Premium
PHOTOPEROXIDATION OF CHOLESTEROL IN HOMOGENEOUS SOLUTION, ISOLATED MEMBRANES, AND CELLS: COMPARISON OF THE 5α‐ AND 6β‐HYDROPEROXIDES AS INDICATORS OF SINGLET OXYGEN INTERMEDIACY
Author(s) -
Korytowski Witold,
Bachowski Gary J.,
Girotti Albert W.
Publication year - 1992
Publication title -
photochemistry and photobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.818
H-Index - 131
eISSN - 1751-1097
pISSN - 0031-8655
DOI - 10.1111/j.1751-1097.1992.tb09594.x
Subject(s) - chemistry , singlet oxygen , photochemistry , yield (engineering) , membrane , allylic rearrangement , bilayer , dioxetane , liposome , oxygen , methanol , organic chemistry , catalysis , chemiluminescence , biochemistry , materials science , metallurgy
— Singlet oxygen ( 1 O 2 ) can react with cholesterol (Ch) to give three possible ene‐addition hydroperoxides: 3β‐hydroxy‐5α‐cholest‐6‐ene‐5‐hydroperoxide (5α‐OOH), 3β‐hydroxycholest‐4‐ene‐6α‐hydroperoxide (6α‐OOH), and 3β‐hydroxycholest‐4‐ene‐6β‐hydroperoxide (6β‐OOH). The rates of dye‐sensitized photogeneration and also the fates of 5α‐OOH and 6β‐OOH in membrane bilayers have been studied and compared. Irradiation of unilamellar [ 14 C]Ch/phospholipid vesicles in the presence of aluminum phthalocyanine tetrasulfonate or merocyanine 540 resulted in formation of 5α‐OOH and 6β‐OOH, as determined by high performance liquid chromatography with radiochemical or electrochemical detection. The initial rate of 6β‐OOH formation was 335% that of 5α‐OOH in a variety of liposomal systems. However, after a lag, 5α‐OOH invariably decayed via allylic rearrangement to 7α‐OOH (also known to be a free radical product), whereas 6β‐OOH accumulated in unabated fashion until Ch depletion became limiting. Photooxidation of Ch in an isolated natural membrane (erythrocyte ghost) or in L1210 leukemia cells gave similar results. When the reaction was carried out in pyridine or methanol, the rate of 6β‐OOH formation relative to 5α‐OOH was reduced by approximately half, with essentially no isomerization of the latter to 7α‐OOH. These results suggest that (i) environmental factors in a lipid bilayer somehow make photogeneration of 6β‐OOH more favorable than in homogeneous solution; and (ii) due to its relative stability, 6β‐OOH may be a more reliable probe of 1 O 2 intermediacy than 5α‐OOH.