J. Belkner et al., THE RABBIT 15-LIPOXYGENASE PREFERENTIALLY OXYGENATES LDL CHOLESTEROL ESTERS, AND THIS REACTION DOES NOT REQUIRE VITAMIN-E, The Journal of biological chemistry, 273(36), 1998, pp. 23225-23232
The oxidation of low density lipoprotein (LDL) by mammalian 15-lipoxyg
enases (15-LOX) was implicated in early atherogenesis, We investigated
the molecular mechanism of 15-LOX/LDL interaction and found that duri
ng short term incubations, LDL cholesterol esters are oxygenated prefe
rentially by the enzyme. Even when the LDL particle was loaded with fr
ee linoleic acid, cholesteryl linoleate constituted the major LOX subs
trate. In contrast, only small amounts of free oxygenated fatty acid i
somers were detected, and re-esterification of oxidized fatty acids in
to the LDL ester lipid fraction was ruled out. When LDL was depleted f
rom alpha-tocopherol, specific oxygenation of the cholesterol esters w
as not prevented, and the product pattern was not altered. Similar res
ults were obtained at low (LDL/LOX ratio of 1:1) and high LOX loading
(LDL/LOX ratio of 1:10) of the LDL particle. During long term incubati
ons (up to 24 h), a less specific product pattern was observed. Howeve
r, when the hydroperoxy lipids formed by the 15-LOX were immediately r
educed by the phospholipid hydroperoxide glutathione peroxidase, when
the reaction was carried out with vitamin E-depleted LDL, or when the
assay sample was diluted, the specific pattern of oxygenation products
was retained over a long period of time. These data suggest that mamm
alian 15-LOX preferentially oxidize LDL cholesterol esters, forming a
specific pattern of oxygenation products. During long term incubations
, free radical-mediated secondary reactions, which lead to a more unsp
ecific product pattern, may become increasingly important. These secon
dary reactions appear to be suppressed when the hydroperoxy lipids for
med are immediately reduced, when alpha-tocopherol-depleted LDL was us
ed, or when the incubation sample was diluted. It may be concluded tha
t 15-LOX-initiated LDL oxidation constitutes a dual-type oxygenase rea
ction with an initial enzymatic and a subsequent nonenzymatic phase. T
he biological relevance of this dual-type reaction for atherogenesis w
ill be discussed.