Effects of methanol on the catalytic properties of porcine pancreatic lipase

The effect of aqueous methanol on the catalytic properties of porcine pancreatic lipase has been investigated. The k_cat values for the hydrolysis of N^α-benzyloxycarbonyl-L-lysine p-nitrophenyl ester at 0°C increased in a linear manner with increasing methanol concentration. However, the K_M values were not influenced at methanol concentrations lower than 30% and then began to increase at higher concentrations in an exponential fashion. Based on product analysis, the increase in k_cat with increasing methanol concentration can be accounted for by nucleophilic competition of methanol for the acyl enzyme intermediate, indicating that the rate-limiting step of the porcine pancreatic lipase-catalyzed reaction is deacylation under current experimental conditions. The exponential increase in K_M at methanol concentrations higher than 30% is attributed to the hydrophobic partitioning effect on substrate binding. There was no loss of lipase activity over a 4 h period in 60% methanol concentration at pH* 5.5 and 0°C. By monitoring the intrinsic fluorescence and absorbance, no evidence for structural changes by methanol was observed.