Activated rat HSL at the lipid particle hydrolyzes triacylglycerol to yield diacylglycerol + fatty acid. In vitro, activated partially purified HSL catalyzes this reaction at only about two times the rate measured with non-activated enzyme (Fredrikson et al. 1981). The much greater rate increase caused by HSL phosphorylation in vivo appears to be due to its phosphorylation-dependent translocation to the surface of the lipid particle (Birnbaum 2003).
HSL-mediated triacylglycerol hydrolysis in humans has not been studied in detail, so the human reaction is inferred from the well-studied rat one.