Herbs, spices and cardiovascular disease 131
components of the extracellular matrix. This trapping increases the residence time of
LDL within the vessel wall where the lipoprotein may undergo chemical modifications.
The LDL becomes oxidised by local free radicals and as oxidised LDL it attracts
circulating monocytes to the vessel wall. The modified or oxidised LDL can be
ingested by macrophages contributing to the development of foam cells.
Following oxidation of the LDL, the next stage is the attraction of leucocytes,
primary monocytes and T lymphocytes. After the monocytes have adhered to the
luminal surface they may penetrate into the subendothelial space by slipping between
the junctions. Once localised beneath the endothelium, monocytes differentiate into
macrophages, the phagocytic cells that are able to ingest oxidised LDL. The macrophages
then become lipid-laden foam cells, the primary constituent of the fatty streak. More
recently oxidised LDL has been recognised as playing a more important role in
vascular dysfunction leading to atherosclerosis rather than native LDL (Battacharyya
and Libby, 1998). Oxidation of the LDL is a key stage in the process of atherosclerosis.
The antioxidant activity of herbs may have an important role at this stage of the
disease.
8.3.2 Metabolic effect of antioxidants
Herbs and spices contain high levels of antioxidants which contribute to their
pharmaceutical value (Dragland et al., 2003). In the plants these compounds are
necessary because they provide a protection against excessive input of solar energy
during photosynthesis. Hazardous excess energy is eliminated and oxidative damage
to the plant cell prevented. During the oxidative process of cellular metabolism
reactive oxygen species and reactive nitrogen species are released. The most reactive
are the free radicals of which the most oxidising and therefore the most reactive is the
hydroxyl radical (OH–) which can oxidise, i.e., remove an electron from almost any
molecule and thus damage cell structures and cell metabolites. The function of the
antioxidant system is to facilitate the donation of electrons to the free radicals thereby
reducing the chemical energy of the hydroxyl radical or other reactive oxygen or
reactive nitrogen species. The antioxidant itself then needs to be progressively reduced
in a step-wise manner until the organic molecule is finally released as oxygen or
carbon dioxide. Plants contain large amounts of many antioxidants compounds such
as polyphenols, carotenoids, tocopherols, glutathione and ascorbic acid that can unite
chemically and non-enzymically with an oxygen donor such as a free radical (Blomhoff,
2005). It is these compounds in herbs and spices that provide the essential antioxidant
component in the diet of animals and humans (McCord, 2000).
In addition to the chemical non-enzymic protection of the antioxidant compounds,
there is an anti-oxidant system that consists of a number of enzymes which are
referred to collectively as phase 2 enzymes (Benzie, 2003). These enzymes remove
the reactive oxygen species and catalyse the conversion of toxic metabolites to easily
excreted compounds. The enzyme, superoxide dismutase, provides for the elimination
of superoxide radicals and catalases and glutathione peroxidases for the elimination
of hydrogen peroxide and organic peroxides. Members of the glutathione transferase
family, g-glutamyl cysteine synthetase and NAD(P)H:quinine reductase are also essential
in antioxidant defence. Breakdown products of the sulphur-containing compounds
from the Allium species may also induce phase 2 enzymes. It has been suggested that
the anti-oxidant compounds and the phase 2 enzymes work together in sequence
(Blomhoff, 2005). Antioxidant compounds such as quercetin may donate an electron