Measurement of Protein Turnover in Rat Brain¹

Degredation rates of rat brain proteins were measured by following the decay in specific radioactivity of carboxyl labelled aspartate and glutamate over a 17‐day period. Initial labelling of these amino acids was achieved by a single intraperitoneal injection 0f NaH 14 CO 3 . The non‐linear decay curve for total brain proteins could be approximated by assuming that the mixture contained two classes of proteins with half‐lives of 3.3 and 8.7 days, respectively. Half‐lives of 2.5 and 7.7 days were estimated for such protein classes in the microsomal fraction. The half‐lives of soluble proteins, synaptic membranes, cell body and synaptic mitochondria were 3.1, 5.8, 5.6 and 8.4 days, respectively. Identical results were obtained if the change in specific activity of intact protein labeled by NaH 14 CO 3 was followed. Two‐fold slower decay rates were obtained when brain proteins were labeled with a pulse of [4,5‐ 3 H]leucine or [l‐ 14 C]leucine. Half‐lives calculated for the two classes of proteins in whole brain were 8.4 and 16.5 days, respectively with [4,5‐ 3 H]leucine and 8.9 and 14.2 days, respectively with [1‐ 14 C]leucine. These results indicate the very significant reutilization of this amino acid in brain. Sodium [ 14 C]bicarbonate is a more satisfactory isotopic precursor for accurate assessment of rates of protein turnover in brain.