On the Rate of Synthesis of Individual Proteins within and between Different Striated Muscles of the Rat

Abstract

The turnover of muscle protein is responsive to different (patho)-physiological conditions
but little is known about the rate of synthesis at the level of individual proteins or whether this
varies between different muscles. We investigated the synthesis rate of eight proteins (actin, albumin,
ATP synthase alpha, beta enolase, creatine kinase, myosin essential light chain, myosin regulatory
light chain and tropomyosin) in the extensor digitorum longus, diaphragm, heart and soleus of
male Wistar rats (352 ˘ 30 g body weight). Animals were assigned to four groups (n = 3, in each),
including a control and groups that received deuterium oxide (2H2O) for 4 days, 7 days or 14 days.
Deuterium labelling was initiated by an intraperitoneal injection of 10 µL/g body weight of 99.9%
2H2O-saline, and was maintained by administration of 5% (v/v) 2H2O in drinking water provided
ad libitum. Homogenates of the isolated muscles were analysed by 2-dimensional gel electrophoresis
and matrix-assisted laser desorption ionisation time of flight mass spectrometry. Proteins were
identified against the SwissProt database using peptide mass fingerprinting. For each of the eight
proteins investigated, the molar percent enrichment (MPE) of 2H and rate constant (k) of protein
synthesis was calculated from the mass isotopomer distribution of peptides based on the amino
acid sequence and predicted number of exchangeable C–H bonds. The average MPE (2.14% ˘ 0.2%)
was as expected and was consistent across muscles harvested at different times (i.e., steady state
enrichment was achieved). The synthesis rate of individual proteins differed markedly within each
muscle and the rank-order of synthesis rates differed among the muscles studied. After 14 days the
fraction of albumin synthesised (23% ˘ 5%) was significantly (p < 0.05) greater than for other muscle
proteins. These data represent the first attempt to study the synthesis rates of individual proteins
across a number of different striated muscles.