Methionine is an amino acid with sulphur methyl group, which is converted into S-adenosylmethionene (SAM) with ATP by methionine adenosyltransferase (EC 126.96.36.199). 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 (EC188.8.131.52) 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. SAH and Hcy connect the processes of providing methyl group to critical biological molecules and recycling methyl group from N-5– methyltetrahydrofolate. The levels of SAH will determine of reflect whether the methionine cycle is normal or not. Therefore, finding a way to better quantify them has valuable practical implications. Elevated SAH can cause damage to vascular endothelial cells, which is related to the levels of global genome methylation. SAH may be a potencial biomarker for atherosclerosis.