It has been proposed that certain cell-surface proteins undergo
redox reactions, that is, transfer of hydrogens and electrons between
closely spaced cysteine thiols that can lead to reduction, formation,
or interchange of disulfide bonds. This concept was tested using a
membrane-impermeable trivalent arsenical to identify closely spaced
thiols in cell-surface proteins. We attached the trivalent arsenical,
phenylarsenoxide, to the thiol of reduced glutathione to produce
4-(N-(S-glutathionylacetyl)amino)phenylarsenoxide
(GSAO). GSAO bound tightly to synthetic, peptide, and protein
dithiols like thioredoxin, but not to monothiols. To identify
cell-surface proteins that contain closely spaced thiols, we attached
a biotin moiety through a spacer arm to the primary amino group of
the γ-glutamyl residue of GSAO (GSAO-B). Incorporation of GSAO-B
into proteins was assessed by measuring the biotin using
streptavidin-peroxidase. Up to 12 distinct proteins were labeled with
GSAO-B on the surface of endothelial and fibrosarcoma cells. The
pattern of labeled proteins differed between the different cell types.
Protein disulfide isomerase was one of the proteins on the endothelial
and fibrosarcoma cell surface that incorporated GSAO-B. These findings
demonstrate that the cell-surface environment can support the existence
of closely spaced protein thiols and suggest that at least some of these
thiols are redox active.