|
CHOLESTEROL AS
A SINGLET OXYGEN DETECTOR IN CELL MEMBRANES
Girotti, Albert1
and Korytowski, Witold2
Medical College of Wisconsin1
Jagiellonian University2
Abstract-
Cholesterol (Ch) is found in all membrane compartments of eukaryotic
cells, most being located in the plasma membrane, where it comprises
40-45 mol% of the total lipid. Oxidation of Ch in homogeneous solution
or membranes affords a relatively small number of chromatographically
separable intermediates/products, which can be used as mechanistic reporters.
Free radical-mediated Ch oxidation (as in type I photoreactions) gives
two prominent primary hydroperoxides, 7 -OOH
and 7 -OOH,
which accumulate at nearly equal initial rates. Singlet oxygen (1O2)-mediated
oxidation (as in type II photoreactions) gives three other hydroperoxides:
5-OOH,
6-OOH,
and 6-OOH,
the first of which accumulates ~10-times faster than the others in membrane
systems. 7-
and 7-OOH
cannot be generated directly by 1O2. However,
unless precautionary steps are taken, some rearrangement of 5-OOH
to 7-OOH
might occur, e.g. during sample handling, leading one to wrongly deduce
that a reaction is partly free radical-mediated. Many of the membrane-targeting
sensitizers examined to date, including merocyanine 540, aluminum phthalocyanine
disulfonate and Photofrin, act primarily as 1O2
generators, producing only traces of 7/7-OOH
compared with 5-OOH
and 6/6-OOH
in model membranes and cells. In the presence of reductants and catalytic
iron, a type II hydroperoxide, 5-OOH,
can undergo 1-electron reduction to free radical species, which trigger
chain reactions not easily distinguished from those initiated by type
I chemistry. In cells these light-independent downstream reactions can
exacerbate the damaging effects of photoperoxidation alone. Although
detection of 5-OOH
or 5-OH
(the alcohol analogue) in an oxidized membrane typically specifies 1O2
intermediacy, recent evidence points to an exception. Peroxide-primed,
iron/ascorbate-induced chain peroxidation of Ch-containing liposomes
gave no detectable 5-OOH
or 5-OH,
yet the latter appeared when the reaction was carried out in the presence
of nitric oxide (.NO). Under investigation is the possibility
that 5-OH
arose via rearrangement of the .NO adduct 7-ONO
to 5-ONO,
followed by reduction. (Supported by NIH: CA70823 and CA72630)
Keywords: Singlet
oxygen, Lipid peroxidation, Cholesterol
|
