Sterol metabolism. XLII. On the interception of singlet molecular oxygen by sterols

Another cholesterol hypothesis: cholesterol as antioxidant

Current emphasis on cholesterol as agency if not cause of human atherosclerosis and subsequent cardiovascular disease ignores the essentiality of cholesterol in life processes. Additionally ignored is the ubiquitous presence of low levels of oxidized cholesterol derivatives (oxysterols) in human blood and select tissues, oxysterols also implicated in atherosclerosis. Whereas such oxysterols may be regarded putatively as agents injurious to the aorta, an alternative view of some of them is here proposed: that B-ring oxidized oxysterols of human blood represent past interception of blood and tissue oxidants in vivo by cholesterol as an ordinary aspect of oxygen metabolism. Such interception and subsequent efficient hepatic metabolism of oxysterols so formed, with biliary secretion and fecal excretion, constitute as in vivo antioxidant system. Whether cholesterol, oxysterols, oxidized lipoproteins, or oxidants in blood, singly or in concert, cause or exacerbate human atherosclerosis remains to be understood.

Lipid peroxidation during the oxidation of haemoproteins by hydroperoxides. Relation to electronically excited state formation

The formation of electronically excited states during hydroperoxide metabolism is analysed in terms of recombination reactions involving secondary peroxyl radicals and scission of the O-O bond of peroxides by haemoproteins, mainly myoglobin. Both processes may be sequentially interrelated, for the cleavage of H2O2 by metmyoglobin leads to the formation of a strong oxidizing equivalent with the capability to promote peroxidation of polyunsaturated fatty acids. The decomposition of lipid hydroperoxides by ferryl-hydroxo complexes, as that formed during the oxidation of metmyoglobin by H2O2, is a source of peroxyl radicals, the recombination of which proceeds with elimination of a conjugated triplet carbonyl or singlet oxygen.