Evidence against the incorporation into protein of amino acids directly from the membrane transport system in rat heart

The possibility suggested recently Hider, R.C., Fern, E.B. and London, D.R. (1969) Biochem. J. 114, 171-178; Hider, R.C., Fern, E.B. and London, D.R. (1971) Biochem. J. 121, 817-827; van Venrooij, W.J., Poort, C., Kramer, M.F. and Jansen, M.T. (1972) Eur. J. Biochem. 30, 427-433; and Adamson, L.F., Herington, A.C. and Bornstein, J. (1972) Biochim. Biophys. Acta 282, 352-365] that protein synthesis takes place using amino acids directly from the membrane transport system and not from an intracellular pool has been investigated in rat heart. The tissue was perfused first for 30 min with either 14C]glycine or 14C]leucine and then for a further 30 min with identical medium containing 3H]glycine or 3H]leucine, respectively. After an initial lag, 14C]glycine was incorporated into protein at a linear rate up to 60 min. The 3H]glycine was accumulated into tissue water and incorporated just as readily as the 14C]glycine had been. The rate of total protein synthesis agrees with literature values only if intracellular and not extracellular specific activity values are used in the calculation. Some glycine was converted to serine or threonine. Leucine influx and efflux were very rapid in contrast to the relatively slow exchange reported for incubated tissues Hider, R.C., Fern, E.B. and London, D.R. (1969) Biochem. J. 114, 171-178; Hider, R.C., Fern, E.B. and London, D.R. (1971) Biochem. J. 121, 817-827; van Venrooij, W.J., Poort, C., Kramer, M.F. and Jansen, M.T. (1972) Eur. J. Biochem. 30, 427-433]. The results are consistent with the existence of an intracellular precursor pool for glycine. Some possible reasons for the discrepancies between this and the other studies are discussed.