Mechanism of phospholipase A2 action toward mixed micelles of detergent and phospholipids

Human Group IIA Phospholipase A2-Three Decades on from Its Discovery

Phospholipase A₂ (PLA₂) enzymes were first recognized as an enzyme activity class in 1961. The secreted (sPLA₂) enzymes were the first of the five major classes of human PLA₂s to be identified and now number nine catalytically-active structurally homologous proteins. The best-studied of these, group IIA sPLA₂, has a clear role in the physiological response to infection and minor injury and acts as an amplifier of pathological inflammation. The enzyme has been a target for anti-inflammatory drug development in multiple disorders where chronic inflammation is a driver of pathology since its cloning in 1989. Despite intensive effort, no clinically approved medicines targeting the enzyme activity have yet been developed. This review catalogues the major discoveries in the human group IIA sPLA₂ field, focusing on features of enzyme function that may explain this lack of success and discusses future research that may assist in realizing the potential benefit of targeting this enzyme. Functionally-selective inhibitors together with isoform-selective inhibitors are necessary to limit the apparent toxicity of previous drugs. There is also a need to define the relevance of the catalytic function of hGIIA to human inflammatory pathology relative to its recently-discovered catalysis-independent function.

The inactivation mechanism of human group IIA phospholipase A(2) by Scalaradial

Secretory phospholipases A(2) (sPLA(2)s) are implicated in the pathogenesis of several inflammation diseases, such as rheumatoid arthritis, septic shock, psoriasis, and asthma. Thus, an understanding of their inactivation mechanisms could be useful for the development of new classes of chemical selective inhibitors. In the marine environment, several bioactive terpenoids possess interesting anti-inflammatory activity, often through covalent and/or noncovalent inactivation of sPLA(2). Herein, we report the molecular mechanism of human group IIA phospholipase A(2) (sPLA(2)-IIA) inactivation by Scalaradial (SLD), a marine 1,4-dialdehyde terpenoid isolated from the sponge Cacospongia mollior and endowed with a significant anti-inflammatory profile. Our results have been collected by a combination of biochemical approaches, advanced mass spectrometry, surface plasmon resonance, and molecular modeling. These suggest that SLD acts as a competitive inhibitor. Indeed, the sPLA(2)-IIA inactivation process seems to be driven by the noncovalent recognition process of SLD in the enzyme active site and by chelation of the catalytic calcium ion. In contrast, covalent modification of the enzyme by the SLD dialdehyde moiety emerges as only a minor side event in the ligand-enzyme interaction. These results could be helpful for the rational design of new PLA(2) inhibitors that would be able to selectively target the enzyme active site.

Thin-layer chromatography combined with MALDI-TOF-MS and 31P-NMR to study possible selective bindings of phospholipids to silica gel

High-performance thin-layer chromatography (HPTLC) is a highly established separation method in the field of lipid and (particularly) phospholipid (PL) research. HPTLC is not only used to identify certain lipids in a mixture but also to isolate lipids (preparative TLC). To do this, the lipids are separated and subsequently re-eluted from the silica gel. Unfortunately, it is not yet known whether all PLs are eluted to the same extent or whether some lipids bind selectively to the silica gel. It is also not known whether differences in the fatty acyl compositions affect the affinities to the stationary phase. We have tried to clarify these questions by using a readily available extract from hen egg yolk as a selected example of a lipid mixture. After separation, the complete lanes or selected spots were eluted from the silica gel and investigated by a combination of MALDI-TOF MS and (31)P NMR spectroscopy. The data obtained were compared with the composition of the total extract (without HPTLC). Although there were significant, solvent-dependent losses in the amount of each lipid, the relative composition of the mixture remained constant; there were also only very slight changes in the fatty acyl compositions of the individual PL classes. Therefore, lipid isolation by TLC may be used without any risk of major sample alterations.

Inhibitory effects of surfactant protein A on surfactant phospholipid hydrolysis by secreted phospholipases A2

Hydrolysis of surfactant phospholipids by secreted phospholipases A(2) (sPLA(2)) contributes to surfactant dysfunction in acute respiratory distress syndrome. The present study demonstrates that sPLA(2)-IIA, sPLA(2)-V, and sPLA(2)-X efficiently hydrolyze surfactant phospholipids in vitro. In contrast, sPLA(2)-IIC, -IID, -IIE, and -IIF have no effect. Since purified surfactant protein A (SP-A) has been shown to inhibit sPLA(2)-IIA activity, we investigated the in vitro effect of SP-A on the other active sPLA(2) and the consequences of sPLA(2)-IIA inhibition by SP-A on surfactant phospholipid hydrolysis. SP-A inhibits sPLA(2)-X activity, but fails to interfere with that of sPLA(2)-V. Moreover, in vitro inhibition of sPLA(2)-IIA-induces surfactant phospholipid hydrolysis correlates with the concentration of SP-A in surfactant. Intratracheal administration of sPLA(2)-IIA to mice causes hydrolysis of surfactant phosphatidylglycerol. Interestingly, such hydrolysis is significantly higher for SP-A gene-targeted mice, showing the in vivo inhibitory effect of SP-A on sPLA(2)-IIA activity. Administration of sPLA(2)-IIA also induces respiratory distress, which is more pronounced in SP-A gene-targeted mice than in wild-type mice. We conclude that SP-A inhibits sPLA(2) activity, which may play a protective role by maintaining surfactant integrity during lung injury.