Mammals have evolved a special cellular mechanism for killing invading
microbes, which is called the phagocytosis. Neutrophils are the first
phagocytosing cells that migrate into the site of infection. In these
cells, hypochlorite (HOCl) and other hypohalites, generated in the
myeloperoxidase (MPO)-hydrogen peroxide (H₂O₂)-halide
system is primarily responsible for oxidative killing. Here, we present
a method for assessing these oxidative mechanisms in an in vitro
cell-free system in a kinetical real-time-based manner by utilizing a
bioluminescent bacterial probe called Escherichia coli-lux. The E.
coli-lux method provides a practical tool for assessing the effects of
various elementary factors in the MPO-H₂O₂-halide
system. Due to the reported versatile intracellular pH and halide
concentration during the formation of the phagolysosome and respiratory
burst, the antimicrobial activity of the MPO-H₂O₂-halide
system undergoes extensive alterations. Here, we show that at a
physiological pH or lower, the antimicrobial activity of MPO is high,
and the system effectively enhances the H₂O₂-dependent
oxidative killing of E. coli by chlorination. The HOCl formed in this
reaction is a prominent microbe killer. During the respiratory burst,
there is a shift to a more alkaline environment. At pH 7.8, the
chlorinating activity of MPO was shown to be absent, and the activity of
the HOCl decreased. At this higher pH, the activity of H₂O₂
is enhanced and high enough to kill E. coli without the participation
of MPO, and the lowered chloride concentration seemed still to enhance
the H₂O₂-dependent killing capacity.