Protective effects of a superoxide dismutase mimetic and peroxynitrite decomposition catalysts in endotoxin-induced intestinal damage

1. The relative contributions of superoxide anion (O2-) and peroxynitrite (PN) were evaluated in the pathogenesis of intestinal microvascular damage caused by the intravenous injection of E. coli lipopolysaccharide (LPS) in rats. The superoxide dismutase mimetic (SODm) SC-55858 and the active peroxynitrite decomposition catalysts 5,10,15,20-tetrakis(2,4,6-trimethyl-3,5-disulphonatophenyl)-por phyrinato iron (III) and 5,10,15,20-tetrakis(N-methyl-4'-pyridyl)-porphyrinato iron (III) (FeTMPS, FeTMPyP respectively) were used to assess the roles of O2- and PN respectively. 2. The intravenous injection of LPS elicited an inflammatory response that was characterized by a time-dependent infiltration of neutrophils, lipid peroxidation, microvascular leakage (indicative of microvascular damage), and epithelial cell injury in both the duodenum and jejunum. 3. Administration of the SODm SC-55858, FeTMPS or FeTMPyP at 3 h post LPS reduced the subsequent increase in microvascular leakage, lipid peroxidation and epithelial cell injury. Inactive peroxynitrite decomposition catalysts exhibited no protective effects. Only, SC-55858 inhibited neutrophil infiltration. 4. Our results suggest that O2 and peroxynitrite play a significant role in the pathogenesis of duodenal and intestinal injury during endotoxaemia and that their remoyal by SODm and peroxynitrite decomposition catalysts offers a novel approach to the treatment of septic shock or clinical conditions of gastrointestinal inflammation. Furthermore, the remarkable protection of the intestinal epithelium by these agents suggests their use during chemo- and radiation therapy, cancer treatments characterized by gastrointestinal damage. Potential mechanisms through which these radicals evoke damage are discussed.

Inhibition of endosome fusion by phospholipase A2 (PLA2) inhibitors points to a role for PLA2 in endocytosis

Fusion of intracellular membrane-bound compartments is a common step in the transport of macromolecules along the endocytic and secretory pathways. A large number of factors active in the fusion process or its regulation have been identified; however, the actual sequence of events leading to membrane fusion is still unknown. In this study, we have assessed a possible role for PLA2 in endosome fusion by using an in vitro reconstitution assay and by examining endocytosis in intact cells. Several PLA2 inhibitors blocked endosome fusion in a broken-cell preparation. Inhibition was reversed by addition of arachidonic acid. At the electron microscope level, endosome clusters were observed even in the presence of inhibitors; however, actual fusion between endosomes was largely reduced. Fusion frequency increased upon the addition of arachidonic acid. A membrane-permeable PLA2 inhibitor blocked mixing of ligands internalized sequentially but did not affect internalization. The results indicate that vesicle fusion along the endocytic pathway requires a PLA2 activity. The effect of this activity would be, at least in part, mediated by arachidonic acid release.

Structural determinants of haloenol lactone-mediated suicide inhibition of canine myocardial calcium-independent phospholipase A2

Haloenol lactones are potent mechanism-based inhibitors of a novel class of calcium-independent phospholipases A2 which have been implicated as the enzymic mediators of membrane dysfunction during myocardial ischemia (Hazen, S. L.; et al. J. Biol. Chem. 1991, 266, 7227-7232). Herein we demonstrate that the ring size, hydrophobic group, and cryptic electrophile in the haloenol lactone moiety are important and modifiable determinants of the inhibitory potency of haloenol lactone-mediated inhibition of calcium-independent phospholipase A2. Direct comparisons between haloenol lactone-mediated inhibition of calcium-independent phospholipase A2 and the absence of inhibition with calcium-dependent phospholipase A2 further underscore the marked differences in the catalytic strategy employed by these two classes of intracellular phospholipases A2.

Kinetic and structural evidence for a sequential ordered Bi Bi mechanism of catalysis by Saccharomyces cerevisiae myristoyl-CoA:protein N-myristoyltransferase

The mechanism of catalysis of Escherichia coli-derived Saccharomyces cerevisiae myristoyl-CoA: protein N-myristoyltransferase (NMT) has been characterized. Previous studies indicated that a high affinity reaction intermediate forms between NMT and myristoyl-CoA in the absence of a peptide substrate. This complex has been further characterized using S-(2-oxo)pentadecyl-CoA, a nonhydrolyzable myristoyl-CoA analog. Binding studies involving this analog, as well as myristoylpeptide and CoA, have indicated that the CoA moiety of the acyl substrate is retained in the acyl-NMT complex prior to peptide addition. These structural data, along with kinetic studies of myristoylpeptide and CoA product inhibition, indicate that the mechanism of catalysis of NMT is ordered Bi Bi, with myristoyl-CoA binding to NMT occurring prior to peptide binding and CoA release taking place before release of acyl peptide. Further analyses of the interactions between NMT, acyl peptide, and CoA demonstrate that NMT is able to deacylate a myristoylpeptide in the presence of CoA.