Chapter 19 - Sulfur Amino Acids Metabolism From Protein Synthesis to Glutathione

Abstract

The sulfur amino acids (SAAs) are methionine and cysteine. Methionine is indispensable thus it must be obtained preformed in the diet. Cysteine is dispensable because it is synthesized from methionine. The SAAs have received increasing attention because of their metabolites homocysteine (Hcy), glutathione (GSH), and more recently hydrogen sulfide (H2S), which has emerged as an important gaseous signaling molecule. Hcy is synthesized from methionine and hyperhomocysteinemia is a predictive marker of cardiovascular disease. GSH, synthesized from cysteine, is the most abundant intracellular antioxidant, and GSH deficiency is associated with poor prognosis in disease. H2S is produced endogenously from cysteine and is involved in blood pressure regulation, cardiac function, neuromodulation, inflammation, cellular energetics, and apoptosis. The SAAs share a complex metabolic pathway. The flux of different metabolites through the pathways is influenced by the dietary intake of methionine and cysteine. Cysteine provides a sparing effect on the methionine requirement and this sparing effect occurs equally with enteral and parenteral feeding. However, the requirement for SAA is decreased in parenteral compared with enteral feeding.