Human milk bile-salt stimulated lipase: further investigations on the amino-acids residues involved in the catalytic site.

The bile-salt-stimulated lipase purified from human skim milk was modified with diisopropyl phosphofluoridate (DFP), N-ethyl-5-phenylisoxazolium-3'-sulfonate and ethoxyformic anhydride. These chemical modifications lead to the following results: (1) the inhibition of the enzyme by DFP is due to the phosphorylation of a single residue, probably a serine residue, which may represent the acylable group of the enzyme; (2) carbethoxylation of histidine residues leads to inhibition of the enzyme activity. Among the nine modified histidine residues, only one is essential for enzyme activity; (3) a free carboxyl group with a pKa of 5.4 is also involved in catalysis. These three essential residues are involved in the enzymatic hydrolysis of substrates whatever their physical state (soluble or emulsified). Upon treatment with DFP as well as with ethoxyformic anhydride, the enzyme remains able to bind to the model interface formed by siliconized glass-beads with almost the same efficiency (Kd between 4.1 and 7.4.10(-8) M) than the native bile-salt-stimulated lipase (Kd = 6.3.10(-8) M). Moreover, the modified and native enzymes occupy the same interfacial area (4000-4600 A2/molecule). By contrast, the enzyme modified by N-ethyl-5-phenylisoxazolium-3'-sulfonate reagent presents an interfacial area close to that of a denatured protein of size (approximately 18,300 A2/molecule) and a Kd more than 20-fold higher than that of the native enzyme. Solvent isotope effects measured on kcat/Km and kcat indicate that the catalytic mechanism of bile-salt-stimulated lipase involves transition states that are stabilized by hydrogen bonds as described in the two-step acylation-deacylation mechanism of serine enzymes.