The specific activities of human digestive lipases measured from the in vivo and in vitro lipolysis of test meals

Background & Aims: The lipolytic potential of digestive lipases in vivo has always been deduced so far from their in vitro activities under nonphysiol ogic conditions, In the present study, the specific activities of human gas tric lipase (HGL) and pancreatic lipase (HPL) were measured on dietary trig lycerides (TGs) during test meal lipolysis, Methods: Healthy human voluntee rs ingested a liquid or solid meal, The specific activities of HGL and HPL were estimated from the lipase and free fatty acid (FFA) outputs at the pos tpyloric and duodenal levels, respectively. Based on the in vivo data, lipo lysis was also performed in vitro by mixing the meal either with gastric ju ice and subsequently with pancreatic juice and bile or with purified HGL an d HPL, FFAs were measured by thin-layer chromatography, and the specific ac tivities of HGL and HPL were expressed as micromoles of FFA per minute per milligram of lipase, Results: In vitro, the specific activities on the liqu id meal TGs were 32 (gastric juice) and 34 (pure lipase) mu mol.min(-1).mg( -1) with HGL and 47 (pancreatic juice) and 43 (pure lipase) mu mol.min(-1). mg(-1) with HPL. The specific activities on the solid meal TGs were 33 (gas tric juice) and 32 (pure lipase) mu mol.min(-1).mg(-1) with HGL and 12 (pan creatic juice) and 15 (pure lipase) mu mol.min(-1).mg(-1) with HPL, The in vivo values obtained were in the same range, The secretory lipase outputs w ere 21.6 +/- 14.5 mg HGL and 253.5 +/- 95.5 mg HPL with the liquid test mea l and 15.2 +/- 5.1 mg HGL and 202.9 +/- 96.1 mg HPL with the solid test mea l, Conclusions: The specific activities of HGL and HPL on meal TGs were muc h lower than those measured in vitro under optimized assay conditions (1300 -8000). However, these low specific activities are enough for the meal TGs to be completely lipolysed, given the amounts of HGL and HPL secreted durin g a meal.