Binding of active and inactive forms of lipoprotein lipase to heparin. Effects of pH

Lipoprotein lipase has been shown to bind to, be internalized by, and perhaps be transferred through, a variety of cells. These processes may involve a heparin-like cell-surface receptor and passage through acidified cell compartments. We have therefore studied effects of low pH on the binding of the lipase to heparin and on its catalytic activity. The rate of inactivation of the lipase in solution was found to increase as the pH was lowered. Addition of heparin stabilized the enzyme. Binding of active lipoprotein lipase to heparin-Sepharose could be demonstrated at pH down to 6.5. At pH below 6, binding could not be studied directly because the lipase was too unstable in solution. Lipase bound to heparin-Sepharose could, however, be exposed to pH 4.5 at 10 degrees C with little loss of activity. Binding to heparin-Sepharose also stabilized under physiological conditions (37 degrees C, 0.15 M-NaCl, pH 5.5-7.4). Catalytically inactive lipoprotein lipase retained the ability to bind to heparin-Sepharose. Higher concentrations of salt were needed to displace both active and inactive lipase from heparin-Sepharose at lower pH, indicating that the affinity increased as pH was lowered. The inactive lipase was, however, displaced by lower concentrations of salt than was active lipase.