N-Hydroxy-N-arylacetamides. V. Differences in the Mechanism of Haemoglobin Oxidation in Vitro, by N-hydroxy-4-Chloroacetanilide and N-Hydroxy-4-Chloroaniline

Abstract

1. Autoxidation of N-hydroxy-4-chloroaniline(I) in buffer pH7.4 was rapid and yielded 4,4′-azoxybischlorobenzene, 4-chloronitrosobenzene, 4-chloronitrobenzene, and 4-chlorophenyl nitroxide. In contrast, autoxidation of N-hydroxy-4-chloroacetanilide(II) was very slow, since in ether and water 78 and 92%, respectively, had decomposed in six months.

2. Haemoglobin(HbO₂)-catalysed autoxidation of (I) occurred at a molar ratio of haemoglobin-Fe²⁺ to (I) of <0.25 and was accompanied by ferrihaemoglobin(HbFe³⁺)-formation and oxygen consumption. Coupled oxidation of HbO₂ with (I) occurred at a molar ratio of >0.2 and was accompanied by liberation of oxygen and the formation of HbFe³⁺, haemoglobin-4-chloronitrosobenzene complex, HbO₂, desoxyhaemoglobin, 4-chloronitrosobenzene, 4-chloronitrobenzene, 4-chloroaniline, 4,4′-azoxybischlorobenzene, and 4-chlorophenyl nitroxide. At an equimolar ratio of 10⁻³m haemoglobin-Fe²⁺ to (I), 96% HbO₂ was converted into HbFe³⁺ (50%) and haemoglobin-4-chloronitrosobenzene complex in the initial fast phase of the reaction, but only 34% of the bound oxygen was liberated, the rest was sequentially reduced to water. (I) completely disappeared, and 4-chloronitrosobenzene was the major metabolite, mainly bound to haemoglobin.

3. Chemical oxidation of (II) by PbO₂ in benzene produced acetyl 4-chlorophenyl nitroxide, whose spontaneous decomposition gave 38% 4-chloronitiosobenzene, 33% N-acetoxy-4-chloroacetanilide, 10% 4-chloroacetanilide, and 8% 4-chloronitrobenzene. Its spontaneous decomposition in water also followed second order kinetics, K=3501mol⁻¹ sec⁻¹ and yielded N-2-acetylamino-5-chlorophenyl)-p-benzoquinoneimine-N-oxide in addition.

4. In the coupled oxidation of 10⁻³M haemoglobin-Fe²⁺ with 10⁻³M (II), 75% HbFe³⁺ was formed after 1 h, but only one third of the equivalent of oxygen was released. and two thirds were reduced to water. Concentration of (II) decreased by 5% only, indicating that one mol of (II) had catalysed the oxidation of 15 equivalents of haemoglobin-Fe²⁺. The identity of the product pattern formed with HbO₂ with that produced by chemical one-electron oxidation indicated that oxygen bound to haemoglobin also functions as an acceptor for electrons from (II) as from (I), but the different redox potentials can explain why the secondary aromatic nitroxide was catalytically active and the primary nitroxide was not.