—A method is described for the preparation of glutamate dehydrogenase in a highly purified form from rat brain. Only one protein band was detected when the enzyme was subjected to electrophoresis on SDS polyacrylamide gels. The rat brain enzyme was essentially identical to the rat liver enzyme with respect to electrophoresis on SDS polyacrylamide gels, immunochemical properties and most kinetic parameters. However, the brain enzyme was much less reactive with glutamate, was more sensitive to inhibition by haloperidol, and was considerably more stable than the liver enzyme.

Abstract— 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 NaH14CO3. 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 NaH14CO3 was followed. Two‐fold slower decay rates were obtained when brain proteins were labeled with a pulse of [4,5‐3H]leucine or [l‐14C]leucine. Half‐lives calculated for the two classes of proteins in whole brain were 8.4 and 16.5 days, respectively with [4,5‐3H]leucine and 8.9 and 14.2 days, respectively with [1‐14C]leucine. These results indicate the very significant reutilization of this amino acid in brain. Sodium [14C]bicarbonate is a more satisfactory isotopic precursor for accurate assessment of rates of protein turnover in brain.

Abstract— Protein turnover in rat brain was measured over a period of 30 days by following the decay in specific radioactivity of acidic amino acids in proteins labelled by a single intraperitoneal injection of [14C]NaHCO3. Two major populations of brain proteins can be identified from the resultant non‐linear decay curve—one with an average half‐life of 4 days and another with an average half‐life of 12 days. The half‐lives of total brain, mitochondrial, microsomal and soluble proteins determined over a period of 5 days were 3.4, 5.8, 2.8, and 2.6 days, respectively. Turnover of these same brain subcellular fractions was also measured by continuous infusion of [14C]tyrosine. The estimated half‐lives were in close agreement with those obtained from the 5 day measurement of radioactive decay following a pulse label of [14C]NaHCO3.

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