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

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Chapter | 7 The Erythrocyte: Physiology, Metabolism, and Biochemical Disorders
FIGURE 7-5 Metabolic pathways of the mature erythrocyte. Abbreviations: HK, hexokinase; GPI, glucose
phosphate isomerase; PFK, phosphofructokinase; TPI, triosephosphate isomerase; GAPD, glyceraldehyde-
3-phosphate dehydrogenase; PGK, phosphoglycerate kinase; MPGM, monophosphoglycerate mutase; DPGM,
diphosphoglycerate mutase; PK, pyruvate kinase; G6PD, glucose-6-phosphate dehydrogenase; 6PGD,
6-phosphogluconate dehydrogenase; LDH, lactate dehydrogenase; LMB, leukomethylene blue; MB, methylene
blue; GR, glutathione reductase; GPx, glutathione peroxidase; TK, transketolase; TA, transaldolase; GSSG, oxidized
glutathione; G6P, glucose 6-phosphate; F6P, fructose 6-phosphate; FDP, fructose 1,6-diphosphate; DHAP,
dihydroxyacetone phosphate; GAP, glyceraldehyde 3-phosphate; 1,3-DPG, 1,3-diphosphoglycerate; 2,3-DPG,
2,3-diphosphoglycerate; 3PG, 3-phosphoglycerate; 2PG, 2-phosphoglycerate; PEP, phosphoenolpyruvate; ADP,
adenosine diphosphate; ATP, adenosine triphosphate; NAD, nicotinamide adenine dinucleotide; NADH, reduced
nicotinamide adenine dinucleotide; NADP, nicotinamide adenine dinucleotide phosphate; NADPH, reduced nicotinamide
adenine dinucleotide phosphate; NADPH-D, reduced nicotinamide adenine dinucleotide phosphate
diaphorase; GSH, reduced glutathione; P i , inorganic phosphate; SOD, superoxide dismutase.
G . Embden-Meyerhof Pathway
Most of the species variations in glucose utilization appear
to result from variations in EMP metabolism, with PPP
metabolism being relatively constant when not stimulated
by oxidants ( Harvey and Kaneko, 1976a ). In addition to the
phosphorylation of glucose, one molecule of ATP is used
to phosphorylate fructose 6-phosphate, and one molecule
of ATP is generated for each three-carbon molecule metabolized
through the phosphoglycerate kinase (PGK) and
pyruvate kinase (PK) reactions ( Fig. 7-5 ). Consequently a
net of two molecules of ATP is produced for each molecule
of glucose metabolized to two molecules of lactate in the
EMP. Because mature RBCs lack mitochondria, the EMP
is the only source of ATP production in these cells.
In human RBCs, reactions catalyzed by HK, phosphofructokinase
(PFK), and PK appear to be rate-limiting steps
in glycolysis, because these reactions are far displaced
from equilibrium ( Minakami and Yoshikawa, 1966 ). Under
physiological steady-state conditions, the PFK enzyme
reaction controls glycolysis through the EMP ( Rapoport,
1968 ). Its activity is influenced by a variety of effectors,
with ATP being the most important inhibitor and AMP and