Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

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Chapter | 7 The Erythrocyte: Physiology, Metabolism, and Biochemical Disorders
goats and some sheep with HbC, a Hb type prominent in the
neonate and induced in anemic and hypoxemic adults (see
Section III.C.4). HbC binds twice as much CO 2 as does HbA
in goats ( Winslow et al. , 1989 ), and CO 2 decreases the oxygen
affinity of HbC in goats and sheep more than it does the
oxygen affinity of normal adult Hbs ( Huisman and Kitchens,
1968 ). The carbamino formation also produces H , which
further lowers Hb oxygen affinity (see Section I.B).
The processes discussed previously are reversed at the
lungs ( Fig. 7-7b ). O 2 enters RBCs and the resultant binding
of O 2 to Hb promotes the release of Hb-bound CO 2 and H
that were buffered by DeoxyHb. The H binds to HCO 3
to form H 2 CO 3 , which dissociates to form CO 2 and water.
The lungs expel the CO 2 , and the resultant decreases in CO 2
and H concentrations increase the Hb oxygen affinity of
RBCs passing through pulmonary capillaries. Because CO 2
and H effects are interrelated and additive, the combined
change in Hb oxygen affinity has been called the “ classical
Bohr effect, ” whereas the change in Hb oxygen affinity
produced only by H is called the “Bohr effect ” (Zhang
et al. , 2003 ).
Increased temperature decreases the oxygen affinity
of Hb, a response that appears physiologically appropriate
considering that increased heat production accompanies
increased oxygen consumption in tissues ( Benesch et al. ,
1975 ). Body temperature increases during prolonged strenuous
exercise ( Hsia, 1998 ), with muscle temperature increasing
more than pulmonary arterial temperature ( Fenger
et al. , 2000 ). Increased muscle temperature, increased CO 2
production, and increased H production (from lactic acidosis
and transport of CO 2 ) decrease the Hb oxygen affinity
and promote the release of O 2 to muscles during prolonged
heavy exercise. Oxygen extraction from the blood of horses
increases from 20% at rest to 80% at maximal exercise
(Fenger et al. , 2000 ). Increased cardiac output and higher
blood Hb concentrations (from splenic contraction) are
equally important for maximal oxygen delivery to muscles
in exercising horses ( Fenger et al. , 2000 ).
Breed differences in P 50 have been described in dogs
(Clerbaux et al. , 1993 ), horses, and goats ( Haskins and
Rezende, 2006 ). The P 50 for greyhound RBCs in whole
blood is lower than that for mongrel dogs, yet the groups
have similar 2,3DPG concentrations ( Sullivan et al. , 1994 ).
The cause of this difference remains to be determined, but it
is suggested that the higher hematocrit found in greyhound
dogs may represent a compensatory response to a higher
oxygen affinity of Hb in this species.
3 . Effects of 2,3DPG
In RBCs from most mammalian species, 2,3DPG decreases
the oxygen affinity of Hb, resulting in an increase in P 50
( Bunn et al. , 1974 ). In contrast, poikilothermic animals
generally use ATP or GTP (primarily fish), and birds typically
use inositol pentaphosphate, to decrease the oxygen
affinity of Hb ( Barvitenko et al. , 2005 ; Val, 2000 ). 2,3DPG
reacts with Hb in a ratio of one molecule per Hb tetramer.
Negatively charged groups of 2,3DPG bind to specific
positively charged groups in the N-terminal region of Hb
beta chains. There is a marked preference for binding to
DeoxyHb as compared to OxyHb because of differences
in the conformation of the molecules. The interaction of
2,3DPG with Hb is represented as follows:
HbDPG O ↔ HbO DPG
+ 2 2
When 2,3DPG is increased the reaction is displaced to
the left, and when pO 2 is increased the reaction is displaced
to the right. ATP has a similar effect on Hb oxygen affinity
but is generally much less important than 2,3DPG in mammals,
because it usually occurs in lower concentration and
is complexed with Mg 2 (Bunn, 1971 ).
When the oxygen affinity of Hb is studied in hemolysates
dialyzed to remove 2,3DPG and ATP, the “ stripped ”
Hb from species with low 2,3DPG RBCs has considerably
lower oxygen affinities than stripped Hb from species with
high 2,3DPG RBCs ( Bunn, 1971 ; Bunn et al. , 1974 ). The
Hb oxygen affinity of most mammalian Hbs is decreased in
the presence of chloride ions and, to a lesser extent, phosphate
ions ( Bårdgard et al. , 1997 ; Fronticelli, 1990 ; Gustin
et al. , 1994 ; Haskins and Rezende, 2006 ). Consequently,
the oxygen affinities of stripped Hbs can vary depending on
the buffer system used for these assays. The oxygen affinity
of stripped cattle Hb is lower in buffers containing NaCl
than in buffers without NaCl. Although the addition of
2,3DPG to stripped cattle Hb causes a prominent increase
in P 50 in the absence of Cl , it causes only a tiny additional
increase in P 50 in the presence of physiological concentrations
of Cl (Marta et al. , 1998 ). In contrast, the addition of
2,3DPG to stripped dog Hb results in a prominent increase
in P 50 , even in the presence of physiological concentrations
of Cl (Bårdgard et al. , 1997 ). Because stripped Hbs from
species such as dogs with high 2,3DPG RBCs naturally
have high oxygen affinities, 2,3DPG is needed within RBCs
of these species to maintain Hb oxygen affinity within a
physiologically useful range ( Benesch et al. , 1975 ).
When blood from many mammalian species is studied,
an inverse linear correlation is recognized between the log
P 50 of whole blood and the log of body weight ( Scott et al. ,
1977 ); however, oxygen affinity of stripped Hb from
various mammals does not correlate with body weight
(Nakashima et al. , 1985 ). The maintenance of 2,3DPG in
mammals is energetically expensive because the ATPgenerating
PGK reaction is bypassed. 2,3DPG apparently
allows for an evolutionary adaption of blood Hb oxygen
affinity to metabolic rate.
Mammals with naturally high 2,3DPG in RBCs may
alter their Hb oxygen affinity to meet metabolic needs. The
significance of (and in some cases the appropriateness of)
alterations in 2,3DPG in disease states is not always clear.