Biomedical Engineering Reference
In-Depth Information
with the abstracted hydrogen atom to form a lipid hydroperoxide
(ROOH). Cyclic peroxides can form when a peroxyl radical attacks a
double bondwithin the same fatty acid residue. (3) Termination: the chain
reaction terminates when two lipid peroxyl radicals combine to produce a
nonradical species, or when a radical is halted by binding to antioxidants
such as
-tocopherol (Halliwell and Gutteridge, 2007).
Lipid peroxidation can produce DNA-damaging aldehydes such as
malondialdehyde (MDA), 4-hydroxynonenal (4-HNE), and F-isopros-
tanes. The decomposition of lipid peroxides by heating or reaction with
metal ions creates a wide variety of cytotoxic products, which can
produce more radicals that can initiate further lipid peroxidation
(Gutteridge and Quinlan, 1983).
MDA is produced either from the peroxidation of polyunsaturated
fatty acids (PUFA s) with more than two double bonds, or enzymatically
during the metabolism of eicosanoids. At physiological pH, most MDA
exists as the enolate ion, which has low reactivity toward amino groups
in proteins (Halliwell and Gutteridge, 2007). At a lower pH, MDA
exists as the undissociated enol form in equilibrium with its keto form
and exhibits a higher reactivity toward proteins, which can attack
residues resulting in intra- and intermolecular cross-links (Esterbauer
et al., 1991). MDA can react with DNA, more specifically guanine
bases, to create G to T transversions, A to G transitions, C to T
transitions, frameshifts, and deletions, with potentially mutagenic
consequences (Marnett, 2000). MDA is metabolized to malonic semi-
aldehyde by aldehyde dehydrogenase, and this product is decarboxy-
lated to acetaldehyde, and is finally metabolized to acetic acid again by
aldehyde dehydrogenase (Halliwell and Gutteridge, 2007).
The thiobarbituric acid (TBA) assay is a colorimetric assay com-
monly used to detect MDA, whereby a sample is heated with TBA in
acid and a pink color develops, reflecting the formation of thiobarbituric
acid-reactive substances (TBARS). Although technically easy to per-
form, the specificity of this assay has been questioned (Gutteridge and
Quinlan, 1983). It has been suggested that much of the MDA measured
may actually be generated during the assay workup since the amount of
free MDA in most lipid peroxidizing systems is too low to be detected
by the assay. As much as 98% of the MDA detected can be generated
a
