Lipid-protein globules of avian egg yolk. Isolation and properties of globules stable in concentrated sodium chloride solution

Abstract

A new type of globular particle, the insoluble yolk globule, was isolated from the egg yolk of 3 avian species [hen, Gallus domesticus, duck, Anas platyrhyndros and emu, Dromaeus novaehollandiae] by centrifugation or gel-filtration chromatography. These globules are stable in NaCl and urea solutions at concentrations that dissolve or disrupt other constituents of yolk. The isolated globules are about 1% of the dry yolk of hen''s and duck''s eggs but about 8% of emu''s-egg yolk. Most of these globules are less than 2 .mu.m in diameter. Electron micrographs of sections show a preponderance of globules in the range 0.125-0.25 .mu.m, each with a thick shell surrounding a featureless interior. Globules with the same appearance were seen in sections of unfractionated yolk. Two kinds of larger particle were also observed: particles with a distinct outer membrane and a vesiculated interior; featureless spheres, possibly of lipid. The insoluble yolk globules comprise protein (8-11% by dry wt.), phospholipid (31-35% total lipid), triacyglycerols (49-53%), cholesterol (8%) and cholesteryl esters (2-3%); the variations being among species. The phospholipid is accessible to phospholipase C. The isolated protein is heterogeneous and resembles the apoprotein from the yolk low-density lipoprotein.