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CHAPTER 28
Hemoglobin
1. The exchange of chloride and bicarbonate across the
erythrocyte membrane (the
chloride shift).
2. The role of H+, and therefore CO
2
and carbonic
anhydrase, in unloading oxygen from oxyhemoglobin.
3. The intermediary role of plasma as a carrier of CO
2
(as HCOJ).
4. The occurrence of oxygen in solution in plasma, in
addition to that bound to hemoglobin; this satisfies
the law of mass action (since oxygen binding to
hemoglobin is an equilibrium reaction) and also
moves oxygen from erythrocytes into tissues. The
concentration of dissolved oxygen can be increased
by breathing oxygen at pressures greater than
atmospheric pressure. Hyperbaric oxygen therapy
has been used to treat carbon monoxide poisoning,
gas gangrene, decompression sickness, and other
disorders in which hemoglobin cannot carry
adequate oxygen to one or more tissues.
5. The occurrence of CO
2
chemically combined with
hemoglobin, as carbamino compounds.
Nitric Oxide (NO) Binding to Hemoglobin
NO binds to hemoglobin at two sites. One is involved
in scavenging NO through its binding to the ferrous iron
of heme. The affinity of NO at the Fe2+-heme site is about
8000 times greater than oxygen affinity at the same site.
The binding of NO at the second site is a reversible process
and occurs at the /193 cysteine residue with the formation
of S-nitrosothiol. This interaction of NO with /193 cys-
teine is linked to the binding of oxygen to hemoglobin
in the lungs and also to release of NO and 0
2
into the
tissues. It is thought that NO reduces regional vascular
resistance. In the blood, NO is also transported by bind-
ing to plasma albumin; the exact physiological role of
the transport of NO by hemoglobin or albumin is not
understood.
Erythropoietin
Erythropoietin is a glycoprotein hormone that regulates
red blood cell production in a feedback loop manner be-
tween kidney and bone marrow based on oxygen tension.
It consists of 165 amino acids and has a molecular weight
of 30,000-34,000; approximately 30% is accounted for
by covalently linked carbohydrate. Erythropoietin is pro-
duced by the fetal liver and shortly after birth production
switches from the liver to the kidney. In the fetus, erythro-
poietin functions in a paracrine-endocrine fashion because
liver is the site of erythropoietin synthesis as well as ery-
thropoiesis. The mechanism of this developmental switch
is unclear. In the liver, erythropoietin synthesis occurs in
the Ito cells and in a subset of hepatocytes. In the kidney,
erythropoietin is synthesized in the peritubular interstitial
fibroblast-like cells. Hypoxia is the primary physiologi-
cal stimulus for the dramatic rise of erythropoietin levels,
which can rise up to
1000
-fold through an exponential in-
crease in the number of erythropoietin-producing cells as
well as in the rate of synthesis. Hypoxic states can occur
due to loss of red blood cells, decreased ambient oxy-
gen tension, presence of abnormal hemoglobin with in-
creased oxygen affinity (discussed later), and other causes
that limit oxygen delivery to the tissues (e.g., chronic ob-
structive lung disease). The expression of the erythropoi-
etin gene is affected by a number of other physiological
and pharmacological agents in addition to hypoxic states.
Transition metal ions, Co2+, Ni2+, Mn2+, and iron chela-
tor desferrioxamine can stimulate erythropoietin gene ex-
pression. Carbon monoxide, nitric oxide, inflammatory
cytokine tumor necrosis factor-a, and interleukin
-1
can
prevent expression of the gene. The latter is implicated
as one of the factors causing anemia in chronic disease
(anemia of chronic disease).
Erythropoietin is the primary
regulator of erythropoiesis and red blood cell mass. Ery-
thropoietin action is mediated by its binding to plasma
cell membrane receptors of erythroid progenitors and pre-
cursors. The sensitivity of erythroid progenitors to ery-
thropoietin appears to be under developmental regulation.
Colony-forming unit erythroid (CFU-E) and proerythrob-
lasts have a peak amount of erythropoietin receptors. In
peripheral reticulocytes, the receptors are undetectable.
Erythropoietin’s function is mediated through its receptor
and includes proliferation, survival, and terminal differen-
tiation of CFU-E cells.
The mechanism of oxygen sensing and signal trans-
duction that leads to activation of the erythropoietin gene
and erythropoietin synthesis in renal cells involves the fol-
lowing steps. The first step in the hypoxia-induced tran-
scription of the erythropoietin gene consists of production
of oxygen free radicals (
0 2
) by cytochrome b-like flavo-
heme NADPH oxidase in proportion to oxygen tension.
Superoxide in the presence of iron is converted to other
reactive oxygen species (e.g., OH‘). During normal oxy-
gen tension, superoxide and other reactive oxygen species
oxidize hypoxia-inducible factor-a (HIF-a) and cause its
destruction in the ubiquitin-proteosome pathway. How-
ever, at low oxygen tension, HIF-a is preserved and forms
a heterodimer with constitutively expressed HIF-/J. The
dimer HIF-a/HIF-/i is translocated into the nucleus where
it interacts with hypoxia response elements to activate gene
expression.
Recombinant human erythropoietin has been used in the
correction of anemia of chronic renal failure. It has also
been used in other disorders of anemia such as anemia
