The kinetics of myofibrillar protein breakdown in perfused rat skeletal muscle.

Nt-Methylhistidine, a non-reutilised amino acid present in some myofibrillar proteins, was radioactively labelled in vivo with [Me-3H]methionine. The specific radioactivities of protein-bound methylhistidine and free methylhistidine in perfusate after perfusion of rat hind limbs taken from prelabelled rats was determined. The decrease in urinary methylhistidine activity with time was determined for rats similarly labelled. Comparison of the specific activities of free and bound methylhistidine and the non-linear semilogarithmic plot of urinary methylhistidine activity suggest that the myofibrillar protein catabolism, as indicated by methylhistidine release, may not be a simple exponential process. The possibility of non-random decay is discussed and an alternative model proposed.