The incorporation of labeled lysine into the proteins of guinea pig liver homogenate

When C14-labeled lysine is incubated with guinea pig liver homogenate, α-aminoadipic, α-ketoadipic, and glutaric acids are formed from the lysine (1). These transformations were established by finding the radioactivity of the C14 tracer in the metabolic products. The homogenate proteins coagulated by boiling at pH 5 also contained radioactivity. The counts given by the proteins corresponded to about 0.02 to 0.03 per cent of that added as lysine; the extent of lysine incorporation into the proteins was of the same order of magnitude as Melchior and Tarver (2) had found after incubating S35-labeled methionine and Winnick et al. (3, 4) C14-labeled glycine with rat tissue homogenates. Yet we could not satisfy ourselves that the radioactivity remaining in the proteins in our experiments, although it persisted through exhaustive extraction, did not come from traces of adsorbed radioactive lysine. Some counts were found in the protein when the homogenate was boiled prior to incubation with isotopic lysine