No abstract

Access to the active site of human pancreatic lipase (HPL) is controlled by a surface loop (the lid) that undergoes a conformational change in the presence of amphiphiles and lipid substrate. The question of how and when the lid opens still remains to be elucidated, however. A paramagnetic probe was covalently bound to the lid via the D249C mutation, and electron paramagnetic resonance (EPR) spectroscopy was used to monitor the conformational change in solution. Two EPR spectral components, corresponding to distinct mobilities of the probe, were attributed to the closed and open conformations of the HPL lid, based on experiments performed with the E600 inhibitor. The open conformation of the lid was observed in solution at supramicellar bile salt concentrations. Colipase alone did not induce lid opening but increased the relative proportions of the open conformation in the presence of bile salts. The opening of the lid was found to be a reversible process. Using various colipase to lipase molar ratios, a correlation between the proportion of the open conformation and the catalytic activity of HPL was observed.

This study presents a new method for evaluating the oxidation of lubricating oils. An aging cell adapted to a Fourier transform infrared (FT-IR) spectrometer allows the continuous and direct study of the oxidative aging of base oils. During the test, oxidation bands appeared in the spectra (carbonyl bands around 1730 cm(-1)). The graphic representation of the carbonyl band modification--using a spectroscopic index--makes it possible to monitor the evolution of the lubricant composition. Comparing the oxidation constants, determined from the kinetic plots of several base oils, makes it possible to evaluate their relative sensitivity.