Triacylglycerol hydrolysis by cells isolated from lactating rat mammary gland

The hydrolysis of exogenous trioleoylglycerol emulsions by suspensions of cells prepared from lactating rat mammary gland has been investigated. Cell integrity remains high throughout short (at least 30 min) incubations, during which extracellular hydrolysis of trioleoylglycerol proceeds at a mean rate (11 preparations) of 1.9 nmol oleate (and 0.6 nmol glycerol) released/min per mg protein. This hydrolysis shows partial dependence upon added serum and partial inhibition by protamine sulphate - both characteristic properties of lipoprotein lipase-catalyzed lipolysis. One of more monoacylglycerol hydrolase enzymes may also contribute to the measured lipolysis. Evidence is presented consistent with the hypothesis that a surface-located lipoprotein lipase is responsible for the observed lipolysis. Very little lipoprotein lipase activity is released from the cell surface by heparin. During trioleoylglycerol hydrolysis, non-esterified oleate does not accumulate in the cells or in the medium in quantities stoicheiometric with glycerol release. Analyses indicate that it passes into the cells without prior equilibration with the extracellular oleate pool(s). Once inside the cells, oleate is rapidly re-esterified into the triacylglycerol fraction. The possible relevance of these findings to the physiological mechanism of fatty acid uptake from triacylglycerol at the capillary endothelium is discussed.