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

IV. Mature RBC
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occurs in chickens, in which erythroid cells in bone marrow
transport glucose, but circulating RBCs do not ( Johnstone
et al. , 1998 ). With the exception of cats, fetal and neonatal
RBCs studied from humans and animals have higher glucose
transport than RBCs from adults ( Mooney and Young,
1978 ; Widdas, 1955 ).
6 . Adenine, Adenosine, and Inosine
RBC membranes from most animal species have a nucleoside
transporter ( Young, 1983 ). The adenosine transporter
from human and pig RBCs migrates in the band 4.5 region
on SDS-PAGE ( Kwong et al. , 1986 ). Rabbit, pig, and
human RBCs exhibit substantially more adenosine uptake
than those of other species studied ( Van Belle, 1969 ). RBCs
from dogs exhibit more adenosine uptake than cats, goats,
or cattle, and RBCs from horses and most sheep appear to
be nearly impermeable to adenosine. A low percentage of
sheep have RBCs with a high affinity nucleoside transport
system with a broad specificity for both purine and pyrimidine
nucleosides ( Young, 1983 ). RBCs from most horses
have a Na -dependent hypoxanthine transporter. Although
dog RBCs are permeable to adenosine, they are impermeable
to inosine ( Duhm, 1974 ). Dog and cat RBCs exhibit
adenine uptake and incorporation into nucleotides, but values
are much lower than those of human, rabbit, or rodent
RBCs ( Lalanne and Willemot, 1980 ).
E . Metabolism of Adenine Nucleotides
Adenine nucleotides in RBCs contain adenine, ribose, and
one or more phosphate groups. Mature RBCs cannot synthesize
adenine nucleotides de novo but can produce these
compounds utilizing so-called salvage pathways ( Brewer,
1974 ; Eaton and Brewer, 1974 ). AMP can be synthesized
from adenine or from adenosine, both of which may be
supplied to RBCs as they pass through the liver. One molecule
of ATP interacts with one molecule of AMP to generate
two molecules of ADP in the adenylate kinase reaction.
ATP is generated from ADP in glycolysis.
AMP is synthesized from adenine and phosphoribosyl
pyrophosphate (PRPP), utilizing the adenine phosphoribosyltransferase
enzyme. Adenine is converted to ATP at a
slower rate in dog and cat RBCs than in those of humans,
rodents, or rabbits ( Lelanne and Willemot, 1980 ). AMP
degradation to inosine monophosphate and ammonia is
catalyzed by AMP-deaminase. The activity of this enzyme
is generally lower in mammalian RBCs compared to
nucleated RBCs from birds, reptiles, amphibians, and fish
( Kruckeberg and Chilson, 1973 ).
Adenosine can be phosphorylated to AMP using ATP
in the adenosine kinase reaction. A competing reaction,
adenosine deaminase, converts adenosine to inosine, which
cannot be incorporated into AMP. The uptake or deamination
of adenosine varies considerably by species ( Van
Belle, 1969 ). Not only are dog, cat, and cattle RBCs poorly
permeable to inosine, but inosine produced by adenosine
deamination cannot be readily used for energy in these
species because of low purine nucleoside phosphorylase
activity, which converts inosine to ribose 1-phosphate and
hypoxanthine ( Duhm, 1974 ).
NAD and NADP can apparently be synthesized from
nicotinate by way of a series of reactions in RBCs ( Eaton
and Brewer, 1974 ). In addition to ATP, PRPP and NH 3 , or
glutamine, are required. Comparative studies of the synthesis
of these compounds in domestic animals have not been
reported.
F . Carbohydrate Metabolism
RBCs require energy in the form of ATP for maintenance
of shape and deformability, phosphorylation of membrane
phospholipids and proteins, active membrane transport of
various molecules, partial synthesis of purine and pyrimidine
nucleotides, and synthesis of GSH ( Nakao, 1974 ;
Reimann et al. , 1981 ). Reducing potential in the form of
NADH and NADPH is needed to counteract oxidative
processes. Although substrates such as ribose, fructose,
mannose, galactose, dihydroxyacetone, glyceraldehyde,
adenosine, and inosine may be metabolized to some extent,
depending on the species, glucose is the primary substrate
for energy needs of RBCs from all species except the pig
( Agar and Board, 1983 ; Kim, 1983 ).
RBCs from adult pigs utilize glucose at lower rates than
other species ( Magnani et al. , 1983 ) because they lack a
functional glucose transporter ( Craik et al. , 1988 ; Zeidler
and Kim, 1982 ). Inosine appears to be the major substrate
for pig RBCs; its production by the liver is sufficient to meet
their energy requirements ( Young et al. , 1985 ; Zeidler et al. ,
1985 ). Inosine can be used because nucleoside phosphorylase
converts it to ribose 1-phosphate and hypoxanthine
(Sandberg et al. , 1955 ). Ribose 1-phosphate is converted to
ribose 5-phosphate, an intermediate of the pentose phosphate
pathway (PPP), by phosphoribomutase ( Brewer, 1974 ).
Glucose utilization rates of RBCs vary by species ( Table
7-1 ). Factors such as pH, phosphate concentration, temperature,
and leukocyte and platelet contamination of RBC incubations
can have substantial effects on glucose utilization
rates measured in vitro . Consequently, species comparisons
of values determined in different laboratories may be misleading.
Harvey and Kaneko (1976a) approximated physiological
conditions in vitro and measured mean glycolytic
rates of 0.64, 0.94, 1.33, and 1.48 μmoles/hour/ml RBC for
the horse, cat, dog, and human, respectively. Once glucose
enters the cell, it is phosphorylated to glucose 6-phosphate
(G6P) utilizing the hexokinase (HK) enzyme. The G6P is
then metabolized through either the Embden-Meyerhof
pathway (EMP) or the pentose phosphate pathway (PPP) as
shown ( Fig. 7-5 ).