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Type I recessive congenital methaemoglobinaemia (RCM), caused by the reduced form of nicotinamide adenine dinucleotide (NADH)-cytochrome b(5) reductase (cytb(5)r) deficiency, manifests clinically as cyanosis without neurological dysfunction. Two mutations, E255- and G291D, have been identified in the NADH-binding lobe of cytb(5)r in previously reported patients, and we have detected a further novel mutation, D239G, in this lobe in two unrelated Irish families. Although one family belongs to the genetically isolated Traveller Community, which separated from the general Irish population during the 1845-48 famine, the D239G mutation was present on the same haplotype in both families. Three known cytb(5)r mutations were also identified, including the R159- mutation, which causes loss of the entire NADH-binding lobe and had previously been reported in an individual with type II RCM. Characterization of the three NADH-binding lobe mutants using a heterologous expression system revealed that all three variants retained stoichiometric levels of flavin adenine dinucleotide with spectroscopic and thermodynamic properties comparable with those of native cytb(5)r. In contrast to the E255- and G291D variants, the novel D239G mutation had no adverse impact on protein thermostability. The D239G mutation perturbed substrate binding, causing both decreased specificity for NADH and increased specificity for NADPH. Thus cytb(5)r deficient patients who are heterozygous for an NADH-binding lobe mutation can exhibit the clinically less severe type I phenotype, even in association with heterozygous deletion of the NADH-binding lobe.

Deficiency of folate during pregnancy is associated with megaloblastic anaemia. Lower levels of folate and vitamin B12 have been reported in mothers whose offspring had neural tube defects compared to unaffected controls. Increased methylmalonic acid levels are a sensitive indicator of mild vitamin B12 deficiency and elevated homocysteine levels denote vitamin B12 or folate deficiency. We have investigated the relationship between serum concentration of total homocysteine, methylmalonic acid, vitamin B12 and folate in pregnancy. A significant inverse correlation was found between homocysteine and red cell folate and, to a lesser extent, serum folate. In addition, a significant inverse correlation was found between methylmalonic acid and vitamin B12. No significant relationship was found between homocysteine and vitamin B12. The relationship between red cell folate and serum folate and homocysteine may be useful for detecting borderline folate deficiency in pregnancy and indicate pregnancies at risk of neural tube defect. These sensitive assays are useful tools for the further investigation of folate vitamin B12 and metabolism in normal and abnormal pregnancy.

In pyruvate kinase (PK)-deficient blood with high levels of reticulocytes, the degree of haemolysis after incubation at 37 degrees C for 48 h was halved by the inclusion of adenine in the medium. the decrease in haemolysis was associated with a higher ATP level but was not related to the pH of the incubation mixture. The incorporation of adenine into nucleotides studied over a 3 h period was similar in normal and PK-deficient blood. The observed increase in cellular ATP was equivalent to that shown by radioactive measurements to have been synthesized from added adenine. Inhibition of mitochondrial oxidative phosphorylation in the reticulocytes of the PK-deficient blood by KCN reduced the amount of adenine taken up by the cells by a factor of 3 and altered the pattern of incorporation into the nucleotides. Only 25% of the adenine which entered the blood cells was converted into ATP compared with 85% in the absence of cyanide. Despite the synthesis of small amounts of ATP from labelled adenine, the overall ATP content fell to less than 50% of its original level. It is suggested that incubation with adenine increased the ATP level in the reticulocytes by virtue of mitochondrial oxidative phosphorylation thereby reducing the haemolysis of these cells.