Prostaglandin endoperoxides. 6. A polar transition state in the thermal rearrangement of 2,3-dioxabicyclo[2.2.1]heptane

Isolevuglandin adducts in disease

SIGNIFICANCE

A diverse family of lipid-derived levulinaldehydes, isolevuglandins (isoLGs), is produced by rearrangement of endoperoxide intermediates generated through both cyclooxygenase (COX) and free radical-induced cyclooxygenation of polyunsaturated fatty acids and their phospholipid esters. The formation and reactions of isoLGs with other biomolecules has been linked to alcoholic liver disease, Alzheimer's disease, age-related macular degeneration, atherosclerosis, cardiac arythmias, cancer, end-stage renal disease, glaucoma, inflammation of allergies and infection, mitochondrial dysfunction, multiple sclerosis, and thrombosis. This review chronicles progress in understanding the chemistry of isoLGs, detecting their production in vivo and understanding their biological consequences.

CRITICAL ISSUES

IsoLGs have never been isolated from biological sources, because they form adducts with primary amino groups of other biomolecules within seconds. Chemical synthesis enabled investigation of isoLG chemistry and detection of isoLG adducts present in vivo.

RECENT ADVANCES

The first peptide mapping and sequencing of an isoLG-modified protein present in human retina identified the modification of a specific lysyl residue of the sterol C27-hydroxylase Cyp27A1. This residue is preferentially modified by iso[4]LGE2 in vitro, causing loss of function. Adduction of less than one equivalent of isoLG can induce COX-associated oligomerization of the amyloid peptide Aβ1-42. Adduction of isoLGE2 to phosphatidylethanolamines causes gain of function, converting them into proinflammatory isoLGE2-PE agonists that foster monocyte adhesion to endothelial cells.

FUTURE DIRECTIONS

Among the remaining questions on the biochemistry of isoLGs are the dependence of biological activity on isoLG isomer structure, the structures and mechanism of isoLG-derived protein-protein and DNA-protein cross-link formation, and its biological consequences.

Oxidized phospholipids, isolevuglandins, and atherosclerosis

Autoxidation of polyunsaturated phosphatidylcholines (PCs) generates isolevuglandins (isoLGs) through rearrangements of isoprostanoid endoperoxides. Within seconds, isoLGs are sequestered by covalent adduction with proteins. Murine plasma isoLG-protein levels increased at least 2.5-fold in response to inflammation. IsoLG-protein adducts accumulate in vivo providing a convenient dosimeter of oxidative stress. Elevated blood isoLG-protein levels present in atherosclerosis (AS) patients point to an independent defect that is not associated with total cholesterol levels, which results in an abnormally high level of oxidative injury in AS. Protein adduction and cross-linking caused by isoLGs can obstruct protein function. For example, it interferes with proteosomal degradation of proteins and, consequently, may result in apoptotic death of smooth muscle cells and destabilization of atherosclerotic plaques. Phospholipid autoxidation also generates biologically active oxidatively truncated PCs through fragmentation of dihydroperoxydienes that can be promoted by alpha-tocopherol. The oxidatively truncated PCs in oxidized low-density lipoprotein (oxLDL) contribute to the etiology of AS by inhibiting enzymatic activities required for normal processing of oxLDL by macrophages. They promote interactions of monocytes with endothelial cells that may foster migration of monocytes into the subendothelial space. They are also ligands for unregulated receptor-mediated uptake of oxLDL by monocyte macrophages leading to foam cell formation.

