Lipoprotein and hormone-sensitive lipases in porcine adipose tissue

Lipoprotein lipase (LPL) is an adipocyte enzyme that cleaves fatty acids from circulating lipoproteins. Fatty acids enter the cell to be oxidized or esterified. Hormone-sensitive lipase (HSL) is an adipocyte enzyme that cleaves fatty acids from intracellular triacylglycerol. The HSL is activated by phosphorylation. Assays for the two lipases are complex because the hydrophobic substrate, triacylglycerol, must be presented as a gum-based suspension or as a detergent-based emulsion to a relatively hydrophilic enzyme. A convenient, stable glycerol/phospholipid suspension of the substrate was used for measurement of porcine adipose tissue LPL and HSL in vitro. This substrate was excellent for LPL. It produced rates five times those using a more complex and less convenient gum-based substrate suspension. The LPL activity was released by heparin, had a pH optimum of approximately 8.5, was activated by serum, and was inhibited by NaCl and protamine. This LPL assay measures enzyme capacity. The same substrate was used to measure an adipose tissue lipase activity that had a pH optimum below 7, was not activated by serum, and was not inhibited by NaCl or protamine. These are all characteristics of HSL. Despite the convenience, this substrate was not appropriate for porcine adipose tissue HSL because the rates were only 30 to 50% of those produced with a more complex, less convenient gum-based substrate suspension. Furthermore, incubation of enzyme or tissue slices with insulin, or agents that elevate cAMP concentration, did not modulate this lipase activity, as expected. These incubations poorly modulated LPL activity.