Blood and air pollution: state of knowledge and research needs.

The ready access to blood (plasma and formed cellular elements) makes it unusually susceptible to the deleterious effects of pollutants whose origins may be in the air. The red blood cells' hemoglobin may be rendered useless for oxygen transport by combination with carbon monoxide or conversion to methemoglobin or sulfhemoglobin. Lead and arsine can damage the erythrocytes' membranes, resulting in anemia. Metabolites of benzene and other volatile polycyclic hydrocarbons are implicated in the causation of leukemias. The extensive use of pesticides and herbicides may be associated with the development of Hodgkin's disease, non-Hodgkin's lymphoma, and aplastic anemia. The carcinogenic risks from ionizing radiation, especially for leukemia, are well known. More information is needed concerning the epidemiology of environmental factors responsible for damage to blood. Enhanced knowledge about the molecular biology of toxins' effects on the hematopoietic system and improved detection and prevention technologies are needed to answer environmentally related health questions.

Enzymopenic hereditary methemoglobinemia: a clinical/biochemical classification.

Recessively inherited NADH-cytochrome B5 reductase deficiency, when present in the homozygous or doubly heterozygous form, is manifested by two different clinical presentations, depending on the nature and cellular distribution of the mutant enzyme. The observations supporting a clinical and biochemical classification of enzymopenic hereditary methemoglobinemia are summarized. Type I, with deficiency demonstrable only in the erythrocytes, presents as uncomplicated, benign methemoglobinemia. Type II, generalized cytochrome B5 deficiency demonstrable in all of the tissues that have been examined, is accompanied by severe, lethal, progressive neurological disability, in addition to methemoglobinemia. Type III deficiency is limited to hematopoietic cells and resembles Type I clinically. Type IV, also clinically like Type I, is associated with deficiency of the cofactor, cytochrome B5. Except for Type IV, the different types appear to be the result of mutations in paired alleles of a gene on chromosome 22 that affect the catalytic activity or stability of the cytochrome B5 reductase.

Enzymopenic hereditary methemoglobinemia.

The normal erythrocyte is well endowed with a system to convert useless methemoglobin to functional hemoglobin. The major mechanism for this reductive capacity resides in the soluble cytochrome b5/NADH cytochrome b5 reductase of the cytosol which presumably arise from the microsomal proteins of the endoplasmic reticulum through proteolytic cleavage of the proteins' hydrophobic tails during the maturation of nucleated erythrocyte precursors. NADH cytochrome b5 reductase is coded for by a gene on the human chromosome 22. Inheritance of a pair of abnormal alleles which specify an enzyme with decreased activity or stability occurs only rarely, but leads to enzymopenic hereditary methemoglobinemia. Type I, uncomplicated, benign methemoglobinemia is attributed to mutation in paired alleles that affect primarily the catalytic capacity, stability, or solubilization of the polar, soluble segment of the enzyme. It does not appear to affect significantly the well being or life expectancy of the homozygous subject. The cosmetic affliction or the minimal symptoms can rather easily be controlled with methylene blue, ascorbic acid, or riboflavin. The heterozygote is entirely asymptomatic, but may have an increased tendency to develop methemoglobinemia on exposure to methemoglobin-inducing drugs or chemicals. Type II, severe lethal methemoglobinemia is a generalized disorder in which the NADH cytochrome b5 reductase is apparently defective in all tissues. It is thought to result from either gene deletion or mutation in paired alleles that determine the function, stability, or attachment to the endoplasmic reticulum of the entire enzyme, both the polar and the hydrophobic segments. As in Type I, the heterozygote is asymptomatic, and the homozygote's methemoglobinemia is readily controlled. The generalized disorder including the neurologic dysfunction, however, is not amenable to treatment at this time. Prenatal diagnosis by examination of amniotic fluid cells is both feasible and useful.

Methemoglobin pathophysiology.

The biochemical processes involved in the formation of methemoglobin, in the protection of hemoglobin against oxidation to methemoglobin, and in the reduction of methemoglobin to hemoglobin are reviewed. Special emphasis is directed to the major and minor metabolic pathways in human erythrocytes that are involved in the reduction of methemoglobin, and evidence is presented that the NADH-methemoglobin reductase system, involving a soluble cytochrome b5 and NADH-cytochrome b5 reductase, it is the most important reductive mechanism. Toxic methemoglobinemia, methemoglobinemia due to hemoglobin M's, and hereditary methemoglobinemia due to deficiency in NADH-methemoglobin reductase activity are considered. Their clinical manifestations, diagnosis, and treatment are discussed briefly.

Molecular and cellular effects of antisickling concentrations of alkylureas.

Alkylureas are capable of inhibiting sickling in vitro and the gelation of solutions of hemoglobin S at concentrations between 0.05 and 0.1 M with increasing effectiveness that is directly proportional to the length of the alkyl chain (butyl greater than propyl greater than ethyl greater than methyl). 6The inhibitory effect is independent of pH between 6.5 and 7.5 and is a process driven by entropy. The alkylureas at concentrations of 0.1 M have minimal effects on several erythrocyte functions. Oxygen equilibria, osmotic fragility, reduced glutathione content, and glutathione reductase activity are totally unaffected, while pyruvic kinase activity is decreased only by butylurea by about 20%, and glucose-6-phosphate dehydrogenase activity is decreased progressively to a maximum of 30% in direct proportion to the length of the alkyl chain. Alkylureas not only inhibit sickling but are also capable of desickling erythrocytes that have been maintained in the deoxygenated state. They have little effect on several erythrocyte functions at antisickling concentrations, but their toxicity must be evaluated before they can be examined as potential therapeutic agents for the treatment or prevention of acute episodes in sickle cell anemia.

Metabolic effects of antisickling amounts of nitrogen and nor-nitrogen mustard on rabbit and human erythrocytes.

Nitrogen mustard (NH2) and Nor-nitrogen mustard (Nor-HN2) both inhibit the polymerization of deoxyhemoglobin S in solution and in intact erythrocytes. Metabolic studies were undertaken to determine the feasability of an extracorporeal treatment with these or related agents. Glucose utilization, hexose monophosphate shunt activity, methemoglobin reduction, and incubation with acetylphenylhydrazine for Heinz body formation were performed, as well as specific assays for hexokinase, pyruvate kinase, glucose-6-phosphate dehydrogenase, glutathione reductase, ATP, reduced glutathione (GSH), and survival of autologous mustard-treated cells in rabbits. HN2 was found to enter red cells rapidly and bind to intracellular contents. Metabolic studies revealed no significant inhibition or alteration of function by Nor-HN2 at 10 mg/ml of whole blood. Rabbit red cell survival was also normal. HN2, however, inhibited glutathione reductase and blocked the free sulfhydryl group of GSH by forming serveral addition products of alkylated GSH. Heinz body test with acetylphenylhydrazine became positive in HN2-treated cells, and rabbit red cell survival was shortened considerably in the concentration range used to inhibit sickling. Ascorbic acid stimulation of the hexose shunt pathway was inhibited by HN2, but methylene blue stimulation remained unaffected. 14-C-HN2 remains bound to red cells in vivo, and the disappearance of radioactivity is similar to that found with 14-C-DFP (disopropylfluorophosphate). Oxygen affinity of both HN2 and Nor-HN2 treated human red cells remains virtually the same as that found in control samples. It is concluded that Nor-HN2 may be a suitable agent for an extracorporeal therapy, and that each mustard needs to be evaluated individually for its antisickling effects and its suitability for extracorporeal use.