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692 Part VI: The Erythrocyte Chapter 47: Erythrocyte Enzyme Disorders 693

Glucose H 2 O 2 H 2 O
Glutatione Peroxidase
ATP GSH GSSG
Hexokinase Glutathione Reductase
+
NADP NADPH
ADP
Glucose 6-P
Hexose
Glucosephosphate isomerase monophosphate CO 2
pathway
Fructose 6-P
ATP
Phosphofructokinase
ADP
Fructose 1,6-DIP

Aldolase
T riosephosphate
Glyceraldehyde 3-P Dihydroxyacetone P
NAD Isomerase
Hemoglobin P i
Methemoglobin Glyceraldehydephosphate
reductase dehydrogenase
Methemoglobin
NADH
1,-3 DIP glycerate
ADP Diphosphoglyceromutase
Phosphoglycerate 2,3-BiP glycerate
kinase
Diphosphoglycerate
ATP phosphatase
3-P-glycerate

Phosphoglyceromutase

2-P-glycerate

Enolase

Phosphoenol-pyruvate
ADP
Pyruvate kinase
ATP
Pyruvate
NADH
Lactate dehydrogenase
NAD
Lactate

Figure 47–2. Glucose metabolism of the erythrocyte. The details of the hexose monophosphate pathway are shown in Fig. 47–3.

exquisitely sensitive to pH: a rise in pH causes a rise in 2,3-BPG levels, in reducing methemoglobin to hemoglobin, the end product of glucose
whereas acidosis results in 2,3-BPG depletion. It may be that the ratio of metabolism is pyruvate. If NADH is not reoxidized by methemoglobin,
oxyhemoglobin to deoxyhemoglobin also influences 2,3-BPG synthesis however, pyruvate is reduced in the lactate dehydrogenase (LDH) step,
by virtue of the fact that only deoxyhemoglobin binds this compound, forming lactate as the final end product of glucose metabolism. The lac-
thus affecting the concentration of free 2,3-BPG that is available for feed- tate or pyruvate formed is transported from the red cell and is metab-
back inhibition of the enzymes that lead to its formation. However, the olized elsewhere in the body. Thus, the erythrocyte has a flexible EMP
available evidence suggests that the pH is the primary controlling factor. that can adjust the amount of ADP phosphorylated per mole of glucose
Metabolism of glucose by way of the EMP may also yield reducing according to the requirement of the cell.
energy in the form of the reduced form of NAD (NADH). The reduction The regulation of red cell glycolytic metabolism is very complex.
of NAD to NADH occurs in the GAPDH step. If NADH is reoxidized Products of some reactions may stimulate others. For example, the PK
+

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