Myofibrillar protein turnover. Synthesis of protein-bound 3-methylhistidine, actin, myosin heavy chain and aldolase in rat skeletal muscle in the fed and starved states

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Abstract
The turnover of 3-methylhistidine (N.tau.-methylhistidine) and in some cases actin, myosin H chain and aldolase in skeletal muscle was measured in a number of experiments in growing and adult rats in the fed and overnight-starved states. In growing fed rats in 3 separate experiments, measurements of the methylation rate of protein-bound 3-methylhistidine by either [14C]- or [3H]-methyl-labeled S-adenosylmethionine show that 3-methylhistidine synthesis is slower than the overall rate of protein synthesis indicated by [14C]tyrosine incorporation. Values ranged from 36-51%. In 1 experiment with rapidly growing young fed rats, acute measurements over 1 h showed that 3-methylhistidine synthesis could be increased to the same rate as the overall rate. After overnight starvation in these rats, the steady-state synthesis rate of 3-methylhistidine was 38.8% of the overall rate. This was a similar value to that in adult non-growing rats, in which measurements of the relative labeling of 3-methylhistidine and histidine after a single injection of [14C]histidine indicated that 3-methylhistidine synthesis was 37% of the overall rate in the fed or overnight-starved state. According to measurements of actin, myosin H chain and aldolase synthesis in the overnight-starved state with young rats, with a variety of precursors, slow turnover of 3-methylhistidine results from the specific slow turnover of actin, since turnover rates of myosin H chain, mixed protein and aldolase were 2.5, 3 and 3.4 times faster respectively. However, in the fed state synthesis rates of actin were increased disproportionately to give similar rates for all proteins. 3-Methylhistidine turnover in muscle is less than half the overall rate in both young and adult rats, slow 3-methylhistidine turnover reflects the specifically slow turnover of actin compared with myosin H chain and other muscle proteins, and during growth the synthesis rate of actin is particularly sensitive to the nutritional state and can be increased to a similar rate to that of other proteins.