FIG. 1.

Basal mammalian S-metabolism. Major cellular pathways are shaded to distinguish those that are NADPH dependent (blue), those in which the S must come entirely from extracellular Met sources (red), those in which the sulfur can come from either Met in an NADPH-independent process or from cystine in an NADPH-dependent process (green), and those in which the S can come from Met, cystine, or inorganic sulfite or sulfate (brown). Red boxes denote the following: [1] the Met cycle, [2] trans-sulfuration, and [3] the polyamine synthesis (methionine salvage) pathway. Dashed arrows denote multistep processes. Abbreviations are listed in Abbreviations Used section. Different cell types will have different activities of the various pathways; arrow weights reflect predictions of approximate flux in liver hepatocytes under basal states. As discussed in the text, stresses will variously affect flux rates at many points in these pathways. Color images are available online.