Gastric peroxidase (GPO) was purified to apparent homogeneity to chara
cterize its major physiological electron donor. The enzyme (RZ = 0.7),
with a subunit molecular mass of 50 kDa, is a glycoprotein, with a re
lative abundance of aspartic and glutamic acid over arginine and lysin
e. It has a Soret maximum at 412 nm, which is shifted to 426 nm by H2O
2 due to formation of compound II. Although the physiological electron
donors I-, Br- SCN-, but not Cl-, are oxidized by GPO optimally at ac
id pH, only I- and SCN- are oxidized appreciably at physiological pH.
Considering that the I- concentration in stomach is less than 1 mu M,
whereas the SCN- concentration is about 250 mu M, SCN- may act as a ma
jor electron donor for GPO. Moreover, SCN- oxidation remains unaltered
in the presence of physiological concentrations of other halides. The
second-order rate constant for the reaction of GPO with H2O2 (k(1)) a
nd compound I with SCN- (k(2)) at pH 7 was found to be 8 x 10(7) M(-1)
.s(-1) and 2 x 10(5) M(-1).s(-1) respectively. GPO has significant pse
udocatalase activity also in the presence of I- or Br-, but it is bloc
ked by SCN-. The SCN- oxidation product OSCN- may be reduced back to S
CN- by cellular GSH, and GSSG may be reduced back to GSH by glutathion
e reductase and NADPH. In a system reconstituted with pure glutathione
reductase, NADPH, GSH, SCN- and H2O2, GPO-catalysed SCN- oxidation co
uld be coupled to NADPH oxidation. This system where GPO utilizes SCN-
as the major physiological electron donor may operate efficiently to
scavenge intracellular H2O2.