Methionine is an amino acid with sulphur methyl group, which is converted into S-adenosylmethionene (SAM) with ATP by methionine adenosyltransferase (EC 188.8.131.52). SAM is the sole methyl group donor for over 50 kinds of biologically active substances such as DNA, RNA, protein, phospholipids, hormones and neuro-transmitters, etc. After methyl group is transferred away from SAM under a methyltransferase, S-adenosylhomocysteine is formed, which is further metabolized into homocysteine (Hcy) after adenosine is removed. The homocysteine is metabolized to methionine thought N5-methyl tetrahydrofolate methyltransferase (EC184.108.40.206) and coenzyme vitamin B12 pathway accepting methyl form N5– methyltetrahydrofolate. This is methionene cycle. Methylation index is defined and the ratio of SAM and SAH. Methylation index is a better marker for methylation status and methylation capability. SAM is used as medicines or nutritional supplements for treating depression, liver diseases, arthritis and joint pain, etc.