Except for tyrosine, the amino acid requirement of parenterally fed (PN) human neonates has not been derived. Methionine and cysteine are indispensable and dispensable sulphur amino acids respectively. Cysteine is synthesized from methionine. Cysteine is unstable in solution, and is left out or added in very small amounts to amino acid solutions. Methionine is added to compensate for the lack of cysteine, assuming that the neonate will convert methionine to cysteine to meet the body’s metabolic demand. Methionine is hepatotoxic and there is evidence that the neonate has limited ability for its conversion to cysteine. To determine the requirement of the neonate for methionine, PN-fed, stable, post-surgical neonates received graded intakes of methionine. The mean methionine requirement was estimated to be 49 mg.kg-1.day-1, which is 48 to 90% of the methionine content of current commercial amino acid solutions. Because cysteine is the rate limiting substrate for glutathione (GSH) synthesis and current methods of determining amino acid requirement measure requirement for protein synthesis, SAA requirements for maintenance of GSH status was deleniated in healthy adult males and in PN-fed human neonates. GSH kinetics was measured in healthy men receiving the mean methionine requirement and graded intakes of cysteine. GSH synthesis did not change with the addition of cysteine. Additionally, PN-fed post-surgical neonates recieved a methionine-adequate cysteine-free PN followed by cysteine supplemented PN for two 3-day periods and GSH kinetics measured on days 3 and 6. There was no change in GSH synthesis in response to cysteine supplementation. It is concluded that the PN-fed human neonate is capable of synthesizing enough cysteine from methionine not only for protein synthesis but for GSH synthesis. For both healthy men and stable post-surgical neonates, the requirement for GSH synthesis is met at the sulphur amino acid requirement derived using the indicator amino acid technique

Except for tyrosine, the amino acid requirement of parenterally fed (PN) human neonates has not been derived. Methionine and cysteine are indispensable and dispensable sulphur amino acids respectively. Cysteine is synthesized from methionine. Cysteine is unstable in solution, and is left out or added in very small amounts to amino acid solutions. Methionine is added to compensate for the lack of cysteine, assuming that the neonate will convert methionine to cysteine to meet the body’s metabolic demand. Methionine is hepatotoxic and there is evidence that the neonate has limited ability for its conversion to cysteine. To determine the requirement of the neonate for methionine, PN-fed, stable, post-surgical neonates received graded intakes of methionine. The mean methionine requirement was estimated to be 49 mg.kg-1.day-1, which is 48 to 90% of the methionine content of current commercial amino acid solutions. Because cysteine is the rate limiting substrate for glutathione (GSH) synthesis and current methods of determining amino acid requirement measure requirement for protein synthesis, SAA requirements for maintenance of GSH status was deleniated in healthy adult males and in PN-fed human neonates. GSH kinetics was measured in healthy men receiving the mean methionine requirement and graded intakes of cysteine. GSH synthesis did not change with the addition of cysteine. Additionally, PN-fed post-surgical neonates recieved a methionine-adequate cysteine-free PN followed by cysteine supplemented PN for two 3-day periods and GSH kinetics measured on days 3 and 6. There was no change in GSH synthesis in response to cysteine supplementation. It is concluded that the PN-fed human neonate is capable of synthesizing enough cysteine from methionine not only for protein synthesis but for GSH synthesis. For both healthy men and stable post-surgical neonates, the requirement for GSH synthesis is met at the sulphur amino acid requirement derived using the indicator amino acid technique