N-Chlorination and oxidation of procainamide by myeloperoxidase: toxicological implications

Abstract

In previous studies we had shown that procainamide is metabolized to reactive metabolites by activated leukocytes, and evidence pointed to involvement of myeloperoxidase (MPO). In this study we examine the metabolism of procainamide by MPO/H2O2, in the presence and absence of chloride ion. In the absence of chloride ion, the metabolism was very similar to that seen with activated leukocytes. The major metabolite was formed by oxidation of the arylamine group to a hydroxylamine. In the presence of chloride ion, a much greater degree of metabolism occurred, and the major product (40% of the starting procainamide) was a reactive species that could not be isolated. This metabolite spontaneously rearranged to 3-chloroprocainamide, and from its mass spectrum and chemical reactions, we deduce its structure to be N-chloroprocainamide. The N-chloroprocainamide metabolite reacted very rapidly with reducing agents, such as ascorbate, and also reacted with protein such as albumin, the major product in both cases being procainamide. This metabolite also chlorinated phenylbutazone. When radiolabeled procainamide was oxidized by MPO/H2O2 in the presence of albumin, covalent binding of the radiolabel to albumin occurred, and binding was greater under conditions in which N-chloroprocainamide was formed. It is probable that the failure to observe N-chloroprocainamide, when procainamide is oxidized by activated leukocytes, is due to its rapid reaction with the cells. We propose that modification of neutrophils (or neutrophil precursors in the bone marrow) by these reactive metabolites is responsible for procainamide-induced agranulocytosis. In a similar manner, procainamide-induced lupus could be due to modification of monocytes by monocyte-generated reactive metabolites.

Procainamide, but notN-Acetylprocainamide, Induces Protein Free Radical Formation on Myeloperoxidase: A Potential Mechanism of Agranulocytosis

“…PA has previously been shown to form an N-hydroxylamine and N-chloro-PA when oxidzed by activated neutrophils or MPO/H 2 O 2 , and MPO/H 2 O 2 /Cl − , respectively (40,41). It was shown previously that approximately 20% of PA results in N-chloro-PA in the presence of MPO/H 2 O 2 /Cl − and PA at pH 6 (41).…”

Polymorphisms for Aromatic Amine Metabolism in Humans: Relevance for Human Carcinogenesis

“…In addition, this enzyme has been shown to carry out the metabolic activation of several arylamine drugs and carcinogens (64,65) and has been suggested to be involved in several kinds of bone marrow toxicity induced by drugs such as procainamide, dapsone, and sulfamethoxazole (66,67). Recently, we characterized a peroxidase in human lung microsomal preparations that activates benzidine, ABP, 2-naphthylamine, 2-aminofluorene, and 4,4'-methylenebis(2-chloroaniline) and appears to be identical with myeloperoxidase, on the basis of cofactor requirements, inhibition, and solubilization with high salt (S. J. Culp and F. F. Kadlubar, unpublished studies).…”

Peroxidative metabolism of carcinogenic N-arylhydroxamic acids: implications for tumorigenesis.

“…However, the presumed intermediary nitroso compound was not detected from any of the drugs investigated. Simulations of these oxidations with MPO and H202/Cl-yielded the same metabolites as above and also N-chloramino derivatives that rearranged to o-chlorocompounds (24). The MPO-derived oxidation products from drugs containing the free -NH2 group have been implicated in adverse effects such as agranulocytosis and generalized hypersensitivity reactions during drug therapy (25).…”