Binding of spin-labelled analogues of endotoxin to serum proteins

The study describes an electron spin resonance (ESR) spin-labelling examination of the interaction of endotoxin (LPS) with serum proteins. Endotoxin from E. coli O111. B4, J5 and lipid-A have been spin-labelled at specific sugar residues and found to bind to serum proteins. The sugar moieties play no intrinsic part in this binding. An analogue of lipid-A, 5-doxyl stearate glucosamine-1-phosphate, spin-labelled in the acyl chain, was synthesized and bound to bind High-Density Lipoproteins (HDL), Transferrin and Serum Albumin (BSA). The ESR spectra showed that the acyl chain was directly involved in this binding, with high binding affinity to all the proteins. The presence of iron inhibited binding of the compound to transferrin, and small quantities of BSA (0.2%) were required before binding to HDL was observed. The results suggest that in vivo, BSA might enable endotoxin to complex to HDL and transferrin and be removed from the circulation.