PHYSICAL AND CHEMICAL SCAVENGING OF SINGLET MOLECULAR-OXYGEN BY TOCOPHEROLS

Singlet molecular oxygen (1O2) arising from the thermal decomposition of the endoperoxide of 3,3′-(1,4-naphthylidene) dipropionate was used to assess the effectiveness of α-, β-, γ-, and δ-tocopherol in the physical quenching as well as the chemical reaction of 1O2. The relative physical quenching efficiencies of the tocopherol homologs were found to decrease in the order of α ≥ β > γ > δ-tocopherol. The ability of physical quenching depends on a free hydroxyl group in position 6 of the chromane ring. Chemical reactivity of the tocopherol homologs with 1O2 was low, accounting for 0.1-1.5% of physical quenching with β-tocopherol showing particularly low reactivity, resulting in the sequence α > γ > δ > β-tocopherol. Tocopheryl quinones were products of all tocopherol homologs, and in addition a quinone epoxide was a major product from γ-tocopherol. This quinone epoxide was not cleaved by rat liver microsomal epoxide hydrolase; however, it reacted further with 1O2. It is concluded that methylation in position 5 of the chromane ring enhances physical quenching of 1O2, whereas chemical reactivity is favored by a methylated position 7. In view of the fact that β-tocopherol is as effective as α-tocopherol in physical quenching of 1O2 but shows very low chemical reactivity, this tocopherol homolog might be particularly suitable for biological conditions in which an accumulation of oxidation products might weaken the antioxidant defense. © 1990.