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Mazumdar
Characterization of sheep lacrimal-gland peroxidase and its major physiological electron donor.Mazumdar A, Chatterjee R, Adak S, Ghosh A, Mondal C, Banerjee RK. Biochem J. 1996 Mar 1;314 ( Pt 2):413-9.
"A soluble sheep
lacrimal-gland peroxidase was purified to apparent homogeneity.
It had a native molecular mass of 75 kDa with a subunit molecular
mass of 82 kDa and an isoelectric point of 6.5. Western blotting
showed that it shares some of the enzyme antigenic determinants
in common with other soluble peroxidases. The enzyme exhibits a
Soret peak at 410 nm which is shifted to 431 nm by 5 equiv. of
H2O2 due to the formation of compound II. The latter is, however,
unstable and gradually returns to the native state. The enzyme
forms complexes with CN- and N3- and is reduced by dithionite
showing a characteristic reduced peroxidase spectrum. Although
the enzyme oxidizes I-, SCN- and Br- optimally at pH 5.5., 5.25
and 5.0 respectively, at physiological pH, it oxidizes I- and
SCN- only. Since extracellular SCN- concentration is much higher
than I-, SCN- may act as the major electron donor to the enzyme.
The second-order rate constants for the reaction of the enzyme
with H2O2 (k+1) and of compound I with SCN- (k+2) were 4 X 10(7)
M-1 X s-1 and 8.1 X 10(5) M-1 X s-1 respectively. A plot of log
Vmax against pH yields a sigmoidal curve consistent with a single
ionizable group on the enzyme with a pK(a) value of 5.75,
controlling thiocyanate oxidation. In a coupled system with the
peroxidase, H2O2, SCN-, GSH, NADPH and glutathione reductase,
peroxidase-catalysed SCN- oxidation by H2O2 could be coupled to
NADPH consumption. The system is proposed to operate in vivo for
the efficient elimination of endogenous H2O2." |
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