Glycoproteins of the Human Milk Fat Globule Membrane: Ultrastructure and Relation to Fat Absorption
Evidence is presented that unique high molecular weight glycoproteins of human milk fat globules may be involved in milk fat absorption by the infant. Milk fat globules from a number of species were evaluated for surface structure by freeze-etch and thin-section electron microscopy, and for glycoprotein composition by SDS-gel electrophoresis. Globules of the human, but not of cow, goat, sheep and rat, showed a filamentous surface coat. These filaments could be removed from the globule by heating (80°C-10 min), and were identified by gel electrophoresis as 3 high molecular weight glycoproteins of the globule membrane. This group of proteins was missing from globules of the other species. Several studies (eg. Atkinson et al. J. Pediatr. 99:617, 1981) show that heating milk lowers absorption of its fat in the preterm infant. We suggest that heat destruction of bilesalt-stimulated lipase may not be the explanation of this phenomenon since fat in an infant formula containing no lipase was well absorbed (Shenai et al. Pediatrics 66:233, 1980). Heat-induced changes in the globule surface, such as loss of high molecular weight glycoproteins, affords an alternative or additional explanation.