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chapter 15
Carbohydrate Metabolism II: Gluconeogenesis, Glycogen Synthesis and Breakdown, and Alternative Pathways
Phagocytosis and the Pentose Phosphate Pathway
The pentose phosphate pathway is crucial to the survival of
erythrocytes because of its ability to provide NADPH for
reduction of toxic, spontaneously produced oxidants. In
phagocytic cells, the pentose phosphate pathway generates
oxidizing agents that participate in the killing of bacteria
and abnormal cells engulfed by the phagocytes.
In humans, the two principal types of phagocytic cells
are polymorphonuclear leukocytes (PMN leukocytes, neu-
trophilic granulocytes, or neutrophils) and those of the
mononuclear phagocyte system (MPS). The MPS, also
called the reticuloendothelial system (RES), includes
peripheral blood monocytes, tissue macrophages (inflam-
matory macrophages, Kupffer cells, histiocytes, alveolar
macrophages, and others), and their precursor cells in bone
marrow. These cells are normally active against bacteria,
foreign cells, and particulate matter such as asbestos fibers,
silicon dioxide particles, and coal dust. Neutrophils are
particularly important in protecting against acute bacte-
rial infections.
Phagocytosis is the process whereby phagocytes move
toward, engulf, and digest foreign material. Attraction
of phagocytes is mediated by chemotactic factors such
as C5a (derived from complement; Chapter 35), bacte-
rial endotoxins (Chapter 16), kallikrein, fibrinopeptide B
(Chapter 36), and leukotrienes (Chapter 18). The contrac-
tile systems, which enable the phagocytes to extend pseu-
dopods for movement and engulfment, are discussed in
Chapter 21.
Following invagination of the phagocyte plasma mem-
brane to surround a particle or microorganism, the edges
of the vesicle fuse to form a cytoplasmic vacuole or phago-
some. Lysosomes or, in the case of neutrophils, primary
(azurophilic) and secondary (specific) cytoplasmic gran-
ules then fuse with the phagosome to form a phagolyso-
some. The contents of these organelles are emptied into the
phagolysosome and contribute to the killing and digestion
of the phagocytized material. Most of the cytocidal activ-
ity of phagocytes is due to the generation of highly reactive
forms of oxygen from NADPH and molecular oxygen. In
the absence of stimulation, the activity of the pentose phos-
phate pathway in phagocytes is low. Within 15-60 sec-
onds after a phagocyte is alerted to the presence of foreign
material, there is a 20- to 200-fold increase in oxygen
consumption, the
respiratory burst,
and a 10- to 20-fold
increase in pentose phosphate pathway activity. These
changes are initiated by interactions between receptors on
the plasma membranes of the phagocytes and a wide vari-
ety of stimuli, including immune complexes, chemotactic
peptides, and opsonized bacteria. Phosphatidyl inositol,
prostaglandins, cAMP, and changes in cytosolic [Ca2+]
appear to be essential for this process. The magnitude of
the response is related to the type of inducing signal and
to the cell’s ability to respond to that signal.
Most of the oxygen consumed in the respiratory burst is
converted to superoxide anion (O
2
)■
This highly reactive
oxygen radical anion is one of the substances responsible
for oxidative damage to red cells. In erythrocytes, superox-
ide anion occurs spontaneously, as a byproduct of heme-
oxygen interactions. In the phagocyte, it is formed by the
NADPH oxidase on the outer surface of the plasma mem-
brane. The reaction involves transfer of a single electron
from NADPH to oxygen:
NADPH
oxidase
,
.
NADPH + 202 ----- ►
2
O
2
+ NADP+ + H+
NADPH oxidase is thought to be an electron transport
chain that includes a flavoprotein, and cytochrome bsss
(Chapter 14).
The
NADPH
is
provided
by
the
pentose
phos-
phate pathway. Glucose-6-phosphate dehydrogenase and
6-phosphogluconate
dehydrogenase
are
inhibited
by
NADPH at the concentrations found in the unstimu-
lated phagocyte. As NADPH is consumed by NADPH
oxidase,
inhibition
of the
pentose
phosphate
path-
way is reduced, increasing the rate of formation of
NADPH.
Phagosomes contain some of the activated NADPH
oxidase from the cell surface, and superoxide is produced
within the phagolysosome. Although superoxide is only
bactericidal against organisms that lack superoxide dis-
mutase, it is the substrate for formation of a number of
cytotoxic compounds.
Formation of phagolysosomes is accompanied by a de-
crease in the pH of their interior, which causes spontaneous
conversion of superoxide anion to oxygen and hydrogen
peroxide:
2 0 2
- + 2H+ ^ H20 2 + 0 2
Hydrogen peroxide is a substrate for myeloperoxidase,
a multisubunit heme protein of M.W. ~ 150,000, present
in primary neutrophilic granules. The active prosthetic
groups are two hemes covalently attached to the apoen-
zyme. This enzyme catalyzes many kinds of oxidation
reactions, but oxidation of halide ions to hypohalite ions
appears to be the most important. Hypochlorite ion is the
principal compound formed, although Br“ , I , and SCN-
(a pseudohalide) can also serve as substrates. The reaction
catalyzed is
c r +
H20 2
-+
C1CT
+
h 2o
The hypochlorite ion has an antimicrobial potency 50-fold
greater than that of hydrogen peroxide. It oxidizes a variety
