EFFECTS OF DIETARY-PROTEIN RESTRICTION ON THE FRACTIONAL RATES OF PROTEIN-SYNTHESIS IN PERFUSED RAT HINDLIMB

The effects of dietary protein restriction on protein synthesis were i nvestigated in perfused rat hindlimb. The fractional rate of protein s ynthesis was measured with [H-3]phenylalanine in young adult (7-week-o ld) rats fed a low protein (5% casein) diet and a protein-free diet fo r 3 weeks. The low protein diet (LPD) allowed a moderate gain in body weight. The fractional rate of protein synthesis fell to 70% of the co ntrol value in LPD group and further fell to less than a half in the p rotein-free diet (PFD) group. Thus, the protein synthesis rate decreas ed as the dietary protein content was reduced. The fall of protein syn thesis was mainly accompanied by the reduction of RNA activity (mg pro tein/mg RNA/day) rather than RNA concentration (RNA/protein). The rate of protein breakdown was calculated by subtracting growth rate from p rotein synthesis rate. The breakdown rate was decreased in LPD group a nd increased slightly in PFD group. From the low rates of protein synt hesis and breakdown, it appears that dietary protein restriction, at l east allowing a gain in body weight, makes the turnover rate slow down . The overall changes in protein synthesis obtained in the perfused hi ndlimb are consistent with the reported results in vivo.