Growth, N tau-methylhistidine excretion and muscle protein degradation in growing beef steers

The urinary excretion of N tau-methylhistidine (N tau-MH) was studied quantitatively in growing Hereford steers to measure muscle protein degradation (MPD) at different stages of growth. The amount of MPD and the fractional rate of protein breakdown (FBR) on 28, 42, 56 and 63 d of the experiment were calculated from the 24-h urinary N tau-MH excretion. The steers grew rapidly during the initial phase of the study; the growth rate was higher on d 56 (P less than .01) than on d 42 of the study. On d 63, there was a reduction in the growth rate compared with d 56 (P less than .05). Daily urinary N tau-MH excretion increased gradually as the animals gained weight and on an average, it was 1,957 +/- 88 mumol/d during the entire experimental period. Urinary creatinine excretion was not different at different growth stages, but the urinary N tau-MH: creatinine ratio was higher (P less than .05) on d 56 than on the other days. The amount of MPD per day increased gradually as the animals gained weight and was higher on d 56 than on d 42. Mean MPD and FBR values during the entire experimental period were 557 +/- 25 g/d and 2.44 +/- .09%/d, respectively. Half-life of the myofibrillar proteins decreased as the steers gained weight. These results indicate that the rapid growth in steers is accompanied by a high rate of MPD and demonstrate the usefulness of urinary N tau-MH excretion as a rapid, nondestructive method for measuring muscle protein degradation in large animals.