IMMUNONUTRITION - ROLE OF SULFUR AMINO-ACIDS, RELATED AMINO-ACIDS, AND POLYAMINES

Pro-inflammatory cytokines mediate widespread changes in protein metab olism. Amino acids released from peripheral tissues fulfill a number o f functions. They act as substrate for acute phase protein and immunog lobulin synthesis and, together with polyamines, in the replication of immune cells. Demands for specific amino acids may outstrip the suppl y from endogenous sources. A number of strands of evidence suggest tha t sulphur amino acids, and amino acids that are metabolically related to them, may be required in increased amounts. Protein deficiency impa irs the acute phase response. However, sulfur amino acid insufficiency compromises glutathione synthesis, to a greater extent than hepatic p rotein synthesis, in the presence and absence of an inflammatory stimu lus. The resulting effect may be compromised antioxidant defences. Fun ctioning of T cells is dependent on intracellular,glutathione concentr ations and may also be affected by sulphur amino acid insufficiency. I t has been suggested that the increased N excretion, which occurs duri ng the immune response, is a reflection of a relative imbalance in the profile of amino acids released from peripheral tissues and the requi rements imposed by the synthesis of substances involved in the acute p hase response. Phenylalanine, tyrosine, tryptophan serine, and cystein e are released in amounts closest to requirements. Polyamine synthesis may be important for the fidelity of the enhanced level DNA transcrip tion and RNA translation that occurs in response to infection and duri ng tissue repair, gut growth after surgery, and in gut barrier functio ns. Although synthesized de novo from ornithine, arginine and S-adenos yl methionine (SAM), substantial recycling is a key feature of polyami ne metabolism. The recycling may be a reflection of the need to mainta in adequate tissue SAM during periods of rapid cell growth. During an immune/inflammatory response the combination of enhanced utilization o f cysteine for glutathione synthesis and cell replication may lead to depletion of cellular SAM. A relatively small addition of polyamines t o the diet may improve gut-associated aspects of the hosts' antibacter ial defenses. Nutrition 1998; 14:605-610. (C) Elsevier Science Inc. 19 98.