On the mechanism of the oxidation of human and rat hemoglobin by propylene glycol dinitrate.

The v.inetics and stoichiometry of the oxidation of human and rat oxyhemoglobin (02Hb) by propylene glycol dinitrate CGDN) have been investigated in vitro in both hemolysate and in intact red blood cell preparations. In hemolysate fractions from both these species, the rate constant for oxidation is nearly constant between pH 7-0 and 9-0, but it increases dramatically at lower pH values. The reaction is molecular and approximately first-order in both [PGDN] and in [02Hb]. The ra.e of oxidation is related complexly to the 02 concentration. No oxidation occurs at zero 02 concentration or at very high 02 concentra'icr.j. Maximal rates are observed at 02 concentrations where the hemoglobin is only partially saturated with 02. The stoichiometry appears to be 1-5 hemes oxidized per ester bond broken. In whole cells, the reaction is still molecular, is approximately first-order in both reactants, and has a stoichiometry of 1 9 to 2-3 moles heme oxidkrd per mole of reacted ester. THE ABILITY of organic polynitrates to oxidize hemoglobin was recognized t tfore the turn of the century.Numerous investigations of the pharmacologic and toxicologic properties of these nitrates have been conducted since that time, yet it is not known whether the hemoglobin is oxidized by the nitrate itself or by nitrite produced upon the hydrolysis of the nitrate ester. Even the question of whether the reaction is molecular or enzymatic is still open. On the basis of the amounts of methemoglobin (met-Hb) formed as a function of the amount of organic nitrate administered to the cat, Wilhelmi suggested that the reaction was molecular. However, no systematic chemical analysis of this reaction has yet appeared and Wilhelmi's data are by no means conclusive. The metabolism of polynitrates in blood has received considerable attention. Clark and Litchfield found that propylene glycol dinitrate (PGDN) when incubated with rat blood was broken down to inorganic nitrite (N02~), inorganic nitrate (N03~), and propylene glycol mononitrate (PGMN), with the 2-isomer predominating. Ethylene glycol dinitrate (EGDN) was known to be metabolized in a similar fashion, and with both of these dinitrates, the hydrolysis occurred in the erythrocytes, not in the plasma. We recently conducted a detailed comparison of the toxicities of two organic dinitrates, PGDN and triethylene glycol dinitrate (TEGDN). In preliminary experiments with rats, it was observed that PGDN produced larger amounts of met-Hb ■ * The opinions expressed herein are those of the authors and do not necessarily reflect the view o the Navy Department or the naval service at large.