DIFFERENTIAL PROPERTIES OF LIPASES ACTIVE AS MEMBRANE-BOUND ENZYMES IN ISOLATED FAT-CELLS

Whole isolated fat cells hydrolyzed at similar reaction rates water-insoluble esters oleoylethanol and trioleoylglycerol, added in emulsified form to the extracellular medium. Both lipolytic activities were bound to the cell membrane during hydrolysis. About 50% of the amount of oleic acid formed with each substrate was taken up by the cells and incorporated into cellular lipids, mainly as triacylglycerol. A number of properties proved to be different for each activity. The active form of triester lipase required activation by serum whereas monoester lipase, assayed in membrane-bound and free form, was respectively unchanged and inhibited by this reagent. Neither enzyme was released into the medium by serum alone. Exposure of the cells to serum plus heparin greatly stimulated the release of triester lipase, whereas the release of monoester lipase was only slightly affected. Under the experimental conditions used, 5 mM ATP and 50 mM NaF inhibited triester lipase, without affecting monoester lipase. Taken together these data, along with differences in degree of susceptibility of the two activities to high ionic strength and protamine sulfate, suggest that they are referable to distinct catalytic proteins. The physiological site of monoester lipase might be the plasma membrane, to which the enzyme is firmly anchored, and its substrate monoacylglycerol, a product of triacylglycerol hydrolysis in capillaries. In turn, the looseness with which triester lipase is bound to the cell gives support to the view that this enzyme, which had the characteristic properties of lipoprotein lipase, could be secreted by fat cells and delivered to its site of action in capillaries. © 1979.