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Das
Thiocyanate, a plausible physiological electron donor of gastric peroxidase.Das D, De PK, Banerjee RK. Biochem J. 1995 Jan 1;305 ( Pt 1):59-64.
"Gastric peroxidase
(GPO) was purified to apparent homogeneity to characterize its
major physiological electron donor. The enzyme (RZ = 0.7), with a
subunit molecular mass of 50 kDa, is a glycoprotein, with a
relative abundance of aspartic and glutamic acid over arginine
and lysine. It has a Soret maximum at 412 nm, which is shifted to
426 nm by H2O2 due to formation of compound II. Although the
physiological electron donors I-, Br- and SCN-, but not Cl-, are
oxidized by GPO optimally at acid pH, only I- and SCN- are
oxidized appreciably at physiological pH. Considering that the I-
concentration in stomach is less than 1 microM, whereas the SCN-
concentration is about 250 microM, SCN- may act as a major
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 (k1) and compound I with SCN- (k2) 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 pseudocatalase activity also in the presence
of I- or Br-, but it is blocked by SCN-. The SCN- oxidation
product OSCN- may be reduced back to SCN- by cellular GSH, and
GSSG may be reduced back to GSH by glutathione reductase and
NADPH. In a system reconstituted with pure glutathione reductase,
NADPH, GSH, SCN- and H2O2. GPO-catalysed SCN- oxidation could be
coupled to NADPH oxidation. This system where GPO utilizes SCN-
as the major physiological electron donor may operate efficiently
to scavenge intracellular H2O2." |
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