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A 5'-nucleotidase with unique specificity has been identified in the soluble fraction of normal human erythrocytes. It mediates the hydrolytic dephosphorylation of pyrimidine 5'-ribosemonophosphates but is catalytically ineffective with purine nucleotides or with the 2'-, 3'-, or cyclic isomers of pyrimidine nucleotides. Activities at 37 degrees in dialyzed hemolysates of nromal human erythrocytes averaged 7.3 and 6.2 mumol of Pi liberated per hour per g of hemoglobin for the substrates UMP and CMP, respectively. Activity with TMP as substrate was approximately one-half as much as with UMP or CMP. Apparent Michaelis constants were 0.33 mM UMP, 0.15 mM CMP, and 1.0 mM TMP. Magnesium was required for optimal activity, and this cation could not be replaced by Mn2+. Maximum activity was obtained between pH 7.0 and 7.5 with rapid decreases in more alkaline media and moderate decreases with acidification. The enzyme was quite sensitive to heat and was strongly inhibited by AMP, by some purine bases, and by both purine and pyrimidine nucleosides. Divalent cations of heavy metals were also strongly inhibitory, as were agents active against sulfhydryl groups. The presence of substrates and/or 2-mercaptoethanol provided considerable protection against some of these deleterious agents and conditions. Pyrimidine 5'-nucleotidase activity in hemolysates was clearly distinguishable from erythrocyte acid phosphatase and from leukocyte and serum alkaline phosphatases and nucleotidases.

SUMMARY The heterogeneity of pyruvate kinase (PK) deficiency associated with hereditary haemolytic anaemia is emphasized by studies of a kindred harbouring two distinct mutant forms of this enzyme, both of which were kinetically defective with markedly decreased affinities for the substrate, phosphoenolpyruvate. The two isoenzymes, designated PK-Vancouver1 and PK-Vancouver2, were primarily distinguishable from one another by differences in maximum in vitro activities and by variations in response to fructose-1,6-diphosphate activation. When combined in proband erythrocytes to the exclusion of any normal PK, the isoenzymes were associated with a severe chronic haemolytic process with many of the features of PK deficiency of the more common quantitative type. Clinical laboratory screening tests for detecting PK deficiency may be falsely negative or equivocal in such cases.

The proband with lifelong hemolytic anemia has a high K0.5s phosphoenolypyruvate (PEP) erythrocyte pyruvate kinase (PK) variant substantially but incompletely normalized by the allosteric modifier fructose-1,6-diphosphate (F-1,6-P2) with conversion of sigmoidal to hyperbolic kinetics. Heterozygotes in four generations express qualitatively identical but less severely abnormal kinetics and lack overt hemolysis. Kinetic abnormalities are closely mimicked by sulfhydryl modification of normal PK. Three distinct clinical and metabolic phenotypes characterize the proband and two sisters: variant PK and hemolytic anemia, variant PK without clinical manifestations or hemolysis, and complete normality. Their mother, whose red cell PK is entirely normal except for a questionably slightly low Vmax, is postulated to express the gene products of nonidentical alleles, one encoding a product with mildly less favorable catalytic characteristics. At low PEP concentrations, the proband and heterozygotes for the PK mutant express only a very small fraction of normal PK activity despite apparent inheritance of one normal allele in the latter. Evidence suggests that disproportionately lowered PK activity may be a property of a heterotetrameric PK. Illusory abnormalities in nucleotide specificity are artifacts of diminished substrate affinity characterizing the mutant PK.

The human red cell, biologically speaking, is intriguingly but deceptively simplistic. Freely floating, separable from contaminating cell types, devoid of a nucleus and cellular organelles, restricted in its energy needs and in the mechanisms for meeting them, it can be biopsied repeatedly with safety and offers many lures to the investigator. In recent years its enzymatic machinery has been explored with increasing intensity, and a variety of genetically determined inborn errors have been demonstrated (Valentine, 1, 2; Jaffe, 3). Some have been associated etiologically with hereditary hemolytic disease, and some have clarified mechanisms of what formerly were regarded as idiosyncratic reactions to drugs. Some have provided a convenient diagnostic tool for defining diseases not characterized by he­ matopoietic abnormalities. In still others, the definition of genetically de­ termined enzymopathy has been a curiosity with no counterpart in clinical disease.

