Metabolism of linoleic acid by prostaglandin endoperoxide synthase from adult and fetal blood vessels

Linoleic acid (18:2) is converted by prostaglandin endoperoxide synthase in particulate fractions and homogenates of fetal calf aorta to its 9- and 13-hydroperoxy metabolites. These intermediates are then either dehydrated to the corresponding oxo compounds or reduced to monohydroxy products. Alternatively, the hydroperoxyoctadecadienoic acids can be converted to epoxyhydroxyoctadecenoic acids, which are hydrolyzed to trihydroxy metabolites by epoxide hydrolases present in both particulate and cytosolic fractions from aorta. Linoleic acid (Km, 442 microM) is a much poorer substrate for prostaglandin endoperoxide synthase than is arachidonic acid (20:4) (Km, 48 microM). However, the oxygenation of 18:2 by particulate fractions from aorta is linear with time for at least 5 min, whereas the oxygenation of 20:4 is linear for only 15 s. Arachidonic acid strongly inhibits the conversion of 18:2 to monohydroxy (ID50, 10 microM) and trihydroxy (ID50, 140 microM) products. Linoleic acid has a similar, but much weaker effect on the formation of 6-oxoprostaglandin F1 alpha from 20:4. Substantial amounts of both the monohydroxy (9-hydroxy-10, 12-octadecadienoic acid and 13-hydroxy-9,11-octadecadienoic acid) and trihydroxy (9,10,11-trihydroxy-12-octadecenoic acid, 9,10,13-trihydroxy-11-octadecenoic acid and 9,12,13-trihydroxy-10-octadecenoic acid) metabolites of 18:2 were shown by gas chromatography-mass spectrometry to be formed from endogenous substrate during incubation of slices of fetal calf aorta in physiological medium. This raises the possibility that some of these products or their hydroperoxy precursors may have some biological significance.     

The role of molecular conformation in ion capture by carboxylic ionophores: a circular dichroism study of narasin A in single-phase solvents and liposomes

Conformational and thermodynamic aspects of cation binding by the carboxylic ionophore narasin A were studied by circular dichroism (CD). In single-phase solvents, dramatic increases in the maximum differential absorption (delta epsilon) of the C-11 carbonyl were observed upon the binding of K+, Na+ and protons to the free anionic form. These changes were associated with major shifts in the conformation equilibrium between extended and pseudocyclic conformers of narasin. Similar CD changes observed upon the binding of K+ to narasin A in dimyristoylphosphatidylcholine vesicles provided evidence that in the membrane environment, comparable conformation changes were associated with ion binding. Variation of the polar and protic properties of single-phase solvents was also found to influence the delta epsilon of the cation bound species of narasin A, supporting previous evidence for polarity-mediated modulation of conformation. Comparison of cation binding affinities indicated that in both single-phase solvents and liposomes, narasin had a marked equilibrium selectivity for K+ over Na+.     

The metabolism of leukotrienes in blood plasma studied by high-performance liquid chromatography

The metabolism of leukotrienes (B4, C4, D4, and E4) within human plasma was studied and a simple sample preparation is presented. It was demonstrated that leukotriene E4 and leukotriene B4 were stable during incubation at 37 degrees C using the in vitro system. In contrast, leukotriene C4 was metabolized by gamma-glutamyl transpeptidase activities into leukotriene D4 which was further metabolized by dipeptidase activities of plasma into leukotriene E4. The transition state inhibitor of gamma-glutamyl transpeptidase L-serine-borate decreased the metabolism of leukotriene C4 in plasma. Dilution of plasma demonstrated that the dipeptidase was more active compared to the gamma-glutamyl transpeptidase. The metabolizing activities of plasma were functionally characterized by fractionating the plasma proteins.     

Pathway of acetate assimilation in autotrophic and heterotrophic methanococci.

The autotroph Methanococcus maripaludis contained high levels of acetate-coenzyme A ligase, pyruvate synthase, pyruvate, water dikinase, pyruvate carboxylase, and the enzymes of the incomplete reductive tricarboxylic acid cycle. Phosphoenolpyruvate carboxykinase, citrate synthase, and isocitrate dehydrogenase were not detected. In contrast, the heterotroph Methanococcus sp. strain A3 contained acetate kinase, and acetate coenzyme A ligase was virtually absent.     

Characterization of the specific binding of rat apolipoprotein E-deficient HDL to rat hepatic plasma membranes

We have used a preparation of rat liver plasma membranes to study the binding of rat apolipoprotein E-deficient HDL to rat liver. The membranes were found to bind HDL by a saturable process that was competed for by excess unlabeled HDL. The binding was temperature-dependent and was 85% receptor-mediated when incubated at 4, 22 and 37 degrees C. The affinity of the binding site for the HDL was consistent at all temperatures, while the maximum binding capacity increased at higher temperatures. The specific binding of HDL to the membranes did not require calcium and was independent of the concentration of NaCl in the media. The effect of varying the pH of the media on HDL binding was small, being 30% higher at pH 6.5 than at pH 9.0. Both rat HDL and human HDL3 were found to compete for the binding of rat HDL to the membranes, whereas rat VLDL remnants and human LDL did not compete. At 4 degrees C, complexes of dimyristoylphosphatidylcholine (DMPC) and apolipoproteins A-I, A-IV and the C apolipoproteins, but not apolipoprotein E, competed for HDL binding to the membranes. At 22 and 37 degrees C, all DMPC-apolipoprotein complexes competed to a similar extent, DMPC vesicles that contained no protein did not compete for the binding of HDL. These results suggest that the rat liver possesses a specific receptor for apolipoprotein E-deficient HDL that recognizes apolipoproteins A-I, A-IV and the C apolipoproteins as ligands.