Growth, Nτ-Methylhistidine Excretion and Muscle Protein Degradation in Growing Beef Steers

Abstract

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.

Myofibrillar protein turnover in feed-restricted and realimented beef cattle.

“…In the present study, muscle mass was assumed to be 33% of body weight (Allen et al, 1968). Based on this value, the FBR we observed was similar to data reported by McCarthy et al (1983), 2.46 to 3.42%/d, but slightly higher than data reported by Gopinath and Kitts (1984), 2.07 to 2.84%/d. Results indicate that FBR was higher (P < .05) for RFC than for ALC after realimentation (Table 3).…”

“…Cattle with ad libitum access to feed had a higher (P < .05) urinary W-MH excretion during the first two collection periods; however, it was lower (P < .05) for ALC than for RFC at 118 d and not different at 146 d Fable 1). The quantities of W-MH excreted by our steers (Table 1) were in a range similar to that reported by McCarthy et al (1983) and Gopinath and Kitts (1984) but slightly higher than reported by Nishizawa et al (1979) and Harris and Milne (1981 Body weights of ALC were higher ( P c .05) than those of restricted-fed cattle at the last four sampling periods (Table 2). A linear (P c .05) increase in body weight of ALC with age was observed.…”

“…The calculations of the characteristics of muscle protein metabolism were performed according to Gopinath and Kitts (1984) and McCarthy et al (1983). The W-MH pool in skeletal muscle was calculated by multiplying the estimated skeletal muscle mass (33% of body weight; Allen et al, 1968;Brannang, 1971) by the protein content of skeletal muscle mass (157 mg/g fresh weight; Gopinath and Kitts, 1984) and the W-MH content of skeletal muscle protein (3.5106 pmol W-MHfg muscle protein; Nishizawa et d., 1979). The fractional breakdown rate (FBR) of muscle proteins was calculated by dividing the daily excretion of W-MH in urine by the amount of W-MH in the skeletal muscle pool and multiplying by 100.…”

“…This technique has been validated in cattle (Harris and Milne, 1981;McCarthy et al, 1983). Gopinath and Kitts (1984) utilized W-MH excretion to measure myofibrillar protein degradation in growing steers. They concluded that rapid growth in steers is accompanied by a high rate of myofibrillar protein degradation.…”

Skeletal Muscle Proteases and Protein Turnover

“…Values reported for fractional rates of muscle protein degradation have ranged from 26.5% per day in breast muscle of young chicks (Maruyama et al, 1978) to 3.4 to 4.4% per day in 22-to 45-kg male pigs (Mulvaney et al, 1985) to 2 to 3.3% per day in growing cattle (McCarthy et al, 1983;Gopinath and Kitts, 1984). Table I shows that if the rate of muscle protein degradation in a 454-kg steer were decreased by only 10% from 3.0% to 2.7% per day, the rate of live weight gain in this animal would more than double from 1.20 to 2.55 kg/day.…”