The turnover of
3-methylhistidine (
N tau-methylhistidine) and in some cases actin,
myosin heavy 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 three separate experiments, measurements of the methylation rate of
protein-bound
3-methylhistidine by either [14C]- or [3H]-methyl-labelled
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 to 51%. However, in one 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 labelling 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 heavy-chain and
aldolase synthesis in the over-night-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 heavy 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. These results show that (a)
3-methylhistidine turnover in muscle is less than half the overall rate in both young and adult rats, (b) slow
3-methylhistidine turnover reflects the specifically slow turnover of actin compared with
myosin heavy chain and other
muscle proteins, and (c) 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.