Hydrogen
sulfide (H<sub>2</sub>S) elicits pleiotropic physiological
effects ranging from modulation of cardiovascular to CNS functions.
A dominant method for transmission of sulfide-based signals is via
posttranslational modification of reactive cysteine thiols to persulfides.
However, the source of the persulfide donor and whether its relationship
to H<sub>2</sub>S is as a product or precursor is controversial. The
transsulfuration pathway enzymes can synthesize cysteine persulfide
(Cys–SSH) from cystine and H<sub>2</sub>S from cysteine and/or
homocysteine. Recently, Cys–SSH was proposed as the primary
product of the transsulfuration pathway with H<sub>2</sub>S representing
a decomposition product of Cys–SSH. Our detailed kinetic analyses
demonstrate a robust capacity for Cys–SSH production by the
human transsulfuration pathway enzymes, cystathionine beta-synthase
and γ-cystathionase (CSE) and for homocysteine persulfide synthesis
from homocystine by CSE only. However, in the reducing cytoplasmic
milieu where the concentration of reduced thiols is significantly
higher than of disulfides, substrate level regulation favors the synthesis
of H<sub>2</sub>S over persulfides. Mathematical modeling at physiologically
relevant hepatic substrate concentrations predicts that H<sub>2</sub>S rather than Cys–SSH is the primary product of the transsulfuration
enzymes with CSE being the dominant producer. The half-life of the
metastable Cys–SSH product is short and decomposition leads
to a mixture of polysulfides (Cys–S–(S)<sub><i>n</i></sub>–S–Cys). These in vitro data, together
with the intrinsic reactivity of Cys–SSH for cysteinyl versus
sulfur transfer, are consistent with the absence of an observable
increase in protein persulfidation in cells in response to exogenous
cystine and evidence for the formation of polysulfides under these
conditions
