The relationship of milk phospholipids to membranes of the secretory cell

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Abstract
Forty‐two per cent of the lipid phosphorus in milk was found in skim milk lipoprotein; the other 58% occurs in the milk fat globule membrane (MFGM). Investigation of these two sources of lipid phosphorus revealed that they involve the same individual phospholipids, in essentially the same proportions with similar fatty acid compositions. Both contain sphingomyelin and cerebrosides in levels characteristic of those found in plasma membranes. Other points of resemblance between MFGM and skim milk lipoprotein, have been shown previously. Infusion of (14C) palmitate into the mammary gland of a lactating goat produced more extensive labeling of all the phospholipid classes in the skim milk lipoproteins than in those in the MFGM during the following 24 hr. When (14C) palmitate was infused into the jugular vein of a lactating goat, a precusor‐product‐type relationship was observed between specific activities of the skim milk and MFGM polar lipids. These results render the MFGM an unlikely origin of the skim milk lipoprotein. Other possible sources of this latter lipoprotein are Golgi vesicle membranes or plasma membrane of the lactating cell.