Lysine, homocitrulline, and homoarginine metabolism by the isolated perfused rat liver

The metabolism of lysine, homocitrulline, and homoarginine in the isolated, perfused liver was investigated. Following the addition of 4 mmoles of lysine to the perfusate, homocitrulline but not homoarginine appears in the perfusate and the liver. The liver/perfusate concentration ratio for homocitrulline is approximately 4, an indication that homocitrulline is not rapidly transported out of the cell. Homocitrulline probably is not metabolized by the liver since the concentration of added homocitrulline remains constant during 4 hours of perfusion. The addition of 1-(guanido- 14C)-homoarginine to the perfusate results in the appearance of labeled urea in the liver and perfusate. Apparently the reaction is carried out by liver arginase, since it is demonstrated that homoarginine is hydrolyzed by bovine liver arginase to form urea. The reactions suggest an alternate, minor pathway of ureogenesis in the organism.