The preparation of [35S]homocysteine thiolactone free of [35S]methionine.

The unambiguous study of homocysteine metabolism requires a source of [355]homocysteine completely free from contaminating [35S]methionine. Until now, such a reagent has not been readily available. Homocysteine is an important metabolite involved in methionine, cysteine and methylation metabolism. Homocysteine can be methylated to form methionine with either 5-methyltetrahydrofolate [1] or betaine [2] serving as the methyl donor, it can be condensed with serine to form cystathionine [3] which can be converted to cysteine, or it can be condensed with adenosine [4] to form S-adenosylhomocysteine, a strong inhibitor of all known cellular methylation reactions [5,6]. Additionally, there is evidence that in methionine-dependent tumor lines, methionine synthesized endogenously from homocysteine is used differently by the cell than exogenously supplied methionine [7]. In this report we describe a novel, simple and fast method of recovering methionine-free [35S]homocysteine thiolactone following its synthesis.