ABSTRACT
Apart from a few exceptions, sulfonium compounds and their related salts are by far the most important charged sulfur compounds encountered in biology. In addition, the analogy of their structures to those of many quaternary ammonium compounds contributed to the scientific rationale of their development. The impetus most likely originated from the studies by Brown and Fraser (1866, 1867) relating the chemical constitution and physiological action of substances in which the ‘salts of the ammonium bases derived from strychnia, brucia, thebaia, codeia, morphia and nicotia were used to demonstrate certain principles’. These sub-stances had some of the most striking pharmacological and toxicological actions of known quaternary ammonium compounds. The results raised the question of whether
analogues of these chemicals might possess similar biological properties. The original sulfonium compounds, prepared by Brown and Letts (1878) and by Letts (1878), had relatively simple chemical structures based on carboxymethyldimethylsulfonium salts (1), making them structurally analogous to the naturally occurring quaternary ammonium compound found in plants, betaine (2). In order to describe this new class of compounds the investigators named them thetines, a term that reflected the nature and relationship of these substances to betaine. A quote from the original publication puts this succinctly: ‘We have given the substance the name thetine to recall its relation to betaine and the fact that it contains sulfur’. Since then, the number of known sulfonium compounds has dramatically increased. A small number of these have been found to occur in living organisms and include S-(5’-deoxyadenosyl-5’)-L-methionine (S-adenosylLmethionine, AdoMet, 3), S-methyl-L-methionine (MeMet, 4) and dimethyl-ßpropiothetin (5). The discovery of the biological role of AdoMet, particularly in enzymatic group transfer reactions such as transmethylations which were discovered
during the middle half of this century, has led to an overwhelming amount of published material which reflects the status and unique nature of this molecule in biochemistry. Indeed, as AdoMet is involved in so many of the cellular metabolic reactions, its function and status in cellular terms has been compared to that of adenosine triphosphate (ATP, 6).
