HALOGENATED SPECIES - Oxidation of Amino Acids, Peptides, and Proteins: Kinetics and Mechanism - page 78

HALOGENATED SPECIES

Myeloperoxidase (MPO), a heme enzyme, when reacted with H 2 O 2 and halide
(Cl − , Br − ) or pseudohalide (SCN − ) ions produces hypochlorous acid (HOCl),
hypobromous acid (HOBr), and hypothiocyanous acid (HOSCN) (Fig. 3.1) [1].
Initially, the reaction of H 2 O 2 with the resting Fe(III) form of the enzyme
produces compound I, which undergoes either two successive one-electron
reductions, via compound II, to result in a radical (the peroxidase cycle), or a
two-electron reduction with Cl − , Br − , and SCN − ions to yield oxidants. Under
the physiological concentrations of Cl − (100-150 mM), Br − (10-100 μM), and
SCN − (10-100 μM), a combination of MPO and H 2 O 2 can produce HOCl,
HOBr, and HOSCN [1-6]. The enzyme may also undergo a one-electron
reduction with O •− to form compound III, which contributes to the superoxide
dismutase (SOD) mimetic activity of MPO (Fig. 3.1). The generation of HOCl
may also be accomplished by NADPH oxidase [7]. The hypohalous acid and
HOSCN are among the species responsible for the antibactericidal activity of
neutrophils [8]. However, the possibility of damaging the tissues by such oxi-
dants also exists if they are produced in excessive levels at the inappropriate
place or time [9, 10].
The oxidant species exist as acidic and anionic forms [11-13]:

HOCl + H OCl +

−

p a K 7 . (3.1)