Kinetic Studies on Pancreatic Lipase Activity in Micellar Systems. III. Effect of Micellar Size

Abstract

Pancreatic lipase activities toward fatty acid vinyl esters solubilized in micelles of five surfactants, sodium taurodeoxycholate (NaTDC), octa-oxyethylene dodecyl ether (ODE), and three polyoxyethylene nonylphenyl ethers (PNE), were kinetically investigated. The kinetic data for all the micellar systems were well interpreted on the basis of fully competitive inhibition mechanisms, where substrate-free micelles act as an inhibitor. For NaTDC micellar systems, the maximum velocity V, the Michaelis constant K_m, and inhibition constant K₄ were determined with respect to four substrates. The K_m/K₄ values for all the substrates were larger than unity but smaller than those previously obtained for sodium deoxycholate micellar systems. For the micellar systems of ODE and two PNEs with average numbers of oxyethylene units of 10 and 15, the Michaelis plots at constant surfactant concentrations were linear, indicating that the K_m/K₄ value is close to unity. On the other hand, for PNE with an average number of oxyethylene units of 30, a similar plot was concave, which suggests that the K_m/K₄ value is larger than unity. The alteration of the K_m/K₄ value with surfactants was closely correlated with the change in stability of the enzyme-micelle solubilizing substrate complex relative to the enzyme-substrate-free micelle complex ; the relative stability was shown to depend on the micellar size. The maximum velocity was also affected by micellar size.