Electrostatic interactions between hyaluronan and proteins at pH 4: how do they modulate hyaluronidase activity

Hyaluronan (HA) hydrolysis catalyzed by hyaluronidase (HAase) is inhibited at low HAase over HA ratio and low ionic strength, because HA forms electrostatic complexes with HAase, which is unable to catalyze hydrolysis. Bovine serum albumin (BSA) was used as a model to study the HA-protein electrostatic complexes at pH 4. At low ionic strength, there is formation of (i) neutral insoluble complexes at the phase separation and (ii) small positively-charged or large negatively-charged soluble complexes whether BSA or HA is in excess. According to the ionic strength, different types of complex are formed. Assays for HA and BSA led to the determination of the stoichiometry of these complexes. HAase was also shown to form the various types of complex with HA at low ionic strength. Finally, we showed that at 0 and 150 mmol L(-1) NaCl, BSA competes with HAase in forming complexes with HA and thus induces HAase release resulting in a large increase in the hydrolysis rate. These results, in addition to data in the literature, show that HA-protein complexes, which can exist under numerous and varied conditions of pH, ionic strength and protein over HA ratio, might control the in vivo HAase activity.