Molecular mechanisms of 4-hydroxy-2-nonenal and acrolein toxicity: nucleophilic targets and adduct formation

Abstract

Acrolein and 4-hydroxy-2-nonenal (HNE) are byproducts of lipid peroxidation and are thought to play central roles in various traumatic injuries and disease states that involve cellular oxidative stress, for example, spinal cord trauma, diabetes, and Alzheimer's disease. In this review, we will discuss the chemical attributes of acrolein and HNE that determine their toxicities. Specifically, these aldehydes are classified as type 2 alkenes and are characterized by an alpha,beta-unsaturated carbonyl structure. This structure is a conjugated system that contains mobile pi-electrons. The carbonyl oxygen atom is electronegative and can promote the withdrawal of mobile electron density from the beta-carbon atom causing regional electron deficiency. On the basis of this type of electron polarizability, both acrolein and HNE are considered to be soft electrophiles that preferentially form 1,4-Michael type adducts with soft nucleophiles. Proteomic, quantum mechanical, and kinetic data will be presented, indicating that cysteine sulfhydryl groups are the primary soft nucleophilic targets of acrolein and HNE. This is in contrast to nitrogen groups on harder biological nucleophiles such as lysine or histidine residues. The toxicological outcome of adduct formation is not only dependent upon residue selectivity but also the importance of the targeted amino acid in protein function or structure. In attempting to discern the toxicological significance of a given adduct, we will consider the normal roles of cysteine, lysine, and histidine residues in proteins and the relative merits of corresponding adducts in the manifestations of diseases or toxic states. Understanding the molecular actions of acrolein and HNE could provide insight into many pathogenic conditions that involve initial cellular oxidative stress and could, thereby, offer new efficacious avenues of pharmacological defense.

Figure 1

This figure presents line structures for acrolein, HNE and several structurally related α,β-unsaturated carbonyl derivatives of the type-2 alkene chemical class.

Figure 2

The concept of soft electrophilicity is illustrated in this figure, which shows the color-coded electrostatic potential maps for formaldehyde (2A), ethylene (2B) and acrolein (2C; calculated using Spartan ’04, Wavefunction Inc., Irvine, CA). The line structures for each chemical are inserted within the corresponding potential figure. The color gradient for each map illustrates how charge is distributed across the molecule and, therefore, indicates the relative degree to which the corresponding atoms attract oppositely charged atoms. Accordingly, red signifies the most negative electrostatic potential and is used for regions that attract positively charged molecules most strongly. Blue denotes areas with the most positive electrostatic potential and is used for regions that attract negatively charged molecules most strongly. The orange-yellow-green spectrum indicates intermediate (from negative to positive, respectively) electrostatic potential. For each chemical, a numerical example of local electron density or distribution (expressed as kJ/mol) is provided for each color. If we first consider the relatively simple polar covalent bond of formaldehyde (2A), it is clear that the highly electronegative oxygen atom (Pauling electronegativity = 3.44) draws electron density as indicated by the localized red-colored zone (-47.04) from the less electronegative carbon (2.55) and hydrogen (2.20) atoms. Here, the resulting electron deficiency of the carbon-hydrogen bonds is reflected in the green-blue gradient; i.e., respective electron density from -11.63 to 27.57. We next consider the carbon-carbon double bond of ethylene (2B). A red-yellow gradient (from –22.52 to –12.56, respectively) is centered over the double bond, which indicates the covalent sharing of electron density between two atoms (carbon) of equal electronegativity. Acrolein (2C) combines the carbonyl of formaldehyde and the carbon-carbon double bond of ethylene, which is considered to be a conjugated system. As the corresponding red color-coding indicates, the electronegative carbonyl oxygen atom has withdrawn electron density (-51.40) from the normally electron rich carbon-carbon double bond (see the ethylene double bond; 2B). As a result, the β-carbon atom becomes an electron deficient or electrophilic center (green = -8.32). Such electron delocalization is possible because the π orbitals of the conjugated α,β-unsaturated carbonyl structure overlap. Consequently, the respective π electrons are mobile or polarizable and can, therefore, relocate to the electronegative oxygen atom. The quantum mechanical parameter softness (σ), is an index of π electron mobility and, based on their respective σ values (Table 1), acrolein and HNE are relatively soft electrophiles that will rapidly form adducts with sulfhydryl groups.