Transsulfuration in parenterally nourished beagle pups

Interactions of plasma and tissue sulfur-containing amino acid were studied in beagle pups that received either total parenteral nutrition (TPN) or intravenous (IV) glucose from 21 to 35 days of age and compared with those in pups nursed from birth to 35 days of age. The TPN infusate provided 6 g/kg/day of crystalline amino acids (2 mmol/kg/day of DL-methionine; less than 0.1 mmol/kg/day of cysteine) and 180 kcal/kg/day of glucose. The IV glucose regimen provided 60 kcal/kg/day. Plasma methionine, cystathionine, and cysteine concentrations of the pups that received TPN were similar to those of controls. Cerebral methionine and cystathionine concentrations μmol/100 g tissue), however, were greater while cerebral cyst(e)ine concentration was less than controls. The hepatic content (μmol/organ) of all three amino acids in these animals was similar to that of controls. Although plasma methionine, cystathionine, and cyst(e)ine concentrations of pups that received the IV glucose regimen were significantly less than control values, both cerebral concentrations and hepatic contents were similar. Cerebral specific activity and hepatic total cystathionine synthase activity as well as total hepatic cystathionase activity of animals that received TPN were similar to those of control animals. The activities of these enzymes in animals that received only IV glucose, however, were less than those of controls. These data demonstrate that the relationship between plasma and tissue concentrations of the sulfur-containing amino acids during a period of altered nutrition in beagle pups is neither simple nor direct. During TPN, cerebral methionine and cystathionine concentrations increase and cerebral cysteine concentration falls, due in part to the inability of the brain to metabolize methionine further than cystathionine; competition among cysteine and other plasma amino acids for transport from plasma to brain may also contribute to the low cerebral cysteine concentration. In contrast, both methionine and cystathionine are metabolized readily by the liver, and hepatic cyst(e)ine content is maintained.