A 5'-nucleotidase with unique specificity has been identified in the soluble fraction of normal human erythrocytes. It mediates the hydrolytic dephosphorylation of pyrimidine 5'-ribosemonophosphates but is catalytically ineffective with purine nucleotides or with the 2'-, 3'-, or cyclic isomers of pyrimidine nucleotides. Activities at 37 degrees in dialyzed hemolysates of nromal human erythrocytes averaged 7.3 and 6.2 mumol of Pi liberated per hour per g of hemoglobin for the substrates UMP and CMP, respectively. Activity with TMP as substrate was approximately one-half as much as with UMP or CMP. Apparent Michaelis constants were 0.33 mM UMP, 0.15 mM CMP, and 1.0 mM TMP. Magnesium was required for optimal activity, and this cation could not be replaced by Mn2+. Maximum activity was obtained between pH 7.0 and 7.5 with rapid decreases in more alkaline media and moderate decreases with acidification. The enzyme was quite sensitive to heat and was strongly inhibited by AMP, by some purine bases, and by both purine and pyrimidine nucleosides. Divalent cations of heavy metals were also strongly inhibitory, as were agents active against sulfhydryl groups. The presence of substrates and/or 2-mercaptoethanol provided considerable protection against some of these deleterious agents and conditions. Pyrimidine 5'-nucleotidase activity in hemolysates was clearly distinguishable from erythrocyte acid phosphatase and from leukocyte and serum alkaline phosphatases and nucleotidases.

A number of non-glycolytic metabolic abnormalities may occur in erythrocytes without significantly altering cell function or life span. They include deficiencies of adenine or hypoxanthine-guanine phosphoribosyltransferases, adenosine deaminase, nucleoside phosphorylase, and hyperactivity of ribosephosphate pyrophosphokinase. Three principal enzyme defects are causally associated with hemolytic anemia: hyperactive adenosine deaminase and deficiencies of adenylate kinase and pyrimidine nucleotidase. These produce hemolytic syndromes of variable severity ranging from mild or subclinical in the adenosine deaminase defect to severe in adenylate kinase deficiency. Pyrimidine nucleotidase deficiency is much more common and is associated with intermediate degrees of anemia. Acquired nucleotidase deficiency may occur secondary to lead toxicity and produces a syndrome virtually identical to the hereditary deficiency states.

Erythrocytes from 11 patients with presumptive diagnoses of transient erythroblastopenia of childhood were evaluated retrospectively (six) or prospectively (five) for a possible relationship between erythrocyte adenosine 5'-monophosphate aminohydrolase, adenylic acid deaminase (AMP deaminase) activity and intracellular concentrations of adenine nucleotides. Older red blood cell (RBC) cohorts in these patients consistently exhibited significantly decreased activities of AMP deaminase (approximately 5% to 70% of normal control mean) in association with increased concentrations (up to threefold) of adenosine triphosphate (ATP) and total adenine nucleotides. We postulate that the latter is a direct consequence of the former, since diminishing AMP deaminase activity in aging cells should reduce the drain on the adenine nucleotide pool imposed by irreversible deamination of AMP to inosine 5'-monophosphate. Consistent reductions in AMP deaminase activity indicate that this enzyme should also serve as a reliable marker of mean RBC age useful in diagnostic confirmation of transient erythroblastopenia. The observed increases in ATP and total adenine nucleotides in older RBCs require a reevaluation of the traditional view that age-related losses of these compounds mediate the ultimate demise of senescent erythrocytes. Similar alterations in the balance of degradative and salvage pathways in RBC nucleotide metabolism may also underlie certain cases of so-called "high ATP syndrome."

Residual 5'-nucleotidase activities in hemolysates from nine subjects with severe hereditary deficiency of pyrimidine nucleotidase (PyrNase) were compared to those in normal and reticulocyte-rich controls. Dephosphorylation rates of 12 potential ribo- and deoxyribomononucleotide substrates were measured as a function of pH. Data confirmed the existence of at least two isozymes of 5'-nucleotidase, PyrNase, and 2'-deoxy-5'-ribonucleotide phosphohydrolase (dNase) distinguishable by differences in maximal velocities, substrate preferences and restrictions, and pH optima. PyrNase was confirmed to be active principally with pyrimidine substrates (UMP = dCMP greater than CMP much greater than dTMP greater than dUMP) at a pH optimum of 7.5 +/- 0.1. dNase activity occurred with both purine and pyrimidine substrates and was maximal with deoxy analogs (dIMP much greater than dUMP greater than dGMP greater than dTMP = dAMP much greater than dCMP) at a pH optimum of 6.2, but slight cross-reactivity occurred with some nondeoxy substrates (IMP greater than GMP greater than UMP = XMP greater than CMP). PyrNase and dNase may be complementary systems that serve physiologically to clear the cytosol of RNA and DNA degradation products during maturation of erythroid elements by conversion of nucleotide monophosphates to diffusible nucleosides.

