Abstract
Photooxidation of cholesterol in liposomes with hematoporphyrin sensitization has been studied. With liposomal samples in which the hematoporphyrin is incorporated in the membrane, the yield of the characteristic singlet oxygen product, 3β-hydroxycholest-6-ene 5α-hydroperoxide, was approximately 6 times greater than that observed in the samples in which the hematoporphyrin was outside the membrane. Small amounts of 3β-hydroxycholest-5-ene 7α- and 7β-hydroperoxides, radical autoxidation products, were formed in both samples. Photolysis of a dispersion of cholesterol in an aqueous solution of hematoporphyrin gave no singlet oxygen products. It is concluded from these results that endogenous singlet oxygen when formed in the phospho-lipid membrane has a sufficiently long lifetime to effect oxygenation of cholesterol; whereas exogenous singlet oxygen generated outside the membrane is quenched by solvent before appreciable diffusion into the membrane can occur.
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