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698 Part VI: The Erythrocyte Chapter 47: Erythrocyte Enzyme Disorders 699

rare benign condition without hematologic consequences. It has been phosphotransferase properties, suggesting an additional role of this
247
suggested, though, that it may act as a complicating factor in age and enzyme in nucleotide metabolism. The crystal structure of mouse
234
233
diseases such as diabetes and oxidative stress-related conditions. P5′N1 has been published, providing a framework for understanding
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Deficiencies of peroxiredoxin 2, the major red cell peroxiredoxin, the kinetics of both nucleotidase and phosphotransferase activities of
have not been reported in humans. Peroxiredoxin 2 null mice, how- human P5′N1. 248
236
ever, develop severe hemolytic anemia with Heinz body formation, P5′N1 is encoded by the NT5C3A gene on chromosome 7p14.3.
237
and show signs of abnormal erythropoiesis. Based on mouse models, It comprises 11 exons, and produces three distinct mRNAs by alterna-
it has been postulated that glutathione peroxidase, catalase, and perox- tive splicing. Red cell P5′N1 is translated from the mRNA lacking exons
iredoxin each have distinct roles in the scavenging of hydrogen peroxide 2 and R. 249,250 It is a 286-amino-acids-long monomeric protein with an
and antioxidative defense. 238–240 apparent molecular weight of 34 kDa. 249
A second P5′N is present in red blood cells, the activity of which is
Nucleotide Metabolism of the Erythrocyte generally measured together with that of P5′N1. This enzyme (P5′N2)
Approximately 97 percent of the total nucleotide content of the mature is encoded by a separate gene, shows little homology to P5′N1, and is
red blood cell consists of interconvertible adenosine phosphates (Chap. not strictly pyrimidine-specific. It is unable to compensate for deficient
31). Less than 3 percent of total nucleotides are guanosine phosphates. function of P5′N1. 247,251 P5′N1 deficiency is one of the most common
ATP is the most abundant adenosine phosphate (comprising roughly 84 causes of hereditary nonspherocytic hemolytic anemia.
percent of total adenosine ribonucleotides), whereas ADP (14 percent) Human red blood cells have been found to express low NAD syn-
and adenosine monophosphate (AMP, 1 percent) are present in con- thesis activity, mediated by nicotinamide mononucleotide adenylyl-
siderably lower amounts. The interconversion of adenine nucleotides is transferase. It appears that the predominant isozyme in red blood
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modulated by adenylate kinase (AK): cells is nicotinamide mononucleotide adenylyltransferase-3. Human
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dysfunction of this enzyme has not been reported but the deficiency in
Mg + ATP + AMP → Mg + ADP + ADP
2+
2+
mice blocks glycolysis at the GAPDH step and is associated with hemo-
By catalyzing the reversible phosphoryl transfer among ATP, ADP, lytic anemia. 254
and AMP, AK contributes to cellular adenine nucleotide homeostasis.
Red cells contain the AK1 isozyme, which is present in the cytosol as GENETICS
a monomeric enzyme composed of 194 amino acids. The recombinant
purified enzyme has a molecular mass of approximately 22 kDa. The The great majority of red cell enzyme deficiencies that cause hemolytic
241
AK1 gene is localized on chromosome 9q34.1, and consists of 7 exons of anemia are hereditary. Most are inherited as autosomal recessive disorders,
2
which exon 1 is noncoding. AK activity depends on the presence of Mg . but G6PD deficiency and PGK deficiency are X-chromosome-linked. The
ATP serves as a cofactor in a number of reactions, such as the vast majority of the genes encoding for the red cell enzymes have been
phosphorylation steps mediated by HK and PFK in glycolysis, the syn- identified, making the molecular diagnosis of hereditary red cell enzyme
thesis of GSH, and ATPase-dependent function of membrane pumps. deficiency possible. Occasionally, acquired forms of enzyme deficiencies,
Therefore, ATP is crucial in maintenance of the red cell’s structure and particularly PK deficiency, have been encountered, usually in patients
function. Because the mature red cell is unable to synthesize adenosine with hematologic neoplasia. 255–259
phosphates from precursor molecules, it relies on salvage pathways to
preserve adenosine ribonucleotides. This is of particular importance for ENZYME DEFICIENCIES—BIOCHEMICAL
AMP because this adenosine ribonucleotide is at risk of being lost from
the adenine pool by dephosphorylation to adenosine and, subsequent, GENETICS AND MOLECULAR BIOLOGY
irreversible deamination to inosine by the enzyme adenosine deami- Table 47–2 lists the erythrocyte enzyme deficiencies shown to cause
nase (ADA). ADA thus plays a regulatory role in the concentration hemolytic disease. Other red cell enzyme deficiencies (Table 47–3) do
of adenosine ribonucleotides in the red cell. The gene encoding ADA not appear to cause hemolysis or other functional abnormality of the
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(ADA) is located on chromosome 20q13.12. It comprises 12 exons that erythrocyte. For example, acatalasemia, the state in which there is
encode a 363-amino-acid protein. a virtually total absence of red cell catalase, is devoid of hematologic
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Deficiency of AK and hyperactivity of ADA are both very rare manifestations. Similarly, red cells without acetylcholinesterase sur-
causes of hereditary nonspherocytic hemolytic anemia. vive normally in most cases. 260
Additional enzymes of purine metabolism are also present in the The lack of clinical manifestations is not always clearcut. In some
red cell. Although disorders of these enzymes are associated with a instances, hemolytic anemia is reported in some individuals with a
number of metabolic diseases, their function does not appear to be rel- given deficiency but not in others. For example, most subjects with
evant for the red blood cell as these disorders are without hematologic LDH deficiency have no anemia, but cases with hemolysis have been
136
consequences. 242 reported. Such ambiguity could result from differences in environ-
Pyrimidine-5′-Nucleotidase-1 Pyrimidine ribonucleotides are mental and genetic factors or from bias of ascertainment. Erythro-
found only in trace amounts in the mature red blood cell. They are lost cyte enzyme assays are usually performed on patients with hemolytic
from the cell together with the degradation of ribosomes and RNA anemia. Thus, a benign enzyme defect may be thought, mistakenly, to
during reticulocyte maturation. Pyrimidine-5′-nucleotidase-1 (P5′N1) cause hemolysis because it is found in a patient with hemolytic ane-
mediates this loss by catalyzing the dephosphorylation of pyrimidine mia caused by an unrelated and undetected defect. Deficiencies of
nucleoside monophosphates into the corresponding nucleosides (cyti- PGK, GS, or AK are usually associated with hereditary nonspherocytic
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dine and uridine), which are freely diffusible across the membrane. hemolytic anemia, but cases have been reported in which these defi-
P5′N1 is specific for pyrimidine nucleotides and does not use purine ciencies were unassociated with any hematologic manifestations. 261–263
2+
244
nucleotides as substrate. The enzyme requires Mg for it activity, and At times it has been suggested that moderate decreases in the activity
is inhibited by a number of heavy metals, including Pb . Like other of glutathione peroxidase causes hemolytic anemia, but the best avail-
2+ 245
red blood cell enzymes, P5′N1 activity is much higher in reticulocytes. able evidence indicates that this enzyme is not ordinarily rate limiting
264
246
The enzyme declines in activity during red cell aging. P5′N1 also has in erythrocyte metabolism and not associated with hemolytic anemia.

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