Summary. The synthesis of thymine DNA from deoxyuridine and uridine was studied by culturing human bone marrow for 1–3 hours in the presence of radioactive‐labelled nucleosides. In the current study, this in vitro system appeared useful to test the potential efficacy of agents (such as methionine) which might be used to improve or retard improvement of patients with megaloblastic anaemia. In the bone marrow from patients with B12‐deficient or folate‐deficient megaloblastic anaemia, there was defective incorporation of deoxyuridine into thymine DNA. This defect was partially corrected by added B12 in the B12‐deficient but not the folate‐deficient marrows, and completely corrected by folic acid (pteroylglutainic acid) in both types of deficient marrow. The folate antagonist rnethotrexate blocked the corrective effect of B12. Incorporation of labelled uridine into DNA of B12‐deficient megaloblastic marrow was enhanced less by B12 than by PGA. Vitamin B12 antagonists failed to depress the incorporation of labelled uridine into DNA in bone marrows from both normal and B12‐deficient patients. These results support the concept that inadequate DNA synthesis in B12 deficiency is due in large measure to blockade in folate metabolism brought about by lack of B12. 5‐Methyl‐tetrahydrofolate, which may accumulate in B12 deficiency, failed to correct the defect in DNA synthesis, adding further evidence to the concept of metabolic trapping of this form of folate in B12 deficiency. Thus, riboside reduction in man may be independent of B12, as it is in E. coli, rather than dependent on B12, as it is in L. leichmannii.

Cyanocobalamin supplementation of 50 microg but not 10 microg daily produced a significant increase in serum vitamin B12. This result has implications for the management of patients with subnormal or borderline serum vitamin B12 concentrations and for food fortification with vitamin B12.

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Objective To determine the effect of prospective monitoring on appropriateness of transfusions of red cells, platelets and fresh frozen plasma (FFP). Design Prospective interventional study. Setting Royal Melbourne Hospital (a tertiary teaching hospital), Melbourne, Victoria, March‐May 1996. Intervention The blood product request form was modified to Incorporate indications for transfusion and clinical and laboratory data. Requests were monitored by blood bank laboratory staff for conformation with hospital transfusion guidelines; non‐conforming requests were discussed with the requesting medical practitioner by the Haematology Registrar before blood products were issued. In cases of disagreement, blood products were always issued. Subjects 200 consecutive transfusion episodes for each product (red cells, platelets and FFP). Outcome measures Appropriateness of transfusion, assessed by a Consultant Haematologist according to hospital guidelines. Rates of inappropriate transfusion episodes after intervention were compared with rates in a previous study. Results After intervention, rates of inappropriate transfusion episodes fell significantly (red cells, 16% to 3% [P= 0.004]; platelets, 13% to 2.5% [P=0.02]; and FFP, 31% to 15% [P=0.02]). Almost all Inappropriate FFP transfusion episodes post‐intervention were due to failure to demonstrate prolongation of prothrombin or activated partial thromboplastin times more than 1.5 times the control value. Conclusion Prospective monitoring of request forms can reduce rates of inappropriate transfusions. High rates of Inapproriate FFP transfusions possibly reflect uncertainty about appropriate laboratory criteria for FFP transfusion. While results of large prospective randomised controlled clinical trials of FFP transfusions are awaited, current laboratory criteria can be retained, but should be applied with flexibility.

Neuropathy commonly complicates cobalamin (Cb1) deficiency in humans, monkeys, fruit bats, and pigs. The neuropathy is characterized by demyelination of the posterolateral columns of the spinal cord (subacute combined degeneration). The lesion was thought to arise primarily from impairment of the adenosylcobalamin-dependent methylmalonyl CoA mutase reaction, leading to the formation of abnormal odd-chain and branched-chain fatty acids and their incorporation into myelin with resultant demyelination. Data from recently developed animal models of the Cb1 neuropathy induced by exposure to nitrous oxide do not substantiate this hypothesis, but rather identify impairment of the methylcobalamin-dependent methionine synthetase reaction as the more important basic defect. The key evidence for this hypothesis is the ability of methionine to delay the onset of Cb1 neuropathy in experimental Cb1 deficiency. In the Cb1-deficient pig, adenosylhomocysteine accumulates in neural tissue, presumably owing to the inability to recycle homocysteine via the defective methionine synthetase reaction. Accumulation of adenosylhomocysteine results in a fall in the adenosylmethionine:adenosylhomocysteine methylation ratio, and this change is believed to cause defective methylation and demyelination in the nervous system. However, in the Cb1 neuropathy in the fruit bat, adenosylhomocysteine does not accumulate in the nervous system, the methylation ratio does not change, and no defect can be demonstrated in the methylation of myelin lipid or basic protein. Although a central role for methionine in the pathogenesis of the Cb1 neuropathy has been established, defective methylation attendant upon impairment of the methionine synthetase reaction may not be the universal defect underlying the Cb1 neuropathy. This would suggest that the methionine effect could be mediated via its role in formate metabolism or polyamine synthesis, or by some as yet unidentified pathway.

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To determine the significance of the commonly observed fall in serum vitamin B12 levels during pregnancy, serum levels of the B12 metabolites methylmalonic acid (MMA) and homocysteine (Hcy) were measured in a group of 50 pregnant patients with subnormal serum B12 (range 45-199 pg/ml) and the results compared with those of 25 pregnant controls (serum B12(208-580) pg/ml). Mean values for serum MMA and total Hcy in the subnormal B12 group were 445.4 nmol/L and 7.03 mumol/L, respectively, which were not significantly different from the mean MMA of 440.5 nmol/L and Hcy of 6.88 nmol/L in the controls. For the total group of patients, neither serum MMA nor serum Hcy levels correlated with serum B12. One-third of pregnant patients showed elevated serum MMA values, independent of B12 status. Significant elevation of serum Hcy was detected in only two patients, both with subnormal serum B12 and hematological evidence of B12 deficiency. We conclude that the usual fall in serum B12 concentration in pregnancy does not reflect B12 deficiency at the biochemical level. In establishing true B12 deficiency in pregnancy, the serum Hcy level (in the absence of folate deficiency) but not serum MMA, is of value.