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cess appears to be essential for making the sulphur cyanolysable as evidenced by the acquisition of cyanolysability when ,-mercaptopyruvate is converted to the disulphide; only the latter can give the -S(S)form. This tautomerization can also occur in polysulphides and the lability of the sulphur in polysulphides with n > 3 compared with the stability of dialkyldisulphides (n = 2) again reflects the capacity to tautomerize

The understanding of sulfur bonding is undergoing change. Old theories on hypervalency of sulfur and the nature of the chalcogen-chalcogen bond are now questioned. At the same time, there is a rapidly expanding literature on the effects of sulfur in regulating biological systems. The two fields are inter-related because the new understanding of the thiosulfoxide bond helps to explain the newfound roles of sulfur in biology. This review examines the nature of thiosulfoxide (sulfane, S0) sulfur, the history of its regulatory role, its generation in biological systems, and its functions in cells. The functions include synthesis of cofactors (molybdenum cofactor, iron-sulfur clusters), sulfuration of tRNA, modulation of enzyme activities, and regulating the redox environment by several mechanisms (including the enhancement of the reductive capacity of glutathione). A brief review of the analogous form of selenium suggests that the toxicity of selenium may be due to over-reduction caused by the powerful reductive activity of glutathione perselenide.

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Several systems which generate persulfide sulfur promote in vitro proliferation of L1210 murine lymphoma cells. The systems include cysteine disulfides and pyridoxal, cystamine and diamine oxidase, beta-mercaptoalcohol disulfides and an alcohol dehydrogenase, and sulfide-treated proteins and a thiol. Persulfide sulfur is very unstable at pH near 7 and an essential feature of the growth-supporting systems is the ability to generate persulfide sulfur at a very low rate for long periods of time. Methyl disulfides (R--S--S--CH3) also support growth of L1210 cells and are more stable than persulfides (R--S--S--H). The requirement for these sulfur groups by L1210 cells may be related to the fact that these cells are defective in at least two enzymes of sulfur metabolism, cystathionase and 5'-methylthioadenosine phosphorylase. These findings provide the first evidence that persulfide sulfur may have a physiological role.