Lateral hapten mobility and immunochemistry of model membranes.

Abstract

A study was made of the interaction of specific anti-nitroxide (anti-spin label) [rabbit] IgG [immunoglobulin G] antibodies, Fab fragments, complement and liposomes containing dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine and dipalmitoylphosphatidylcholine plus 0-50 mol % cholesterol, together with various concentrations of a head group spin-labeled phospholipid (0.5-0.01 mol % in the plane of the membrane). The spin-labeled lipid is the amide formed from the reaction of an iodoacetamide spin label, N-(1,oxyl-2,2,6,6-tetramethyl-4-piperidinyl)iodoacetamide, with dipalmitoylphosphatidylethanolamine. Complement fixation depends on the lateral mobility of the lipid hapten at low hapten concentrations in the plane of the membrane (.ltorsim. 0.1 mol %) and does not depend strongly on this mobility at high hapten concentrations. Cholesterol may have 2 distinct effects on complement fixation, one arising from an enhancement of hapten exposure to antibody binding sites and a 2nd due to a modulation of membrane fluidity.