EFFECTS OF AGING ON REGIONAL RATES OF CEREBRAL PROTEIN-SYNTHESIS IN THE SPRAGUE-DAWLEY RAT - EXAMINATION OF THE INFLUENCE OF RECYCLING OF AMINO-ACIDS DERIVED FROM PROTEIN-DEGRADATION INTO THE PRECURSOR POOL

The quantitative autoradiographic L-[1-C-14]leucine method for determi nation of regional rates of cerebral protein synthesis (lCPS(leu)) req uires knowledge of the degree of recycling of leucine derived from pro tein degradation into the precursor pool for protein synthesis. The in fluence of recycling can be evaluated by measuring lambda, the steady state ratio of the leucine specific activity in the precursor amino ac id pool (tRNA-bound leucine) to that in the arterial plasma. To define the changes in lCPS(leu) during the process of normal aging in the ra t we have evaluated lambda in middle-aged (14 months) and aged (24 mon ths) rats and compared its values with those obtained previously in yo ung adult rats (two months of age). The results show that the value of lambda, is the same in all three age groups, and that there is no cha nge with aging in the fraction of leucine in the precursor pool derive d from protein degradation. Our previously reported regional rates of protein synthesis in young adult and aged rats were based on the assum ption that there was no recycling of leucine derived from protein degr adation into the precursor pool for protein synthesis [Ingvar M. C., M aeder P., Sokoloff L. and Smith C. B. (1985) Brain 108, 155-170]. Thes e values have been recalculated in the present study in order to take into account the appropriate correction for recycling. The recalculate d rates are higher than those reported previously, but the effects of aging in the brain as a whole and in some specific brain regions are c onfirmed. Decreased lCPS(leu) was observed by middle-age, and in this cross-sectional study did not appear to decrease further. Of the 39 br ain regions examined decreases were found throughout the brain with so me proclivity for the brain stem. In comparison with young adults the weighted average rate of protein synthesis in the brain as a whole was found to be decreased by 16 and 11% in the middle-aged and aged group s, respectively.