The lipoprotein particles in cow's milk

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Abstract
The lipoprotein complex of cow''s milk was separated from the casein complex by using the marked tendency of the milk-fat globules to absorb the lipoprotein. Electron micrographs show that both components occur in milk as discrete particles, diminishing in size from a max. of about 200 m[mu]. to less than 30 m[mu]. The brown lipoprotein particles contain about 22% total lipid (largely phospholipid), nucleic acid, a hemochromo-gen (probably cytochrome c) and these enzymic activities: alkaline phosphatase, xanthine oxidase, diaphorase, and DPN-cytochrome c reductase. Succinic dehydrogenase and cytochrome oxidase activities were absent. The lipoprotein particles have properties essentially similar to those of microsomes from lactating mammary gland and other animal tissues and, therefore, are called "milk microsomes". They are released into milk from the mammary gland during the normal secretory process, and occur partly in association with the fat globule.