(PROTEIN) WATER MOLECULE AMINO GROUP

Figure 9–17
Formation of free radicals and highly
reactive oxygen compounds: superoxide
(O 2 – or O-O ˜ ), hydroxyl radical (OH •˜ )
and hypochlorus acid (HOCl ˜ ) by respiratory
burst. ➤ These compounds
readily donate their electrons to membrane
lipids and proteins.
2O2 (+ NADPH)
oxygen
NADP oxidase 2O2 - . (+ NADP + + H +)
superoxide
Fe
H2O (+ 2 O )
2
hydrogen
peroxide
+2 (or Cu +1 )
OH .
hydroxyl
OCl
radical
-
* *
myeloperoxidase
*spontaneous,
no enzyme used
Ocular Immunochemistry • 265
superoxide
dismutase
Cl -
HOCl
hypochlorous
acid (biological
"chlorox")
1 O2 (+ H 2 O + Cl - )
singlet
oxygen
The antigens become surrounded by the leucocyte membranes
forming a phagosome (Greek: fagosoma- phagosoma—literally: a body
for eating). The phagosome itself is internalized and fused with lysosomes
(Greek lusosoma: literally: a body for breaking) where the antigenic
particles or even whole bacteria or viruses are destroyed. Note that
the fused bodies are this point are called phagolysosomes.
The destruction of antigenic organisms begins in the phagosome
with the formation of highly active forms of oxygen. This process is initiated
with a respiratory burst in which leucocytes take up large quantities
of oxygen (O 2) to form superoxide radicals (O 2 –• ). This is shown in
Figure 9–17. Superoxide radicals have unpaired electrons that are highly
reactive and unstable. The radicals are rapidly converted to other compounds
(hydrogen peroxide, hydroxyl radicals, and singlet oxygen) that
are themselves highly reactive with tissue proteins and membrane lipids.
Some examples are shown in Figure 9–18.
In the initial reaction of Figure 9–17, superoxide is dismuted or disproportioned
(one molecule is reduced while the other is oxidized) to
hydrogen peroxide by the enzyme superoxide dismutase. The need for
the enzyme, in spite of the highly reactive species, has been shown to be
necessary due to the relatively low concentrations of superoxide that are
present (Babior, 2000). Some of the hydrogen peroxide is also converted
to hypochlorous acid (commercially known as “chlorox”), a very highly
reactive and destructive chemical for nearly any living organism (see
Figure 9–17). All these oxidative species attack the membranes of the
organism (see Figure 9–18) within the phagosome and then spill out into
the surrounding cellular environment to cause collateral damage to
nearby cells or even the phagocytes themselves eventually. The enzyme
that catalyzes the formation of hypochlorous acid, myeloperoxidase, has
a heme group at its active site that has a green color. The green color is
the cause of the green hue that is associated with pus formation. Pus formation
itself is the buildup of cellular debris from the oxidative destruction
(described here) and enzymatic onslaught (described below) that has
taken place.