Lysine metabolism in the human and the monkey: Demonstration of pipecolic acid formation in the brain and other organs

Metabolism ofl-U-¹⁴C]lysine was studied in the human autopsy tissues and the intact monkeys through intracerebroventricular and intravenous injections. The human tissues were more active in the metabolism ofl-¹⁴C]lysine to ¹⁴C]pipecolate than the rat tissues previously reported. This metabolism was equally active in the phosphate (pH 7) and the glycyl-glycine (pH 8.6) buffers with the brain and the kidney having higher activity than the liver. Besides ¹⁴C]pipecolate, traces of ¹⁴C]saccharopine and -¹⁴C]aminoadipate were also detected in the liver incubation. Twenty-four hr after intraventricular injection ofl-¹⁴C]lysine to the monkey, substantial labeling of pipecolate and -aminoadipate was observed in the brain and spinal cord, with the kidney, liver and the plasma having much reduced levels. Radioactivity levels of these two compounds were found low in the organs and plasma of the intravenously injected monkey. The urine of both monkeys contained only traces of ¹⁴C]pipecolate, even though it contained high levels ofl-¹⁴C]lysine and -¹⁴C]aminoadipate. It was concluded thatl-lysine is actively metabolized to pipecolate and -aminoadipate in the human and the monkey, that this reaction is most active in the brain whenl-lysine is intraventricularly administered, and that in contrast to the rat, the monkey may have an effective renal reabsorption for pipecolate which is similar to the human.