Lipoprotein lipase. Localization on plasma membrane fragments from lactating rat mammary tissue

Subcellular fractionation studies were done using lactating rat mammary tissue. Lipoprotein lipase (E.C. 3.1.1.34) was found to be associated with plasma membrane-enriched and endoplasmic reticulum-enriched fractions. Considerable amounts of soluble lipoprotein lipase were found after homogenization of this tissue. The membrane-associated lipoprotein lipase activities described here were found to be resistant to release, by heparin, from their membrane sites. Buoyant density gradient fractionation experiments suggest that the lipoprotein lipase activity of lactating mammary tissue is located in part on the secretory epithelial cell plasma membrane. This conclusion is discussed in the context of the known site of action, in vivo, of mammary gland lipoprotein lipase, and of probable routes of its transport to that site.