A method has been developed for the simultaneous in vivo measurement of local rates for methionine incorporation into cerebral protein in the rat. It is based on the use of L-[35S]methionine as a tracer for reflecting the bidirectional exchange of methionine between plasma and brain and its incorporation into cerebral protein, using a dynamic three-compartment model. An operational equation based on this model has been derived in terms of determinable variables. The method has been applied to the normal freely moving rat and to the rat under chloral hydrate anesthesia. In the freely moving rat, the values of methionine incorporation into cerebral protein in the gray matter vary widely from structure to structure (50-300 nmol/100 g/min), with the highest values in structures related to neurosecretory functions, e.g., supraoptic and paraventricular nuclei. The values for white matter are more uniform (24-28 nmol/100 g/min) at levels approximately six- to seven-fold lower than for gray matter. Chloral hydrate anesthesia depresses the rate of methionine incorporation in all the structures examined. Anesthesia did not reduce the heterogeneity normally present within gray matter.

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The method previously developed for the measurement of rates of methionine incorporation into brain proteins assumed that methionine derived from protein degradation did not recycle into the precursor pool for protein synthesis and that the metabolism of methionine via the transmethylation pathway was negligible. To evaluate the degree of recycling, we have compared, under steady-state conditions, the specific activity of L-[35S] methionine in the tRNA-bound pool to that of plasma. The relative contribution of methionine from protein degradation to the precursor pool was 26%. Under the same conditions, the relative rate of methionine flux into the transmethylation cycle was estimated to be 10% of the rate of methionine incorporation into brain proteins. These results indicate the following: (a) there is significant recycling of unlabeled methionine derived from protein degradation in brain; and (b) the metabolism of methionine is directed mainly towards protein synthesis. At normal plasma amino acid levels, methionine is the amino acid which, to date, presents the lowest degree of dilution in the precursor pool for protein synthesis. L-[35S]-Methionine, therefore, presents radiobiochemical properties required to measure, with minimal underestimation, rates of brain protein synthesis in vivo.

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