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The intrinsic overexpression of secretory phospholipase A2 (sPLA 2 ) in various pro-inflammatory diseases and cancers has the potential to be exploited as a therapeutic strategy for diagnostics and treatment. To explore this potential and advance our knowledge of the role of sPLA 2 in related diseases, it is necessary to systematically investigate the molecular interaction of the enzyme with lipids. By employing a Langmuir trough integrated with X-ray reflectivity and grazing incidence X-ray diffraction techniques, this study examined the molecular packing structure of 1,2-palmitoyl- sn-glycero -3-phosphocholine (DPPC) films before and after enzyme adsorption and enzyme-catalyzed degradation. Molecular interaction of sPLA 2 (from bee venom) with the DPPC monolayer exhibited Ca 2+ dependence. DPPC molecules at the interface without Ca 2+ retained a monolayer organization; upon adsorption of sPLA 2 to the monolayer the packing became tighter. In contrast, sPLA 2 -catalyzed degradation of DPPC occurred in the presence of Ca 2+ , leading to disruption of the ordered monolayer structure of DPPC. The interfacial film became a mixture of highly ordered multilayer domains of palmitic acid (PA) and loosely packed monolayer phase of 1-palmitoyl-2-hydroxy- sn-glycero -3-phosphocholine (lysoPC) that potentially contained the remaining un-degraded DPPC. The redistribution of lipid degradation products into the third dimension, which produced multilayer PA domains, damaged the structural integrity of the original lipid layer and may explain the bursting of liposomes observed in other studies after a latency period of mixing liposomes with sPLA 2 . A quantitative understanding of the lipid packing and lipid-enzyme interaction provides an intuitive means of designing and optimizing lipid-related drug delivery systems.
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Organic Enrichment at Aqueous Interfaces: Cooperative Adsorption of Glucuronic Acid to DPPC Monolayers Studied with Vibrational Sum Frequency Generation
Surface tension, surface-specific vibrational spectroscopy and differential scanning calorimetry measurements were all used to test cooperative adsorption of glucuronic acid (GU) to DPPC monolayers adsorbed to the aqueous/vapor interface. Experiments were performed using GU solutions prepared in Millipore water and in carbonate/bicarbonate solutions buffered to a pH of 9.0. The effects of GU on DPPC monolayer structure and organization were carried out with tightly packed monolayers (40 Å2/DPPC) and monolayers in their liquid condensed phase (55 Å2/molecule). Surface tension data show that GU concentrations of 50 mM lead to expanded DPPC monolayers with diminished surface tensions (or higher surface pressures) at a given DPPC coverage relative to monolayers on pure water. With unbuffered solutions, GU induces significant ordering within liquid condensed monolayers although the effects of GU on tightly packed DPPC monolayers are less pronounced. GU also induces a second, higher melting temperature in DPPC vesicles implying that GU (at sufficiently high concentrations) strengthens lipid-lipid cohesion, possibly by replacing water solvating the DPPC headgroups. Together, these observations all support a cooperative adsorption mechanism. In buffer solutions, the effects of dissolved GU on DPPC structure and organization are muted. Only at sufficiently high GU concentrations (when the solution’s buffering capacity has been exceeded) do the data again show evidence of cooperative adsorption. These findings place limits on cooperative adsorption’s ability to enrich interfacial organic content in alkaline environmental systems such as oceans.
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- Award ID(s):
- 1757351
- PAR ID:
- 10098614
- Date Published:
- Journal Name:
- The journal of physical chemistry. A
- ISSN:
- 1089-5639
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1021/acs.jpca.9b02255
