Investigation of the chemical basis of nitroalkane toxicity: tautomerism and decomposition of propane 1- and 2-nitronate under physiological conditions.
Unlike primary nitroalkanes, such as 1-nitropropane, the secondary nitroalkane 2-nitropropane is geno- and hepatotoxic. Nitroalkanes exist in equilibrium with alkane nitronates. In order to investigate the relationship between nitroalkane toxicity and generation and stability of nitronates, propane 1- or 2-nitronate (4-6 mM) were incubated in buffer (pH 3.8 -7.4) in the absence or presence of cysteine. Equilibrium formation and degradation were studied by 1H-NMR spectroscopy and ion pair HPLC chromatography. Propane 1-nitronate generated 1-nitropropane rapidly and almost quantitatively. In the case of propane 2-nitronate equilibrium at pH 7.4 was reached within 8 h, when 48% of initial nitronate had tautomerised to 2-nitropropane. The pKa of the reaction 2-nitropropane less than--greater than propane 2-nitronate measured by HPLC was 7.63. Equilibrium formation, hydrolysis and reduction of nitronates were pH-dependent and, in the case of propane 2-nitronate, yielded mainly acetone, nitrite and acetone oxime, apart from 2-nitropropane. Hydrolysis of propane 2-nitronate (4 mM) to nitrite was modulated by cysteine (4 mM) and p-methoxyphenol (0.4 mM). At pH 7.4 they increased nitrite generation by 300 and 28%, respectively, at pH 4.8 they decreased nitrite formation by 91 and 82%, respectively, probably by scavenging radical intermediates. Differences between nitroalkanes in terms of content of nitronate tautomer at equilibrium are probably an important chemical determinant of their toxic potential.
Department of Pharmaceutical Sciences, Aston University, Birmingham, U.K.
I
I
Linhart
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A
Gescher
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B
Goodwin
D000818:Animals; D002851:Chromatography, High Pressure Liquid; D006868:Hydrolysis; D007536:Isomerism; D009682:Magnetic Resonance Spectroscopy; D051379:Mice; D008862:Microsomes, Liver / Q000187:drug effects; D009595:Nitroparaffins / Q000737:chemistry / Q000633:toxicity*; D011407:Propane / Q000031:analogs & derivatives* / Q000737:chemistry / Q000633:toxicity
D009595:Nitroparaffins; C035314:1-nitropropane; C016617:2-nitropropane; D011407:Propane