The human body is endowed with a wide range of antioxidants to protect cells from damage induced by free radicals and other reactive species. Glutathione (GSH) is one of the most important endogenous hydrophilic antioxidants 
. It is synthesized in many different cell types from its constituting amino acids glutamic acid, cysteine and glycine, and is therefore not required in the human diet 
. The actual antioxidant property of GSH is attributable to the thiol group that is present in its cysteine moiety. As an effective nucleophile, GSH also plays an important role in the protection against electrophilic compounds 
Like GSH, ascorbic acid (vitamin C) is also an important hydrophilic antioxidant. In contrast to GSH, ascorbic acid cannot be synthesized by humans and, as a consequence, is required in the human diet 
. It directly scavenges O2•−
OH and various other radicals.
Phenolic antioxidants comprise α-tocopherol (the most active form of vitamin E) and flavonoids. Like ascorbic acid and most other vitamins, α-tocopherol has to be obtained exclusively from the diet. It is the major lipid-soluble lipoprotein antioxidant 
. α-Tocopherol is localized in biomembranes and functions as an efficient inhibitor of lipid peroxidation. Flavonoids, on the other hand, are not essential nutrients but they form an integral part of the human diet as they are found in fruits, vegetables, nuts and plant-derived beverages such as tea and wine 
. They have a wide range of biological activities 
, but are most commonly known for their antioxidant activity. Quercetin is one of the most frequently studied dietary flavonoids 
. It can scavenge highly reactive species, an activity that is implicated in its health benefits 
The flavonoid of interest to us, that closely resembles the chemical structure of quercetin, is the semi-synthetic flavonoid 7-mono-O-(β-hydroxyethyl)-rutoside (monoHER). MonoHER is the most powerful antioxidant constituent of the registered drug Venoruton® 
, which is used in the treatment of chronic venous insufficiency 
. In vitro screening has shown that monoHER is the most potent protector against cardiotoxicity induced by the anticancer agent doxorubicin within a series of flavonoids 
. Preclinical experiments have confirmed that monoHER is indeed a potential protective agent against doxorubicin-induced cardiotoxicity 
. Because of these promising results, clinical trials are being performed to study the protection of intravenously administered monoHER against doxorubicin-induced cardiotoxicity in cancer patients 
. The antioxidant activity of monoHER is supposed to be involved in its protection. Because of its excellent radical scavenging properties monoHER can effectively protect the heart against free radicals produced by doxorubicin.
During the scavenging of highly reactive species, antioxidants donate an electron or a hydrogen atom to the radical involved, thereby converting the radical into a relatively stable non-radical. In this way the reactivity of the radical is annihilated. However, in this reaction the antioxidant itself is converted into an oxidation product that takes over part of the reactivity of the radical. This oxidized antioxidant might cause damage to vital cellular components 
. For example, when α-tocopherol scavenges free radicals it is oxidized to produce the corresponding tocopheroxyl radicals 
. These radicals can recombine with other radicals, such as peroxyl radicals, thereby neutralizing them 
. However, when these tocopheroxyl radicals cannot be eliminated, lipid peroxidation is aggravated, a phenomenon referred to as tocopherol-mediated peroxidation 
To prevent damage by reactive oxidation products of antioxidants, the human body has a refined network of antioxidants that pass over the reactivity from one antioxidant to another in a controlled way, thereby gradually diminishing the reactivity of the radical and recycling the antioxidants. In this way, it has been shown that ascorbate can regenerate α-tocopherol from tocopheroxyl radicals, thereby preventing tocopherol-mediated peroxidation 
. This illustrates that antioxidants act in synergy to annihilate radicals. Besides preventing damage induced by harmful oxidation products, regeneration is important because it restores the antioxidant network.
The regeneration of α-tocopherol by ascorbate is well documented, however, not much is known on the regeneration of flavonoids. When quercetin protects against free radicals, thiol-reactive oxidation products of quercetin are formed that can cause damage to vital cellular components, a phenomenon known as the quercetin paradox 
. Recently it was found that the oxidation product of monoHER is also reactive towards thiols 
. This might have implications for the applicability of monoHER. However, as mentioned above, antioxidants do not act in isolation to protect against oxidative damage.
The aim of the present study was to determine how monoHER fits into the antioxidant network and to get insight in the regeneration of flavonoids. Particularly, the reactivity of oxidized monoHER towards thiols and ascorbate was investigated. In addition a comparison with quercetin was made.