Structural, histochemical and biochemical observations on horse milk-fat-globule membranes and casein micelles

Horse milk fat globules (MFGs) and casein micelles were studied using freeze fracturing, freeze etching and thin-section electron microscopy, as well as lectin histochemistry, gel electrophoresis, and Western blotting. Horse MFGs were found to be relatively small, their average volume-surface diameter being about 2.75 microns. The MFG membrane is composed of three layers: an inner proteinaceous coat occasionally having a paracrystalline substructure, a unit membrane, and a prominent filamentous glycocalyx. The last is rich in glycoconjugates, as revealed by its binding of various lectins. In addition, the glycocalyx binds cationized ferritin, which indicates the presence of negative electric charges. Gel electrophoresis revealed the presence of high-molecular-weight glycoproteins in the MFG membrane of horse milk. Such glycoproteins are also present in human MFG membranes but are absent in the bovine MFGs. The casein micelles in horse milk are relatively large, their average volume-surface diameter being about 200 nm.