Role of N-acetylamino acids in cerebral protein synthesis. II. Incorporation of radioactivity from labelled acetyl and aminoacyl moieties into RNA

Abstract

Radioactivity from both [ 3H]acetate and [ 14C]aspartate was incorporated into immature rat brain RNA in vivo indicating active de novo RNA synthesis. The incorporation into RNA was 25–30-fold lower than incorporation into protein. In contrast to acetate and aspartate, the acetyl and aspartyl moieties of N-acetylaspartate did not contribute their 3H- and 14C-labels to RNA to a significant extent. This was in conformity with earlier observations based on protein synthesis studies that the metabolic pools receiving acetyl and aspartyl groups from these 2 sources had different dynamic characteristics. Formation of aminoacyl-tRNAs was measured using tRNA, pH 5 enzymes and purified aminoacyl-tRNA synthetases from rat and calf brains in homologous as well as heterologous combinations. Aspartate, methionine, leucine and phenylalanine were incorporated into tRNA at a high level, but no incorporation of N-acetylaspartate was observed under any of the experimental conditions employed. Further, aspartyl- and methionyl-tRNAs were not acetylated by acetate or acetyl-CoA. These results suggest that N-acetylaspartate, which occurs in a high concentration in nervous tissue, is not the initiating amino acid for the synthesis of at least a large majority of brain proteins.