Influence of surface properties of mixed monolayers on lipolytic hydrolysis

Fluorescence microscopy, surface potential, and activity measurements were used to investigate the influence of fatty acids and fatty alcohols on the lipolytic activity of several lipases. We have determined the lateral lipid distribution and interfacial properties of Langmuir mixed monolayers compo sed of 1,2-didecanoylglycerol/eicosanoic acid or 1,2-didecanoylglycerol/1-o ctadecanol molecules and have measured lipase activities toward these films . Enzymatic activities are remarkably influenced by the addition of fatty a cid. Activity decreases continuously up to a mole fraction of approximate t o 0.1 fatty acid, where phase separation and a change in surface potential are observed. Higher concentrations of fatty acid have only marginal effect s on the lipase activities. The relative activity between the different lip ases varies substantially and there is an indication that the level of inhi bition correlates with the isoelectric point (pI) of the enzymes. A simpler mechanism is observed by the addition of fatty alcohol. Within the concent ration range studied, 1-octadecanol is immiscible in the diacylglyceride ma trix, forming liquid-condensed domains. The inhibitory effect is related to the reduction of available diacylglyceride area to the enzyme; Direct imag ing of the lipolytic hydrolysis of these monolayers show that relatively sm all domains are formed, suggesting that the enzyme preferentially acts on p ure diacylglyceride patches.