The rate of incorporation of [35S]methionine and [35S]cystine into proteolipids and proteins of rat brain

Abstract

Measurements were made of the labelling of fractions containing proteolipid, other protein and free amino acids of the rat brain after administering L-[S35]methionine. The rate of incorporation of S35 into the proteolipids was much lower than into the other proteins. The radioactivity of the lipid fraction was present mainly the amino acids of the protein component of the proteolipids; in the sulphatide fraction it was relatively small. Comparison of the apparent turnover rates of [S35] methionine and S35 cystine in brain proteins at short intervals after injection of S35 methionine suggested that demethylation of the methionine was probably not limiting the rate of incorporation of s35 cystine. The apparent turnover rate of S35 methionine in brain proteins increased with time of exchange of the injected S35 methionine. The significance of this observation is discussed.