Garlic has been used for millennia in folk medicine of many cultures to treat cardiovascular diseases and other disorders [1–8]. It has been shown in many cases that the protective effect of garlic is associated with its antioxidant properties [7, 8]. The antioxidant properties of some garlic extracts used in this work have been studied. It has been found that aqueous extract of raw garlic scavenges hydroxyl radicals [20, 31, 32] and superoxide anion , inhibits lipid peroxidation , LDL oxidation , the formation of lipid hydroperoxides [20, 31, 32], and in vivo enhances endogenous antioxidant system  and prevents oxidative stress to the heart [34, 35]. Chronic administration of raw garlic homogenate increases catalase and superoxide dismutase in rat heart  and protects heart against oxidative damage induced by adriamycin  or ischemia and reperfusion . Aqueous extract of garlic powder are also able to scavenge hydroxyl radicals  and superoxide anion . The heated aqueous extract of garlic powder maintains its ability to scavenge hydroxyl radicals . In addition the ability of the aqueous extracts from boiled garlic cloves to scavenge hydroxyl radicals, superoxide anion, and hydrogen peroxide is not altered (unpublished results from our group). To our knowledge, additional antioxidant properties of the extracts from microwave-treated garlic cloves or from pickled garlic have not been studied.
Furthermore, the antioxidant properties of some isolated garlic compounds also have been studied. Allicin, the main component in aqueous extract from raw garlic and garlic powder, scavenges hydroxyl radicals and inhibit lipid peroxidation  and prevents the lung damage induced by ischemia-reperfusion . The antioxidant properties of allicin may explain, at least in part, the ability of these extracts (from raw garlic or garlic powder) to inhibit Cu2+-induced lipoprotein oxidation in human serum. Alliin, the main component in extracts from boiled garlic cloves, microwave-treated garlic cloves and pickled garlic, scavenges hydroxyl radicals , hydrogen peroxide , and inhibits lipid peroxidation  and LDL oxidation . This may explain, at least in part, the ability of boiled garlic, microwave-treated garlic, or picked garlic to inhibit Cu2+-induced lipoprotein oxidation in human serum. Interestingly, it has been shown that another garlic compounds such as S-allylcysteine [10, 13], N-acetyl-S-allylcysteine , S-allylmercaptocysteine , alliin , allixin , and S-ethylcysteine, N-acetylcysteine, diallyl sulfide, and diallyl disulfide  are able to inhibit Cu2+-induced LDL oxidation. The antioxidant properties of S-allylcysteine , S-allylmercaptocysteine , diallyl sulfide , and diallyl disulfide  also have been seen in vivo in an experimental model of nephrotoxicity induced by gentamicin.
Our data strongly suggest that the ability of garlic to prevent Cu2+-induced lipoprotein oxidation in human serum is preserved in spite of inactivation of allinase by boiling, microwave or pickling or by the heating of garlic extracts and that the compound(s) involved in the inhibition of Cu2+-induced lipoprotein oxidation are heat stable. Our data are in contrast with previous studies in the literature showing that the heating may impair significantly several garlic properties. For example, microwave-treatment for 1 min impaired the anticancer properties of garlic  and the heating of garlic cloves by 15 min impairs significantly its ability to inhibit thromboxane B2 synthesis , and platelet aggregation , and the cyclooxygenase activity . The heating by 10 min at 100°C reduced the bactericidal activity against Helicobacter pylori . Interestingly, Kasuga et al.  have found that garlic extracts, prepared from boiled cloves, show efficacy in the following three experimental models: testicular hypogonadism induced by warm water treatment, intoxication of acetaldehyde, and growth of inoculated tumor cells, and Prasad et al.  found that the heating did not modify the ability of garlic extract to scavenge hydroxyl radicals. The data from Prasad et al.  and Kasuga et al.  strongly suggest that some garlic properties may remain unmodified after heating. Our data are in agreement with those of Prasad et al.  suggesting that the ability to inhibit Cu2+-induced lipoprotein oxidation is also preserved after the heating of garlic.
In addition, it was found that the garlic extracts used in our study were unable to chelate Cu2+ suggesting that the ability of these extracts to inhibit Cu2+-induced lipoprotein oxidation is not secondary to Cu2+-chelation. Only RG showed a weak Cu2+-chelation activity, which was more evident at 295 nm. Based on previous data with aged garlic extracts  and some individual garlic compounds such as S-ethylcysteine, N-acetylcysteine, diallyl sulfide, and diallyl disulfide , we expected that our garlic extracts had Cu2+-chelating activity. The discrepancy with our data may simply reflect differences in composition in each garlic extract. This is additionally supported by the fact that the diethyl ether extract from aged garlic extract has no Cu2+-chelating activity . The precise mechanism by which our extracts inhibit Cu2+-induced lipoprotein oxidation remains to be studied.