Figure 7.

Diagram of erythrocytic-renal cycle and erythrocytic-hepatic cycle. In erythrocytes, methionine is converted to SAM, which is a methyl donor for numerous reactions. After losing its methyl group, SAM becomes SAH, which is then converted to homocysteine. Homocysteine exerts its antioxidative effect and converts to homocystine and then releases to extracellular fluid and transport in circulation to the kidney or liver, which shows enriched expression of BHMT and MTR. In the kidney or liver, homocysteine can be converted back to methionine by the addition of a methyl group. Finally, methionine is synthesized and returned to circulation for further utilization.