Antioxidants act as intermediates by picking up the high unselective of radicals and transferring it to other molecules. In this process the reactivity is reduced and becomes selective. This channeling of the can cause selective toxicity. The antioxidant quercetin is known to reactivity towards thiol groups. The present study compares the thiol of quercetin with that of 4'O-methylquercetin (tamarixetin) towards kinase (CK), a vital protein that contains a critical thiol moiety. Our showed that oxidized quercetin and oxidized tamarixetin both adduct CK, then loses its enzymatic function. Ascorbate, an important antioxidant network, is able to prevent adduction to and thus the the enzyme by tamarixetin but not by quercetin. Apparently, tamarixetin thiol toxic than quercetin, because-rather than adduction to CK- quinone prefers to pass reactivity to the antioxidant network, i.e., to ascorbate. The findings exemplify that radical scavenging flavonoids reactivity of radicals and act as a pivot in directing the way the channeled. A mere minor structural difference of only one methyl moiety quercetin and tamarixetin appears to have a high impact on the toxicity.