The Splanchnic Organs, Liver and Kidney Have Unique Roles in the Metabolism of Sulfur Amino Acids and Their Metabolites in Rats

The arterial-venous differences for methionine and cysteine and their metabolites, glutathione, taurine and sulfate, were measured across the splanchnic organs, the liver, the kidney and the hindlimb of fed rats. Methionine and cysteine were released into the blood by the splanchnic organs and removed by the liver. These results indicate that more than half of the sulfur amino acids taken up by the liver were used for synthesis of glutathione for export into the plasma. The kidney removed about half of the glutathione exported by the liver, presumably due to action of gamma-glutamyl transpeptidase and dipeptidase, and released to the circulation a comparable amount of cysteine. Taurine, presumably from deconjugation of bile acids, was released into the plasma by the splanchnic organs; taurine was also released by the liver. The hepatosplanchnic release of taurine into the plasma indicates that the liver is the major site of taurine biosynthesis; taurine was removed by the kidney for excretion in the urine. A small amount of methionine was removed by the kidney, and the hindlimb released a small amount of glutathione and methionine into the plasma. The splanchnic organs seemed to remove substantial sulfate from the plasma in addition to that provided by the diet, and a net release of sulfate from the liver was observed. The relative roles of the various tissues in sulfate production and removal was not clear from these studies, due to the large variability in the arterial-venous differences observed.