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Isolation and Characterization of Intracellular Lipid Droplets from Bovine Mammary Tissue¹

Department of Food Science, Cornell University, Ithaca, New York 14850
and The Lipids Laboratory, The Pennsylvania State University, University Park 16802

ABSTRACT

Intracellular lipid droplets were isolated by differential centrifugation from mammary tissue homogenates from three cows. Several isolation techniques were evaluated. Tissue and milk fat globules from one of the cows were analyzed. Electron microscopy of the intracellular lipid droplet fraction revealed droplets .5 to 6 µ in diameter which were partially enveloped by a discontinuous osmiophilic layer. There was no evidence of a lipoprotein bilayer membrane. Phospholipids comprised .7 to 2.0% of the total lipids. Phosphatidylcholine accounted for 53.7 to 64.0% of the total phospholipid. The phosphatidylcholine :phosphatidylethanolamine ratios for intracellular lipid droplets, tissue, and milk fat globules were 3:1, 2:1, and 1:1, respectively. Cholesterol accounted for .4% of the total lipid. The ratio of unesterified to esterified cholesterol was 3.5:1. Intracellular lipid droplets are stabilized within the cytoplasm by a phospholipid-cholesterol film with a small amount of protein adsorbed at the droplet:cytoplasm interface. Similarity between the phospholipid composition of endoplasmic reticulum and intracellular lipid droplets suggests that phospholipids of the droplets were derived from the reticulum.

¹ Authorized for publication January 8, 1973, as paper 4366 in the journal series of the Agricultural Experiment Station, The Pennsylvania State University.