-4.6) in adults. The daily urinary excretion of 3-Mehis was measured in four young adult males receiving an egg-protein, flesh-free diet. Mean excretion of 3-Mehis was 211 (range 167-252) μmoles/day. From these two sets of data the mean rate of muscle protein breakdown in adult males was estimated to be 50 g/day, or 0.7 ± 0.1 g/kg body weight/day. These results are compared with reported values for the 3-Mehis content of mixed proteins in muscle of various species, and with published estimates, computed by other techniques, of the rate of muscle protein breakdown in human subjects. © 1978.