GSH, the most abundant erythrocyte thiol, is synthesized from its constituent amino acids by two ATP-dependent enzymes present in the mature red cell. Its unusual gamma-glutamyl linkage precludes degradation by any known red cell peptidase. The erythrocyte lacks a complete "gamma-glutamyl cycle" as described by Meister. GSH has important enzymatic and non-enzymatic roles in oxidoreduction reactions. As the specific co-factor of glutathione peroxidase, it participates in the reduction of harmful organoperoxides. Oxidized glutathione is reconverted to GSH via NADPH-dependent, glutathione reductase. NADPH in the red cell is generated solely by the two dehydrogenases of the pentosephosphate shunt. Increased GSH concentrations are normally present in neonatal erythrocytes. For reasons not clear, they are an epiphenomenon in inherited pyrimidine 5'-nucleotidase deficiency. Many syndromes of heterogeneous etiology having in common dyserythropoietic anemia and ineffective erythropoiesis despite a cellular bone marrow also exhibit abnormally high concentrations of red cell GSH as one component of a constellation of metabolic abnormalities. In a single patient with the Lesch-Nyhan syndrome studied by us, erythrocyte GSH was increased. A kindred with dominantly transmitted (or possibly x-chromosome linked) hereditary hemolytic anemia in which the only thus far detected abnormality is increased red cell GSH has also been documented. The fundamental molecular lesion in this syndrome is unknown.

Uncertainties regarding the role of pyrimidine nucleotidase (PyrNase) in AMP catabolism were resolved by studies of erythrocytes from normal controls, controls with young mean cell ages, and patients with hereditary hemolytic anemia due to severe deficiency of PyrNase. Hemolysates from the latter exhibited undiminished capacity to dephosphorylate AMP over a broad range of pH, indicating that PyrNase was not directly involved. In each subject group, the rates of AMP dephosphorylation between pH 5.1 and 8.3 were indistinguishable from those of IMP, suggesting a potential role for AMP-deaminase, an erythrocyte enzyme that was stimulated by coformycin at pH 7.2. Quantitative analysis of catabolites in incubated hemolysates confirmed that AMP degradation preferentially occurred via deamination to IMP with subsequent dephosphorylation by another erythrocyte nucleotidase isozyme, deoxyribonucleotidase. Both AMP-deaminase and deoxyribonucleotidase have acidic pH optima with minimal activities at physiologic pH, suggesting that this pathway of AMP catabolism could accelerate depletion of the adenine nucleotide pool and thereby mediate the demise of senescent erythrocytes sequestered in the spleen.

The maturing reticulocyte degrades ribosomal RNA to constituent ribonucleoside phosphates. Guanosine ribonucleotides are retained only in small amounts and pyrimidine ribonucleotides only in trace quantities. In the mature erythrocyte more than 97% of total nucleotides are the interconvertible adenosine mono-, di-, and triphosphates. High energy ATP fuels most of the reactions required to sustain viability. Unable to synthesize adenosine phosphates from small precursor molecules, the red cell relies on certain salvage pathways to replenish its losses from the adenosine phosphate pool. The most important of these involve adenosine. Adenylate kinase deficiency, when severe, is associated with nonspherocytic hemolytic anemia. A genetically-determined deficiency of pyrimidine 5'-nucleotidase prevents the normal dephosphorylation of pyrimidine ribonucleotides, and hence is characterized by the unique accumulation of pyrimidine phosphates intracellularly. Other features are chronic hemolytic anemia, splenomegaly, and a profound increase in basophilic stippling on the stained blood film. The syndrome is transmitted as an autosomal recessive disorder. A similar syndrome is found in severe lead poisoning as a consequence of nucleotidase inhibition by lead. An inherited, dominantly transmitted hemolytic anemia associated with low red cell ATP and a 45-70 fold increase in the enzymatic activity of adenosine deaminase has also been documented. The undefined molecular lesion appears to involve overproduction of an entirely normal enzyme protein. Severe deficiency of either of two sequential enzymes of purine metabolism, adenosine deaminase anemia, but by excessive accumulations of deoxyribonucleotides within red cells and lymphocytes. The clinical counterpart of each is a severe immunodeficiency state secondary to lymphopenia and lymphocyte dysfunction. Certain other rare clinical syndromes involving disturbed nucleotide metabolism also are detectable by red cell assay procedures.

Carrell and Lehmann have reported a method of precipitating Hb that is attractive in terms of its rapidity, simplicity, reversibility, and lack of rigorous conditions. We have applied the methodology to the concentration and partial purification of certain red cell enzyme activities. Twenty-three glycolytic and nonglycolytic enzymes were studied (Table 1). Under conditions where 97% to 98% of Hb was precipitated, recoveries in the supernatant for 14 enzymes ranged from 40% to 100%, with nine activities virtually completely recovered as measured by routine assay. In six instances, only minimal activity was recoverable either in the supernatant or in the precipitate. In the case of those enzymes whose recovered activities in supernatant were large, more rigorous investigation to detect possible zinc-induced alterations is necessary to determine whether the procedure impairs parameters not readily evaluated by routine assay. The effect of the procedure on PK was studied more intensively. The kinetics of the enzyme with PEP and ADP were unaffected, its thermostability appeared not to differ from native PK, and ATP inhibition of activity was normal. Addition of more zinc to the supernatant precipitated PK, which could then be further partially purified by (NH4)2SO4 precipitation. Inclusion of magnesium and substrate PEP in the hemolysate prior to zinc addition increased recovery in the supernatant from 40% to 80%.

