ABSTRACT
The metabolic interrelationships between methionine, homocyst(e)ine, and cyst(e)ine are extensive. Methionine can be metabolized by either the transsulfuration pathway to form homocysteine and cysteine, or by a transaminative route. In addition to its role in protein synthesis, the methyl group of methionine, as S-adenosyl methionine (AdoMet), participates in numerous methylation reactions, and the carbon skeleton of methionine is utilized for polyamine synthesis. Cysteine is required for both protein and glutathione synthesis. The relative metabolic needs for methionine and cysteine can effect the flow through the transsulfuration pathway. The location of homocysteine at a branch point in transsulfuration makes it a link between these two important sulfur amino acids. The dietary content of sulfur amino acids, labile methyl donors, and cofactors can also alter the flow through the transsulfuration pathway as can metabolic abnormalities in enzyme activity. The following chapter will review the interrelationships between these amino acids and illustrate how the use of animal systems, tissue culture, and human genetic disease have contributed to our knowledge. Focus will be primarily in three areas (1) the regulation of the flow of metabolites through the transsulfuration pathway, (2) the functions and interrelationships of sulfur-containing metabolites in normal and abnormal metabolism, and (3) the importance of the transaminative pathway of methionine metabolism in animals and humans. Special attention has been given to the effect of nutritional manipulation on flux through these pathways and the influence of inborn errors of metabolism on our knowledge of sulfur amino acid interrelationships.
