Red cell metabolism in hereditary pyrimidine 5′‐nucleotidase deficiency: effect of magnesium

Abstract

Since pyrimidine nucleotides avidly bind magnesium, we tested the hypothesis that the haemolytic anaemia in hereditary pyrimidine 5'-nucleotidase (P5N) deficiency is due to a state of functional magnesium depletion in the red cell (RBC). In haemolysates from normal subjects, cytidine triphosphate (CTP) inhibited the activity of pyruvate kinase in a competitive manner for magnesium. The CTP Ki was 0.4 mmol/l. CTP inhibited the activity of hexokinase in a competitive manner for ATP (Mg-ATP2-) with a Ki of 4 mmol/l. The inhibitory effect of CTP on both enzymes was overcome by increasing the magnesium content of the test system. Since CTP appeared to inhibit enzymes which required magnesium as a cofactor or Mg-ATP2- as a substrate, we tested the effect of exogenous magnesium on the metabolism of P5N deficient RBC. The autohaemolysis test, the incubated Heinz body assay and the rate of glucose oxidation by the pentose phosphate shunt were abnormal in the intact RBC from a patient with hereditary P5N deficiency. The addition of MgCl2 (6-10 mmol/l) did not improve these abnormal in vitro measures of metabolism in the P5N deficient RBC. This lack of effect of exogenous magnesium may be due to the slow uptake of magnesium by the human RBC.

Hereditary pyrimidine 5′‐nucleotidase deficiency: from genetics to clinical manifestations

“…In particular, magnesium sequestration by pyrimidine nucleotides is, at least in part, responsible for the reduced activity of phosphoribosyl‐pyrophosphate (PRPP) synthetase found in P5′N‐deficient erythrocytes (Lachant et al , 1989), which may in turn result in an impairment of the PRPP‐dependent adenine salvage pathway (Zerez et al , 1986). CTP has also been shown to inhibit in vitro other Mg‐dependent enzymes, such as PK and hexokinase (Lachant & Tanaka, 1986), although the activity of these enzymes appears to be normal in P5′N‐deficient red cells (Beutler et al , 1980; Dvilansky et al , 1984; David et al , 1991). It has also been suggested that the accumulation of pyrimidine nucleotides may interfere with the glycolytic flux by competing with ATP/ADP‐binding sites of the glycolytic enzymes, such as phosphofructokinase and PK (Oda et al , 1984), but no conclusive evidence for a metabolic block was found.…”

Pyrimidine 5′ Nucleotidase Deficiency

“…Decreased pentose phosphate shunt activity has also been noted in some patients (Tomoda et al , 1982) and it is possible that this is secondary to increased intracellular acidity caused by a shift in permeable anions due to the accumulated nucleotides (Swanson et al , 1983). Pyrimidine nucleotides are also known to be strong chelators of magnesium ions and this may inhibit other enzymes, although in vitro addition of magnesium ions to cells from a P5′N‐deficient subject caused no improvement in haemolysis (Lachant & Tanaka, 1986). It has also been found that P5′N‐1‐deficient red cells have high levels of reduced glutathione (Valentine et al , 1972) and increased pyrimidine nucleoside monophosphate kinase activity (Lachant et al , 1987), although again these may be epiphenomena.…”