A conformational transition between an open and closed form of human pancreatic lipase revealed by a monoclonal antibody.

The interfacial activation of human pancreatic lipase (HPL) probably involves the motion of a lid covering the active site of the enzyme. Here we observed that the presence of either bile salts or a small proportion of water-miscible organic solvents (called activator compounds) considerably enhances the enzymatic activity of HPL on a monomeric solution of tripropionin. This finding suggests that the activator compounds may favor the opening of the lid. This hypothesis was checked by comparing the immunoreactivity of HPL and HPL with a mini-lid (HPL(-lid)) towards anti-HPL monoclonal antibodies (mAbs), in the presence and absence of the activator compounds. A single conformational mAb (248-31) fails to immunoprecipitate HPL in the presence of activator compounds and HPL covalently inhibited with diethyl p-nitrophenyl phosphate (DP.HPL). This loss of recognition of HPL by mAb 248-31 was probably due to the motion of the lid, since HPL(-lid) was always recognized in the presence or absence of activator compounds. Furthermore, two other mAbs (81-23 and 146-40) immunoprecipitated HPL similarly whether or not the activator compounds were present. MAb 248-31 therefore specifically recognizes HPL in the closed but not the open conformation.