Temperature-induced fusion of small unilamellar vesicles formed from saturated long-chain lecithins and diheptanoylphosphatidylcholine

• Published in: Biochemistry (Easton) 1989-10, Vol.28 (20), p.8206-8213
• Main Authors: Eum, Ki Min, Riedy, Gerard, Langley, Kenneth H, Roberts, Mary F
• Format: Article
• Language: English
• Subjects: 550201 - Biochemistry- Tracer Techniques; BASIC BIOLOGICAL SCIENCES; BIOCHEMICAL REACTION KINETICS; CARBOXYLESTERASES; ENERGY TRANSFER; ENZYMES; ESTERASES; ESTERS; FLUORESCENCE; HYDROLASES; ISOTOPE APPLICATIONS; ISOTOPES; KINETICS; Light; LIGHT NUCLEI; LIPASES; LIPIDS; LUMINESCENCE; MAGNETIC RESONANCE; Magnetic Resonance Spectroscopy; Membranes, Artificial; MOLECULAR STRUCTURE; NMR SPECTRA; NUCLEAR MAGNETIC RESONANCE; NUCLEI; ODD-EVEN NUCLEI; ORGANIC COMPOUNDS; ORGANIC PHOSPHORUS COMPOUNDS; Phosphatidylcholines - analysis; Phospholipases - metabolism; PHOSPHOLIPIDS; PHOSPHORUS 31; PHOSPHORUS ISOTOPES; REACTION KINETICS; RESONANCE; SCATTERING; Scattering, Radiation; SIZE; SPECTRA; Spectrometry, Fluorescence; STABLE ISOTOPES; Temperature; TEMPERATURE EFFECTS; TRACER TECHNIQUES
• ISSN: 0006-2960; 1520-4995
• DOI: 10.1021/bi00446a036

Small unilamellar vesicles which form when gel-state long-chain phosphatidylcholines are mixed with micellar short-chain lecithins undergo an increase in size as the long-chain species melts to its liquid-crystalline form. Analysis of the vesicle population with quasi-elastic light scattering shows that the particle size increases from 90-A radius to greater than 5000-A radius. Resonance energy transfer experiments show total mixing of lipid probes with unlabeled vesicles only when the Tm of the long-chain phosphatidylcholine is exceeded. This implies that the large size change represents a fusion process. Aqueous compartments are also mixed during this transition. 31P NMR analysis of the vesicle mixtures above the phase transition shows a great degree of heterogeneity with large unilamellar particles coexisting with oligo- and multilamellar structures. Upon cooling the vesicles below the Tm, the original size distribution (e.g., small unilamellar vesicles) is obtained, as monitored by both quasi-elastic light scattering and 31P NMR spectroscopy. This temperature-induced fusion of unilamellar vesicles is concentration dependent and can be abolished at lower total phospholipid concentrations. It occurs over a wide range of long-chain to short-chain ratios and occurs with 1-palmitoyl-2-stearoylphosphatidylcholine and dimyristoylphosphatidylcholine as well. Characterization of this fusion event is used to understand the anomalous kinetics of water-soluble phospholipases toward these unusual vesicles.