Protein Synthesis Studies in Rats with Methionine

1) Total brain radioactivity was found to be regionally correlated (r = 0.97) with radioactivity incorporated into proteins following a bolus injection of [14C-methyl]methionine. This suggests that regional differences in total label accumulation correspond to differences in the incorporation of label into proteins. 2) Under steady-state conditions (i.e., during continuous infusion of [14C-methyl]methionine), regional brain specific activity (SA) was found to be lower than the plasma SA. Brain SA was diluted by an endogenous source of free methionine likely to be from protein breakdown. Assuming that all endogenous brain methionine can contribute to protein synthesis, measuring labelled methionine incorporation (without accounting for tissue SA), would lead to underestimated rates. 3) In contrast to these results, similar studies carried out on the rat heart have shown a ratio of heart to plasma SA equivalent to unity. According to this, methionine recycling did not became apparent in the heart during the experimental time and under our experimental conditions.