Figure 1.

Absorption and systemic distribution of dietary cysteine and methionine. GI epithelial cells take up cysteine and methionine, with EAAT3 being particularly important for cysteine uptake in the distal ileum. A portion of methionine is converted to cysteine via the methionine cycle and transsulfuration of homocysteine. Absorbed cysteine is taken up by the liver, which releases oxidized cystine. Cystine, but not cysteine, is able to traverse the blood brain barrier and is taken up by astroctyes, which convert it to GSH. Cysteine from astrocyte-derived GSH is available for EAAT3-mediated neuronal uptake, supporting neuronal GSH synthesis.