The role of the plasma membrane in the secretion of milk fat

Abstract

On the basis of electron microscopic evidence, Bargmann & Knoop (1959) proposed that milk fat is secreted from the lactating cell by progressive envelopment of the fat droplet with plasma membrane at the apex of the cell. Ultimately the fully wrapped globule is pinched off from the cell. Support for this hypothesis is presented here in the form of biochemical data together with a rationale for the biophysics of the process. The biochemical evidence indicates that fat droplets within the cell are relatively devoid of certain membrane constituents, particularly phosphatidyl ethanolamine and carotenoids characteristically present in the surface coat of secreted milk fat globules. From a combination of compositional analyses and turnover studies with 14C-fatty acid, the logical origin of these components is the plasma membrane of the cell. The envelopment of the droplet appears adequately explained on the basis of London-van der Waals forces which are estimated to achieve an attraction force of one atmosphere when the separating distance between membrane and droplet is reduced to 20 Å. Milk proteins appear to be secreted by emptying of vacuoles through the plasma membrane. Membranes around such vacuoles may replenish the plasma membrane used in fat secretion, thus relating the two processes.