The metabolic activity of the proteins of the brain

Abstract

The metabolism of the rat brain in vivo has been studied by measuring the rate of incorpora­tion of L-(³⁵S)methionine into the proteins. To avoid complications due to the low permeability of the ‘blood-brain barrier’, the labelled methionine was introduced directly into the cerebro­spinal fluid by intracisternal or subarachnoid injection. It was found that the ³⁵S was quickly distributed throughout the tissues of the body, and under these conditions there was a considerable uptake in the brain. The tissues were separated into lipid, protein and acid-soluble fractions, and it was found that the uptake of ³⁵S in the brain was mainly attributable to the incorporation of (³⁵S)methionine, with a smaller proportion of (³⁵S)cystine, in the brain proteins. The rate of incorporation of methionine in the brain proteins in vivo was approximately 0.4μeq./g protein/h. This indicates a considerably higher turnover for the brain proteins than was suggested by previous investigations which did not take the ‘blood-brain barrier’ into account (Borsook & Deasy 1951). By the use of specific radioactivity ratios, in which the specific activity of the protein S is referred to that of the acid-soluble S, consistent values were obtained for the uptake of ³⁵S into the brain proteins in normal animals. The rate of uptake varied with age, being higher in young animals than in old. The uptake of ³⁵S into the brain proteins, as indicated by the specific activity ratio, was reduced in sodium pentobarbitone and ether narcosis; this effect was greater if the body temperature was allowed to fall. Insulin hypoglycaemia caused a mean decrease of 24% in the uptake of ³⁵S into the brain proteins; electrical stimulation of the brain caused a smaller decrease of 19%.