Skeletal muscle myosin heavy-chain synthesis rate in healthy humans

Mixed muscle protein synthetic rate has been measured in humans. These measurements represent the average of synthetic rates of all muscle proteins with variable rates. We determined to what extent the synthesis rate of mixed muscle protein in humans reflects that of myosin heavy chain (MHC), the main contractile protein responsible for the conversion of ATP to mechanical energy as muscle contraction. Fractional synthetic rates of MHC and mixed muscle protein were measured from the increment of [¹³C]leucine in these proteins in vastus lateralis biopsy samples taken at 5 and 10 h during a primed continuous infusion of L-[1-¹³C]leucine in 10 young healthy subjects. Calculations were done by use of plasma [¹³C]ketoisocaproate (KIC) and muscle tissue fluid [¹³C]leucine as surrogate measures of leucyl- tRNA. Fractional synthetic rate of MHC with plasma KIC (0.0299 ± 0.0043%/h) and tissue fluid leucine (0.0443 ± 0.0056%/h) were only 72 ± 3% of that of mixed muscle protein (0.0408 ± 0.0032 and 0.0603 ± 0.0059%/h, respectively, with KIC and tissue fluid leucine). Contribution of MHC (7 ± 1 mg · kg^-1 · h^-1) to synthetic rates of whole body mixed muscle protein (36 ± 5 mg · kg^-1 · h^-1) and whole body protein (127 ± 4 mg · kg^-1 · h^-1) is only 18 ± 1 and 5 ± 1%, respectively. This relatively low contribution of MHC to whole body and mixed muscle protein synthesis warrants direct measurement of synthesis rate of MHC in conditions involving abnormalities of muscle contractile function.