Mechanisms of scavenging superoxide, hydroxyl, nitrogen dioxide and methoxy radicals by allicin: catalytic role of superoxide dismutase in scavenging superoxide radical

Abstract

The occurrence of free radicals such as superoxide radical anion (${\hbox {O}_{2}}^{\cdot -}$), hydroxyl radical ($\hbox {O}\hbox {H}^{\cdot }$), methoxy radical (${\hbox {OC}\hbox {H}_{3}}^{\cdot }$) and nitrogen dioxide radical (${\hbox {N}\hbox {O}_{2}}^{\cdot }$) inside living cells can be very hazardous as these radicals can modify structures and functions of different biomolecules. Both exogenous antioxidants taken as components of diet and endogenous enzyme antioxidants can scavenge ${\hbox {O}_{2}}^{\cdot -}$ separately. Mechanisms of scavenging ${\hbox {O}_{2}}^{\cdot -}$ by combinations of exogenous and endogenous enzyme antioxidants are not understood properly. In order to understand mechanisms of scavenging ${\hbox {O}_{2}}^{\cdot -}$, ${\hbox {O}\hbox {H}}^{\cdot }$, ${\hbox {OC}\hbox {H}_{3}}^{\cdot }$, and ${\hbox {N}\hbox {O}_{2}}^{\cdot }$ by allicin, and possible roles of superoxide dismutase (SOD) in scavenging ${\hbox {O}_{2}}^{\cdot -}$, density functional theory was employed. Marcus theory was also employed to study scavenging of ${\hbox {O}\hbox {H}}^{\cdot }$, ${\hbox {OC}\hbox {H}_{3}}^{\cdot }$ and ${\hbox {N}\hbox {O}_{2}}^{\cdot }$ by electron transfer. In order to find the most probable mechanisms of scavenging these radicals, different types of reactions such as one and two hydrogen atom transfer (HAT), single electron transfer (SET) and sequential proton loss electron transfer (SPLET) processes were investigated. It is found that allicin can scavenge ${\hbox {O}_{2}}^{\cdot -}$ via double hydrogen atom transfer catalyzed by Fe-SOD efficiently. Further, allicin can scavenge ${\hbox {O}\hbox {H}}^{\cdot }$ by SET, and ${\hbox {OC}\hbox {H}_{3}}^{\cdot }$ and ${\hbox {N}\hbox {O}_{2}}^{\cdot }$ by SPLET mechanisms most efficiently. Our results are in qualitative agreement with the available experimental data wherever these are available.

Graphical Abstract

SYNOPSIS It is shown for the first time, theoretically, that while allicin in the absence of a catalyst would not scavenge $\hbox {O2}\cdot ^{-}$, it would do so efficiently in the presence of the Fe-superoxide dismutase enzyme. It is also shown that allicin is not a scavenger of hydroxyl, nitrogen dioxide and methoxy radicals but its anion would scavenge efficiently all the three radicals by single electron transfer (SET). Further, it is shown that scavenging of nitrogen dioxide and methoxy radicals would occur very efficiently by sequential proton loss electron transfer (SPLET) from allicin.