Kinetic and structural investigations of the allosteric site in human epithelial 15-lipoxygenase-2

Allosteric regulation of human lipoxygenase (hLO) activity has recently been implicated in the cellular biology of prostate cancer. In the current work, we present isotope effect, pH, and substrate inhibitor data of epithelial 15-hLO-2, which probe the allosteric effects on its mechanistic behavior. The Dk(cat)/KM for 15-hLO-2, with AA and LA as substrate, is large indicating hydrogen atom abstraction is the principle rate-determining step, involving a tunneling mechanism for both substrates. For AA, there are multiple rate determining steps (RDS) at both high and low temperatures, with both diffusion and hydrogen bonding rearrangements contributing at high temperature, but only hydrogen bonding rearrangements contributing at low temperature. The observed kinetic dependency on the hydrogen bonding rearrangement is eliminated upon addition of the allosteric effector, 13-(S)-hydroxyoctadecadienoic acid (13-HODE), and no allosteric effects were seen on diffusion or hydrogen atom abstraction. The (k(cat)/KM)AA/(k(cat)/KM)LA ratio was observed to have a pH dependence, which was fit with a titration curve (pKa = 7.7), suggesting the protonation of a histidine residue, which could hydrogen bond with the carboxylate of 13-HODE. Assuming this interaction, 13-HODE was docked to the solvent exposed histidines of a 15-hLO-2 homology model and found to bind well with H627, suggesting a potential location for the allosteric site. Utilizing d31-LA as an inhibitor, it was demonstrated that the binding of d31-LA to the allosteric site changes the conformation of 15-hLO-2 such that the affinity for substrate increases. This result suggests that allosteric binding locks the enzyme into a catalytically competent state, which facilitates binding of LA and decreases the (k(cat)/KM)AA/(k(cat)/KM)LA ratio. Finally, the magnitude of the 13-HODE KD for 15-hLO-2 is over 200-fold lower than that of 13-HODE for 15-hLO-1, changing the substrate specificity of 15-hLO-2 to 1.9. This would alter the LO product distribution and increase the production of the pro-tumorigenic, 13-HODE, possibly representing a pro-tumorigenic feedback loop for 13-HODE and 15-hLO-2.

Rapid sample preparation and simultaneous quantitation of prostaglandins and lipoxygenase derived fatty acid metabolites by liquid chromatography-mass spectrometry from small sample volumes

BACKGROUND

Fatty acid metabolites play a key role in numerous physiological and pathological processes. A rapid liquid chromatography-mass spectrometry assay for the simultaneous determination of prostanoids, isoprostane and lipoxygenase (LOX) derived fatty acid metabolites in a small biological sample of only 20 microL was developed.

METHODS

Human plasma samples were applied to a filter spot, extracted without prior derivatization and analyzed within 13 min. Detection of metabolites was performed on a triple quadrupole mass spectrometer in negative multiple-reaction monitoring detection mode. Application of this assay to various biological matrices was performed.

RESULTS

The validated assay was linear over the concentration range of 5-500 nmol/L for prostanoids and isoprostane, 50-5000 nmol/L for LOX-derived metabolites and 400-40,000 nmol/L for fatty acids. Limits of quantitation were 0.4-233 nmol/L, depending on the metabolite. Plasma samples from diabetic patients and controls showed significant increases in (+/-)9-HODE and 15(S)-HETE with p-values of 0.019 and 0.024, respectively.

CONCLUSIONS

The small amount of 20 microL sample volume used in this assay and the demonstrated application to various sample types makes it an ideal routine analysis method for fatty acid metabolites. The resulting values for LOX-derived metabolites in diabetes mellitus type 2 samples support earlier findings about the role of lipid oxidation products in diabetes.

Substrate specificity changes for human reticulocyte and epithelial 15-lipoxygenases reveal allosteric product regulation

Human reticulocyte 15-lipoxygenase (15-hLO-1) and epithelial 15-lipoxygenase (15-hLO-2) have been implicated in a number of human diseases, with differences in their substrate specificity potentially playing a central role. In this paper, we present a novel method for accurately measuring the substrate specificity of the two 15-hLO isozymes and demonstrate that both cholate and specific LO products affect substrate specificity. The linoleic acid (LA) product, 13-hydroperoxyoctadienoic acid (13-HPODE), changes the ( k cat/ K m) (AA)/( k cat/ K m) (LA) ratio more than 5-fold for 15-hLO-1 and 3-fold for 15-hLO-2, while the arachidonic acid (AA) product, 12-( S)-hydroperoxyeicosatetraenoic acid (12-HPETE), affects only the ratio of 15-hLO-1 (more than 5-fold). In addition, the reduced products, 13-( S)-hydroxyoctadecadienoic acid (13-HODE) and 12-( S)-hydroxyeicosatetraenoic acid (12-HETE), also affect substrate specificity, indicating that iron oxidation is not responsible for the change in the ( k cat/ K m) (AA)/( k cat/ K m) (LA) ratio. These results, coupled with the dependence of the 15-hLO-1 k cat/ K m kinetic isotope effect ( (D) k cat/ K m) on the presence of 12-HPETE and 12-HETE, indicate that the allosteric site, previously identified in 15-hLO-1 [Mogul, R., Johansen, E., and Holman, T. R. (1999) Biochemistry 39, 4801-4807], is responsible for the change in substrate specificity. The ability of LO products to regulate substrate specificity may be relevant with respect to cancer progression and warrants further investigation into the role of this product-feedback loop in the cell.

Differential effect of blood collection tubes on total free fatty acids (FFA) and total triiodothyronine (TT3) concentration: a model for studying interference from tube constituents

BACKGROUND

Besides total triiodothyronine (TT3), total free fatty acids (FFA) concentrations were higher with serum separator tube (SST) than Vacuette tubes.

