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Abstract
The reactions of human hemoglobin and sperm whale myoglobin with ethyl nitrite under aerobic conditions have been examined in kinetic detail. Ethyl nitrite converts two equivalents of oxyhemoglobin or oxymyoglobin to their oxidized counterparts with concurrent production of one equivalent each of molecular oxygen, nitrate ion, and ethyl alcohol. Inverse first order kinetic dependence on the concentration of molecular oxygen has been observed and is interpreted by a mechanism in which oxygen dissociation from the oxyhemoprotein occurs prior to rate-limiting oxidation by ethyl nitrite. The rate constant for ethyl nitrite oxidation of hemoglobin from which the fourth oxygen has dissociated is calculated to be 45 times greater than the corresponding rate constant for oxidation of deoxyhemoglobin. This rate enhancement is proposed to be a reflection of the oxidative susceptibility of the R and T conformational states of hemoglobin. Results obtained for the oxidation of myoglobin confirm this interpretation as do kinetic data for hemoglobin and myoglobin oxidations by iron(III) and copper(II) complexes. The effects of organic phosphates on rates for hemoglobin oxidations are interpreted in terms of oxidation inhibition by molecular oxygen.
Highlights
Vertstwo equivalents of oxyhemoglobin or oxymy- 93 cysteine residues of hemoglobin as having aprotective role oglobin to their oxidized counterpartswith concurrent fortheheme ironagainstoxidation (7)
The autocatalytic osftathgee reaction has been attributed to the actioonf MetHb‘ onHbOz (Z), even though such anunlikely event does not explain the initiation of autocatalytic behavior that is induced by nitrite and does not accountfor the role of hydrogen peroxide that is reported to be generated during the reactionof nitrite with oxyhemoglobin (5)
Oxyhemoglobin Oxidation-The spectral changesobserved during the course of the reaction of ethyl nitrite with oxyhemoglobin A in oxygen-saturated solution are described in Fig
Summary
The reactions of humanhemoglobinand spermwhale attributed to a preference for the R state of hemoglobin in myoglobin with ethyl nitrite under aerobic conditions nitrite oxidations (1). Inverse first order kinetic dependence on the concentration of molecular oxygen has been observed and is interpreted by a mechanism in which oxygen dissociation from the oxyhemoprotein occurs prior to rate-limiting oxidation by ethyl nitrite. The rate constant for ethyl nitrite oxidation of hemoglobin from which the fourth oxygen has dissociated is calculated to be 45 times greater than the corresponding rate constant for oxidation of deoxyhemoglobin. Prior descriptions of oxidative transformations withoxygenated hemoproteins haveprovided two distinctivepathways for molecular oxygen utilization: conversion of bound dioxygen to peroxide by the transferof 1 electron from the chemical oxidant and 1 electron from the iron(I1) to which that oxygen is bound (3,9-11) and inhibition of oxidation by bound dioxygen (12-15) In the former pathway, the rate of oxidation is directly dependent on the concentration of bound dioxygen, and oxygen is consumed in the oxidative process. Interferences from nitrite ion and from the hemoproteins were determined and found to be negligible
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