Transmethylation reactions utilize S-adenosylmethionine (SAM) as a methyl donor and are central to the regulation of many biological processes: more than fifty SAM-dependent methyltransferases methylate a broad spectrum of cellular compounds including DNA, histones, phospholipids and other small molecules. Common to all SAM-dependent transmethylation reactions is the release of the potent inhibitor S-adenosylhomocysteine (SAH) as a by-product. SAH is reversibly hydrolyzed to adenosine and homocysteine by SAH hydrolase. Hyperhomocysteinemia is an independent risk factor for cardiovascular disease. However, a major unanswered question is if homocysteine is causally involved in disease pathogenesis or simply a passive and indirect indicator of a more complex mechanism. A chronic elevation in homocysteine levels results in a parallel increase in intracellular or plasma SAH, which is a more sensitive biomarker of cardiovascular disease than homocysteine and suggests that SAH is a critical pathological factor in homocysteine-associated disorders. Previous reports indicate that supplementation with folate and B vitamins efficiently lowers homocysteine levels but not plasma SAH levels, which possibly explains the failure of homocysteine-lowering vitamins to reduce vascular events in several recent clinical intervention studies. Furthermore, more studies are focusing on the role and mechanisms of SAH in different chronic diseases related to hyperhomocysteinemia, such as cardiovascular disease, kidney disease, diabetes, and obesity. This review summarizes the current role of SAH in cardiovascular disease and its effect on several related risk factors. It also explores possible the mechanisms, such as epigenetics and oxidative stress, of SAH. This article is part of a Directed Issue entitled: Epigenetic dynamics in development and disease.
Keywords: Atherosclerosis; Cardiovascular disease; Epigenetic; Homocysteine; S-adenosylhomocysteine.
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