METHODS

The effects of surfactant, rubber stopper, and separator gel from various tubes were investigated on FFA, beta-hydroxybutyrate (beta-HB), and TT3 with 8 different tube types in blood specimens of apparently healthy volunteers.

RESULTS

Compared to Vacuette tubes, serum FFA and TT3 concentrations were significantly higher in SST than glass tubes. Reformulated SST eliminated the increase in TT3 but not FFA. No significant difference was observed for beta-HB concentration among tube types. Surfactant and rubber stoppers from the different tube types significantly increased TT3 but not FFA and beta-HB concentrations. Agitation of whole blood but not serum or plasma specimens with separator gel from SST, reformulated SST and plasma preparation tube (PPT) tubes compared to Vacuette tubes gave higher FFA but not beta-HB levels.

CONCLUSIONS

Unidentified component(s) from the separator gel in SST, reformulated SST and PPT tubes cause falsely high FFA concentration. In contrast to TT3, falsely high FFA results require exposure of whole blood and not serum to tube constituent(s). The approach employed here may serve as a model for assessing interference(s) from tube constituent(s).

Activation of p38 mitogen-activated protein kinase/cytosolic phospholipase A2 cascade in hydroperoxide-stressed platelets

12-Hydroperoxy-eicosatetraenoic acid (12-HpETE), the main hydroperoxide formed in platelets from arachidonic acid (AA) by 12-lipoxygenase, has been shown to increase the sensitivity of platelets to agonists resulting in increased aggregation. The aim of the present study was to determine the direct effect of low concentrations of 12-HpETE on the signaling pathways leading to AA release from membrane phospholipids and thromboxane A2 (TxA2) formation. Exogenous 12-HpETE activated platelet p38 mitogen-activated protein kinase (p38 MAPK), as assessed by its phosphorylation, at a concentration as low as 100 nM and was much more potent than hydrogen peroxide. Moreover, the incubation of platelets with 100 nM 12-HpETE for 2 min led to the phosphorylation of cytosolic phospholipase A2 (cPLA2). It was associated with a significant decrease in the concentration of AA esterified in phospholipids and an increased concentration of thromboxane B2, the stable catabolite of TxA2. Additionally, decreasing glutathione peroxidase activity pharmacologically favored endogenous 12-HpETE formation and led to an increase in phosphorylated p38 MAPK, while a thiol-reducing agent such as N-acetyl-cysteine fully prevented it. Finally, significant activation of p38 MAPK was also observed in platelets from type 2 diabetic patients with mild hyperglycemia. In conclusion, our data provide a new insight into the mechanism of 12-HpETE-induced platelet priming, suggesting that hydroperoxide-induced p38 MAPK activation could play a relevant role in the exacerbated platelet activation associated with oxidative stress as found in diabetes.

Emmyguyacins A and B: unusual glycolipids from a sterile fungus species that inhibit the low-pH conformational change of hemagglutinin A during replication of influenza virus

Two novel glycolipids, emmyguyacin A (1a) and emmyguyacin B (1b), were isolated at concentrations of 1.51 g/L from a potato dextrose agar fermentation of a sterile fungus species. The compounds inhibit replication of influenza A virus (A/X31) in MDCK cells by inhibiting the pH-dependent conformational change of hemagglutinin A (IC(50) 9 microM). The structures were deduced using one- and two-dimensional NMR techniques and mass spectrometric analyses on both the parent compounds and a host of degradation products and derivatives. A novel and unusual oxalic acid ester of a monohydroxylated fatty acid (5, 17-oxalyloxydocosanoic acid) is reported. The first isolation and characterization of the fatty acid 17-hydroxydocosanoic acid (3) itself is also reported as a saponification product of 1.

Monohydroxylated fatty acid content in peripheral blood mononuclear cells and immune status of people at long times after the Chernobyl accident

The monohydroxylated fatty acid content of peripheral blood mononuclear cells from 23 cleanup workers and 16 unexposed individuals was studied in relation to their immune status after the Chernobyl accident. Men with absorbed doses below 0.32 Gy showed higher levels of free and esterified 12-hydroxyeicosatetraenoic acid (12-HETE) than unexposed men, whereas 15-HETE and the 17-hydroxy derivative of C22 fatty acid (17-OH 22), either free or esterified in phospholipids, were increased in a dose-dependent manner. The percentage of CD4-positive cells was also increased significantly in heavily irradiated men, whereas the percentage of CD8-positive cells tended to decrease with dose. Furthermore, the absolute count of CD4-positive cells was correlated positively with the amount of esterified 15-HETE in the phospholipid fraction of the mononuclear cells and with the total 15-HETE. These results show for the first time that the accumulation of autoxidized/lipoxygenase products of polyunsaturated fatty acids in the mononuclear cells of irradiated individuals was associated with immune imbalance. This may be the basis for certain late effects of radiation such as autoimmune disorders, somatic and neoplastic diseases, and early aging.

Glycosphingolipid changes induced by advanced glycation end-products

The effects of advanced glycation end-products (AGEs) on retinal microvascular cell glycosphingolipids were investigated as a potential pathogenic mechanism of diabetic retinopathy. The results obtained showed that, in microvascular retinal endothelial cells and pericytes, AGEs increased the amount of all glycosphingolipids studied (from 25 to 115% depending on the glycosphingolipid species), except for a specific ganglioside, GD3, which decreased by 35% only in pericytes. Glycosphingolipid profiles and GM3 fatty acid analysis did not show any qualitative differences after incubation with AGEs, suggesting that AGEs only induced quantitative changes in cell glycosphingolipids. These results show a new metabolic effect of AGEs, which could be involved in the microvascular alterations observed in diabetic retinopathy.