title: Detection of liposomal cholesterol and monophosphoryl lipid A by QS-21 saponin and Limulus polyphemus amebocyte lysate.
abstract: Liposomes containing cholesterol (Chol) have long been used as an important membrane system for modeling the complex interactions of Chol with adjacent phospholipids or other lipids in a membrane environment. In this study we utilize a probe composed of QS-21, a saponin molecule that recognizes liposomal Chol and causes hemolysis of erythrocytes. The interaction of QS-21 with liposomal Chol results in a stable formulation which, after injection into the tissues of an animal, lacks toxic effects of QS-21 on neighboring cells that contain Chol, such as erythrocytes. Here we have used liposomes containing different saturated phospholipid fatty acyl groups and Chol, with or without monophosphoryl lipid A (MPLA), as model membranes. QS-21 is then employed as a probe to study the interactions of liposomal lipids on the visibility of membrane Chol. We demonstrate that changes either in the mole fraction of Chol in liposomes, or with different chain lengths of phospholipid fatty acyl groups, can have a substantial impact on the detection of Chol by the QS-21. We further show that liposomal MPLA can partially inhibit detection of the liposomal Chol by QS-21. The Limulus amebocyte lysate assay is used for binding to and detection of MPLA. Previous work has demonstrated that sequestration of MPLA into the liposomal lipid bilayer can block detection by the Limulus assay, but the binding site on the MPLA to which the Limulus protein binds is unknown. Changes in liposomal Chol concentration and phospholipid fatty acyl chain length influenced the detection of the liposome-embedded MPLA.
authors.affiliation: U.S. Military HIV Research Program, Henry M. Jackson Foundation for the Advancement of Military Medicine, 6720A Rockledge Drive, Bethesda, MD 20817 USA; Laboratory of Adjuvant and Antigen Research, US Military HIV Research Program, Walter Reed Army Institute of Research, 503 Robert Grant Avenue, Silver Spring, MD 20910, USA.
authors.forename: Zoltan
authors.initials: Z
authors.lastname: Beck
authors.affiliation: Laboratory of Adjuvant and Antigen Research, US Military HIV Research Program, Walter Reed Army Institute of Research, 503 Robert Grant Avenue, Silver Spring, MD 20910, USA.
authors.forename: Gary R
authors.initials: GR
authors.lastname: Matyas
authors.affiliation: Laboratory of Adjuvant and Antigen Research, US Military HIV Research Program, Walter Reed Army Institute of Research, 503 Robert Grant Avenue, Silver Spring, MD 20910, USA. Electronic address: calving@hivresearch.org.
authors.forename: Carl R
authors.initials: CR
authors.lastname: Alving
mesh_terms: D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry / Q000378:metabolism; D000276:Adjuvants, Immunologic / Q000737:chemistry / Q000378:metabolism; D000818:Animals; D002457:Cell Extracts / Q000737:chemistry*; D002784:Cholesterol / Q000737:chemistry* / Q000378:metabolism; D004134:Dimyristoylphosphatidylcholine / Q000737:chemistry / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D006461:Hemolysis / Q000187:drug effects; D006737:Horseshoe Crabs / Q000737:chemistry / Q000166:cytology / Q000378:metabolism; D006801:Humans; D008050:Lipid A / Q000031:analogs & derivatives* / Q000737:chemistry / Q000378:metabolism; D008081:Liposomes / Q000737:chemistry*; D010713:Phosphatidylcholines / Q000737:chemistry / Q000378:metabolism; D010715:Phosphatidylglycerols / Q000737:chemistry / Q000378:metabolism; D012503:Saponins / Q000737:chemistry* / Q000378:metabolism / Q000494:pharmacology
chemical_list: D000276:Adjuvants, Immunologic; D002457:Cell Extracts; D008050:Lipid A; D008081:Liposomes; D010713:Phosphatidylcholines; D010715:Phosphatidylglycerols; D012503:Saponins; D015060:1,2-Dipalmitoylphosphatidylcholine; C078785:saponin QA-21V1; D002784:Cholesterol; C002773:dimyristoylphosphatidylglycerol; C010942:1,2-distearoyllecithin; C048436:monophosphoryl lipid A; D004134:Dimyristoylphosphatidylcholine
keywords: Cholesterol; Limulus amebocyte lysate; Lipid bilayer heterogeneity; Liposomal model membranes; Monophosphoryl lipid A; QS-21 saponin