H2S and polysulfide metabolism: Conventional and unconventional pathways.

It is now well established that hydrogen sulfide (H2S) is an effector of a wide variety of physiological processes. It is also clear that many of the effects of H2S are mediated through reactions with cysteine sulfur on regulatory proteins and most of these are not mediated directly by H2S but require prior oxidation of H2S and the formation of per- and polysulfides (H2Sn, n = 2-8). Attendant with understanding the regulatory functions of H2S and H2Sn is an appreciation of the mechanisms that control, i.e., both increase and decrease, their production and catabolism. Although a number of standard "conventional" pathways have been described and well characterized, novel "unconventional" pathways are continuously being identified. This review summarizes our current knowledge of both the conventional and unconventional.