Hydrolysis and uptake of an aliphatic fatty ester by whole isolated fat cells

During 9-min incubations at 37 degrees C, whole isolated fat cells are able to hydrolyze a water-insoluble ester, ethyl [3H]oleate, added as an emulsion to the extracellular medium. The monoester lipase exerts its activity as a membrane-bound enzyme and is not releasable by heparin. Variation of the reaction rate with substrate concentration suggests a diffusion-limited process, consonant with the fact that enzyme and substrate are associated with particles of enormous sizes (the fat cell and the lipid droplet, respectively). Variation of lipase activity with increasing temperature in the incubation medium shows a break at 46 degrees C +/-1 degrees C, most probably related to a phase transition in the membrane structure. During incubation, 30-60% of the amount of [3H]oleic chains formed by hydrolysis are incorporated into the cell, mainly as triacylglycerols. Controls of cell viability indicate that under the experimental conditions the framework of the membrane and the internal organization of the cell appear to be maintained. Such an experimental model might duplicate physiological conditions since aliphatic fatty esters are thought to be hydrolyzed at the cell surface during uptake.