In recent years, there has been an upsurge of interest in elevation of the plasma concentration of homocysteine and closely related metabolites as an independent risk factor for cardiovascular disease (reviewed, for example, in References 1 through 3). Homocysteine itself is a thiol-(sulfhydryl-) containing amino acid, but in normal human plasma and other tissues, a variety of related disulfide derivatives may be present. Different authors have written about these compounds and their effects by using differing terminologies. To promote clarity of meaning and to minimize uncertainty, perhaps even confusion, it is important that each article discussing these compounds either defines explicitly the terms and/or abbreviations used or cites a prior publication in which such definitions are provided. Optimally, a more uniform consensus terminology will be developed and adopted by the field. This article describes very briefly the structures of the relevant compounds and sets forth terms and abbreviations that, it is hoped, may provide a basis for such a consensus.

The word “homocysteine” was used first by Du Vigneaud and coworkers 65 years ago when they discovered this compound4 and provided definitive proof that it had the structure of a thiol (ie, sulfhydryl) 4-carbon α-amino acid: HSCH2CH2CH (NH2) COOH. 5 The symmetrical disulfide of homocysteine was termed “homocystine,” both names being