Deficiency states have been well established in nine enzymes of anaerobic glycolysis and suggested, but inadequantely confirmed, in others. In additon, hemolytic anemia has been defined in glucose-6-phosphate dehydrogenase deficiency, in certain enzymes glutathione metabolism of importance to the function of the oxidative hexsemonophosphate shunt, and in certain enzymes of nucleotide metabolism. In heterogeneous dyserythropoietic states with ineffective erythropoiesis, enzyme ratios are grossly distorted and differ greatly from those of either normal or reticullocyte-rich blood. Red blood cell (RBC) enzyme deficiencies show noticeable genetic polymorphism, most apparently recessively transmitted anemias actually resulting from inheritance of two different mutant, allelic genes. In addition to causing hemolytic syndromes in selected instances, deficiencies of enzymes more important to other tissues than to the RBC itself are detectable by assay of hemolystates.

The proband with lifelong hemolytic anemia has a high K0.5s phosphoenolypyruvate (PEP) erythrocyte pyruvate kinase (PK) variant substantially but incompletely normalized by the allosteric modifier fructose-1,6-diphosphate (F-1,6-P2) with conversion of sigmoidal to hyperbolic kinetics. Heterozygotes in four generations express qualitatively identical but less severely abnormal kinetics and lack overt hemolysis. Kinetic abnormalities are closely mimicked by sulfhydryl modification of normal PK. Three distinct clinical and metabolic phenotypes characterize the proband and two sisters: variant PK and hemolytic anemia, variant PK without clinical manifestations or hemolysis, and complete normality. Their mother, whose red cell PK is entirely normal except for a questionably slightly low Vmax, is postulated to express the gene products of nonidentical alleles, one encoding a product with mildly less favorable catalytic characteristics. At low PEP concentrations, the proband and heterozygotes for the PK mutant express only a very small fraction of normal PK activity despite apparent inheritance of one normal allele in the latter. Evidence suggests that disproportionately lowered PK activity may be a property of a heterotetrameric PK. Illusory abnormalities in nucleotide specificity are artifacts of diminished substrate affinity characterizing the mutant PK.

A child with chronic hemolytic anemia since birth was found to have erythrocyte pyruvate kinase (PK) in a highly unusual form relative to other mutant isozymes when characterized by International Committee for Standardization in Hematology criteria. Most properties of the partially purified isozyme (designated PK-Greenville) were altered minimally, if at all, except for nearly total insensitivity to allosteric activation by fructose-1,6-diphosphate (F-1,6-P). One parent appeared to be heterozygous for a null gene and the other for an allele governing production of the mutant isozyme. Apparent restriction of the molecular defect to ineffective activation kinetics suggests that the F-1,6-P binding site on erythrocyte PK is functionally as well as physically allosteric. The magnitude of the metabolic block at the PK step and the clinical severity indicate that allosteric modulation by F-1,6-P is a crucial property of PK in normal erythrocyte metabolism.

Studies of a Mexican kindred present evidence for a unique phenotype of erythrocyte glucosephosphate isomerase, GPI Valle Hermoso. The proband was apparently the homozygous recipient of a mutant autosomal allele governing production of an isozyme characterized by decreased activity, marked thermal instability, normal kinetics and pH optimum, and normal starch gel electrophoretic patterns. Unlike previously known cases, leukocyte and plasma GPI activities were unimpaired. This suggested that the structural alteration primarily induced enzyme instability without drastically curtailing catalytic effectiveness, thereby allowing compensation by cells capable of continued protein synthesis. Age-related losses of GPI, however, were not evident by density-gradient fractionation of affected erythrocytes.

Enzymes of aerobic and anaerobic glycolysis, glutathione cycling, and nucleotide metabolism were assayed on erythrocytes from 7 healthy rhinoceroses, 2 rhinoceroses during periods of intravascular hemolysis, and 1 rhinoceros without clinical signs of illness, which was the mother of 3 offspring with intravascular hemolytic syndrome. Measurements also were made of erythrocyte concentrations of glycolytic intermediates, adenine nucleotides, and glutathione. Although comparison of results for healthy and affected rhinoceroses did not identify an enzyme abnormality as a cause for the hemolytic syndrome, the data provided information regarding the metabolic characteristics of erythrocytes from healthy rhinoceroses.