Small-angle x-ray scattering study of insect lipophorin

The structure of lipophorin in insect blood (hemolymph) was investigated by a small-angle x-ray scattering method over the temperature range 0-45 degrees C. The small-angle x-ray scattering profile of lipophorin exhibited a symmetrical sphere with heterogeneous internal electron density. Cockroach and locust lipophorins, which contain hydrocarbons, demonstrated centrosymmetrical distribution of electron density inside the particles. A previous study suggested that the hydrocarbon-rich region is located in the core of lipophorin particle (Katagiri, C., Kimura, J., and Murase, N. (1985) J. Biol. Chem. 260, 13490-13495). Distance distribution functions, P (r), calculated for a simulated three-layer model (electron-rich shell, middle layer, and electron-deficient core) with radial electron density distribution, show good agreement with those observed experimentally for cockroach and locust lipophorins. The dimensions and electron density obtained for the middle layer reveal that this layer is occupied mainly by diacylglycerol and apolipophorin II. Thus, the present study together with previous reports strongly suggest that insect lipophorin is composed of centrosymmetrical three layers; an outer shell with apolipophorin I and phospholipid, a middle layer with diacylglycerol and apolipophorin II, and a core with hydrocarbons.