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have uncovered a number of clues to the possible mechanism of sulfur mustard skin toxicity. Based on data reported from these systems two postulates have emerged to explain the pathogenesis of sulfur mustard skin toxicity.

DNA Alkylation and Protein Activation. Using cultures of primary epidermal human keratinocytes, Papirmeister and coworkers (1991) have uncovered evidence that alkylation of DNA, reduced tissue levels of oxidized nicotinamide adenine dinucleotide (NAD+), and activation of cellular proteases may account for sulfur mustard-induced blister formation in human skin. According to the authors, the key reaction begins with the intramolecular cyclization of sulfur mustard to form an electrophilic ethylene episulfonium intermediate, which, in the presence of water, alkylates a number of tissue macromolecules including DNA, RNA, and protein. Alkylation of DNA ultimately results in single-strand breaks within basal cells of the epidermis, which in turn trigger activation of DNA  repair enzymes, particularly poly-(adenosine diphosphateribose) polymerase (PADPRP). Excessive PADPRP activity causes inhibition of glycolytic and respiratory enzymes, impaired glucose uptake, and depleted stores of oxidized NAD+ . NAD+ depletion results in inhibition of glycolysis, buildup of glucose-6-phosphate (a substrate in the hexose monophase shunt), and, ultimately, activation of cellular proteases. Proteases released from epidermal cells are thought to cause disruption of dermal-epidermal attachments and resultant blister formation.

In studies using cultured human epidermal cells, Martens (1991) has accumulated some preliminary data to show that after exposure to 0.3 mM sulfur mustard, glucose utilization was 50 percent inhibited, and NAD+ content within cells decreased to 50-60 percent of control. The onset of sulfur mustard-induced NAD+ depletion preceded the inhibition of glycolysis. Martens states that his data support the Papirmeister group's postulated mechanism of sulfur mustard damage to human skin. Smith and Dunn (1991) have stated also that studies at their institute are consistent with the Papirmeister proposal. They add, however that ''the process would appear to require an active inflammatory response and altered fluid dynamics in the affected tissue to generate . . . blisters."

DNA Cross-Linking. A simpler postulate is advanced by Bernstein and colleagues (1985, 1987). Using rat keratinocytes cultivated on nylon membranes, their experiments—in which low-dose sulfur mustard (0.01-0.5 nmol/cm²), caused significant inhibition of 3H



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