Phospholipase activity on N-acyl phosphatidylethanolamines is critically dependent on the N-acyl chain length

Covalent modification of lipids and proteins in rat hepatocytes and in vitro by thioacetamide metabolites

Thioacetamide (TA) is a well-known hepatotoxin in rats. Acute doses cause centrilobular necrosis and hyperbilirubinemia while chronic administration leads to biliary hyperplasia and cholangiocarcinoma. Its acute toxicity requires its oxidation to a stable S-oxide (TASO) that is oxidized further to a highly reactive S,S-dioxide (TASO(2)). To explore possible parallels among the metabolism, covalent binding, and toxicity of TA and thiobenzamide (TB), we exposed freshly isolated rat hepatocytes to [(14)C]-TASO or [(13)C(2)D(3)]-TASO. TLC analysis of the cellular lipids showed a single major spot of radioactivity that mass spectral analysis showed to consist of N-acetimidoyl PE lipids having the same side chain composition as the PE fraction from untreated cells; no carbons or hydrogens from TASO were incorporated into the fatty acyl chains. Many cellular proteins contained N-acetyl- or N-acetimidoyl lysine residues in a 3:1 ratio (details to be reported separately). We also oxidized TASO with hydrogen peroxide in the presence of dipalmitoyl phosphatidylenthanolamine (DPPE) or lysozyme. Lysozyme was covalently modified at five of its six lysine side chains; only acetamide-type adducts were formed. DPPE in liposomes also gave only amide-type adducts, even when the reaction was carried out in tetrahydrofuran with only 10% water added. The exclusive formation of N-acetimidoyl PE in hepatocytes means that the concentration or activity of water must be extremely low in the region where TASO(2) is formed, whereas at least some of the TASO(2) can hydrolyze to acetylsulfinic acid before it reacts with cellular proteins. The requirement for two sequential oxidations to produce a reactive metabolite is unusual, but it is even more unusual that a reactive metabolite would react with water to form a new compound that retains a high degree of chemical reactivity toward biological nucleophiles. The possible contribution of lipid modification to the hepatotoxicity of TA/TASO remains to be determined.

Evidence for a relationship between bovine erythrocyte lipid membrane peculiarities and immune pressure from ruminal ciliates

Erythrocytes of bovines and other ruminants have a strikingly anomalous phospholipid composition, with low or absent phosphatidylcholine (PC) together with high sphingomyelin (SM) content. Here, we report the presence in normal bovine serum of high levels of anti-phospholipid antibodies of IgM isotype against, PC and the phosphono analogue of phosphatidylethanolamine, aminoethylphosphonolipid (AEPL), normally produced by rumen ciliates. In contrast, no antibodies were detected against SM or N-acyl-phosphatidylethanolamine (NAPE), the major components of bovine erythrocytes. In addition, we found that exposure of the ciliate Tetrahymena thermophila to bovine serum results in rapid lysis, an effect that was inhibited by adsorption of the serum with SM/AEPL liposomes. Furthermore, incubation with bovine serum had a similar effect on freshly obtained ruminal ciliates, and the lytic activity was eliminated by pre-adsorption of the serum with SM/PE liposomes. The ruminant mode of life with its concomitant ciliate fauna is hereby linked to the peculiar conformation of bovine erythrocyte membranes. We propose that the unique phospholipid composition of bovine erythrocytes appears as an evolutionary adaptation to tolerate the lytic effects of anti-phospholipid antibodies generated against AEPL, a membrane component of the huge mass of ruminal ciliates, necessary commensals of this group of mammals.

Covalent modification of microsomal lipids by thiobenzamide metabolites in vivo

Thiobenzamide (TB) is hepatotoxic in rats causing centrolobular necrosis, steatosis, cholestasis, and hyperbilirubinemia. It serves as a model compound for a number of thiocarbonyl compounds that undergo oxidative bioactivation to chemically reactive metabolites. The hepatotoxicity of TB is strongly dependent on the electronic character of substituents in the meta- and para-positions, with Hammett rho values ranging from -4 to -2. On the other hand, ortho substituents that hinder nucleophilic addition to the benzylic carbon of S-oxidized TB metabolites abrogate the toxicity and protein covalent binding of TB. This strong linkage between the chemistry of TB and its metabolites and their toxicity suggests that this model is a good one for probing the overall mechanism of chemically induced biological responses. While investigating the protein covalent binding of TB metabolites, we noticed an unusually large amount of radioactivity associated with the lipid fraction of rat liver microsomes. Thin-layer chromatography showed that most of the radioactivity was contained in a single spot more polar than the neutral lipids but less polar than the phospholipid fractions. Mass spectral analyses aided by the use of synthetic standards identified the material as N-benzimidoyl derivatives of typical microsomal phosphatidylethanolamine (PE) lipids. Quantitative analysis indicated that up to 25% of total microsomal PE became modified within 5 h after a hepatotoxic dose of TB. Further studies will be required to determine the contribution of lipid modification to the hepatotoxicity of TB.

Functional analysis of the purified anandamide-generating phospholipase D as a member of the metallo-beta-lactamase family

In animal tissues, bioactive N-acylethanolamines including the endocannabinoid anandamide are formed from their corresponding N-acylphosphatidylethanolamines (NAPEs) by the catalysis of a specific phospholipase D (NAPE-PLD) that belongs to the metallo-beta-lactamase family. Despite its potential physiological importance, NAPE-PLD has not yet been characterized with a purified enzyme preparation. In the present study we expressed a recombinant NAPE-PLD in Escherichia coli and highly purified it. The purified enzyme was remarkably activated in a dose-dependent manner by millimolar concentrations of Mg2+ as well as Ca2+ and, hence, appeared to be constitutively active. The enzyme showed extremely high specificity for NAPEs among various glycerophospholipids but did not reveal obvious selectivity for different long chain or medium chain N-acyl species of NAPEs. These results suggested the ability of NAPE-PLD to degrade different NAPEs without damaging other membrane phospholipids. Metal analysis revealed the presence of catalytically important zinc in NAPE-PLD. In addition, site-directed mutagenesis studies were addressed to several histidine and aspartic acid residues of NAPE-PLD that are highly conserved within the metallo-beta-lactamase family. Single mutations of Asp-147, His-185, His-187, Asp-189, His-190, His-253, Asp-284, and His-321 caused abolishment or remarkable reduction of the catalytic activity. Moreover, when six cysteine residues were individually mutated to serine, only C224S showed a considerably reduced activity. The activities of L207F and H380R found as single nucleotide polymorphisms were also low. Thus, NAPE-PLD appeared to function through a mechanism similar to those of the well characterized members of this family but play a unique role in the lipid metabolism of animal tissues.