Kinetic studies on pancreatic lipase activity in micellar systems. III. Effect of micellar size.

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Abstract
Pancreatic lipase activities toward fatty acid vinyl esters solubilized in micelles of 5 surfactants, sodium taurodeoxycholate (NaTDC), octa-oxyethylene dodecyl ether (ODE) and 3 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, Km, and inhibition constant K4 were determined with respect to 4 substrates. The Km/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 2 PNE with average numbers of oxyethylene units of 10 and 15, the Michaelis plots at constant surfactant concentrations were linear, indicating that the Km/K4 value is close to unity. For PNE with an average number of oxyethylene units of 30, a similar plot was concave, which suggests that the Km/K4 value is larger than unity. The alteration of the Km/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 depended on the micellar size. The maximum velocity was also affected by micellar size.