title: Solvent isotope effects for lipoprotein lipase catalyzed hydrolysis of water-soluble p-nitrophenyl esters.
abstract: Solvent deuterium isotope effects on the rates of lipoprotein lipase (LpL) catalyzed hydrolysis of the water-soluble esters p-nitrophenyl acetate (PNPA) and p-nitrophenyl butyrate (PNPB) have been measured and fall in the range 1.5-2.2. The isotope effects are independent of substrate concentration, LpL stability, and reaction temperature and hence are effects on chemical catalysis and not due to a medium effect of D2O on LpL stability and/or conformation. pL (L = H or D) vs. rate profiles for the Vmax/Km of LpL-catalyzed hydrolysis of PNPB increase sigmoidally with increasing pL. Least-squares analysis of the profiles gives pKaH2O = 7.10 +/- 0.01, pKaD2O = 7.795 +/- 0.007, and a solvent isotope effect on limiting velocity at high pL of 1.97 +/- 0.03. Because the pL-rate profiles are for the Vmax/Km of hydrolysis of a water-soluble substrate, the measured pKa's are intrinsic acid-base ionization constants for a catalytically involved LpL active-site amino acid side chain. Benzeneboronic acid, a potent inhibitor of LpL-catalyzed hydrolysis of triacylglycerols [Vainio, P., Virtanen, J. A., & Kinnunen, P. K. J. (1982) Biochim. Biophys. Acta 711, 386-390], inhibits LpL-catalyzed hydrolysis of PNPB, with Ki = 6.9 microM at pH 7.36, 25 degrees C. This result and the solvent isotope effects for LpL-catalyzed hydrolysis of water-soluble esters are interpreted in terms of a proton transfer mechanism that is similar in many respects to that of the serine proteases.
mesh_terms: D000818:Animals; D002417:Cattle; D003903:Deuterium; D005260:Female; D006863:Hydrogen-Ion Concentration; D006868:Hydrolysis; D007700:Kinetics; D008071:Lipoprotein Lipase / Q000378:metabolism*; D008892:Milk / Q000201:enzymology; D009596:Nitrophenols; D011485:Protein Binding; D012997:Solvents; D013379:Substrate Specificity
chemical_list: D009596:Nitrophenols; D012997:Solvents; D003903:Deuterium; D008071:Lipoprotein Lipase