Isolevuglandins, oxidatively truncated phospholipids, and atherosclerosis

Isolevuglandins (isoLGs) and oxidatively truncated phospholipids are products of lipid peroxidation. Some of these, especially isoLGs and gamma-hydroxyalkenal analogues (e.g., the 5-hydroxy-8-oxo-6-octenoic acid and 9-hydroxy-12-oxo-10-dodecenoic acid esters of 2-lysophosphatidylcholine, HOOA-PC or HODA-PC, respectively) of 4-hydroxy-2(E)-nonenal (HNE), damage proteins by covalent adduction, thereby interfering with their normal functions. These lipid-derived protein modifications may serve as dosimeters of oxidative injury. Elevated plasma levels of isoLG-protein epitopes are associated with atherosclerosis but are independent of total cholesterol, a classical risk factor. Both protein adducts and oxidatively truncated phospholipids (oxPL) can also elicit receptor-mediated cellular responses that include endocytosis of oxidized low-density lipoprotein (LDL) and expression of chemokines, which may foster infiltration of monocyte macrophages into the subendothelial space, where they become foam cells through unregulated endocytosis of oxidatively damaged LDL.

Distinguishing levuglandins produced through the cyclooxygenase and isoprostane pathways

The cyclooxygenase (COX) pathway generates enantiomerically pure levuglandin (LG) E(2) by a rearrangement of the prostaglandin (PG) endoperoxide PGH(2). The isoprostane pathway generates racemic LGE(2) together with stereoisomers, designated collectively as isoLGE(2), through free radical-induced lipid oxidation. Within seconds, both LGs and isoLGs are rapidly sequestered by protein adduction. In theory, the diastereomeric purity of LGE(2)-protein adduct-derived lysyl lactams can reveal the relative contributions of the COX and isoprostane pathways to LGE(2) stereoisomer production in vivo. Notably, however, the detection of LGE(2)-protein adducts does not provide a basis for inferring their formation through the isoprostane pathway in vivo unless the COX pathway can be rigorously excluded. In contrast, LGE(2)structural isomers, designated collectively as iso[n]LGE(2)s, are produced exclusively through the isoprostane pathway. Immunoassays that selectively recognize iso[n]LGE(2)-protein adducts are the only tools available to unambiguously detect and quantify the production of isolevuglandins in vivo through free radical-induced oxidation of arachidonates.

Levuglandins and isolevuglandins: stealthy toxins of oxidative injury

Inspired by a reaction discovered through basic research on the chemistry of the bicyclic peroxide nucleus of the prostaglandin endoperoxide PGH2, we postulated that levulinaldehyde derivatives with prostaglandin side chains, levuglandins (LGs), and structurally isomeric analogues, isolevuglandins (iso[n]LGs), would be generated by nonenzymatic rearrangements of prostanoid and isoprostanoid endoperoxides. Two decades of subsequent studies culminated in our discoveries of the LG and isoLG pathways, branches of the cyclooxygenase and isoprostane pathways, respectively. In cells, PGH2 rearranges nonenzymatically to LGs even in the presence of enzymes that use PGH2 as a substrate. IsoLGs, also known as isoketals or neuroketals, are generated in vivo through free radical-induced autoxidation of polyunsaturated phospholipid esters. Hydrolysis occurs after rapid adduction of isoLG phospholipids to proteins. The proclivity of these reactive species to avidly bind covalently with and cross-link proteins and nucleic acids complicated the hunt for LGs and isoLGs in vivo. The extraordinary reactivity of these "stealthy toxins" underlies much, if not all, of the biological consequences of LG and isoLG generation. They interfere with protein function and are among the most potent neurotoxic products of lipid oxidation known. Because they can accumulate over the lifetimes of proteins, iso[n]LG-protein adducts represent a convenient dosimeter of oxidative stress.

High-field 1H NMR studies of prostaglandin H2 and its decomposition pathways

Prostaglandin H2 displays at 500 MHz a detailed 1H-NMR in which all methylene groups are non-equivalent in C6D6 solution. The spectrum was assigned by analogy to isosteric structures. The dissymmetric perturbation and steric hindrance of the bicyclo [2.2.1] core caused by the side-chains provides a rationale for the selective fragmentations which PGH2 undergoes. Purified PGH2 is considerably more robust than previous literature accounts suggest. The following transformations were monitored by 1H-NMR: 1) O-O bond cleavage by Ph3P , 2) aqueous media fragmentation to PGE2 and PGD2, 3) base catalyzed fragmentation to ketoaldehydes , and 4) thermolysis attempts.