Specific intracellular hyaluronic acid binding to isolated rat hepatocytes is membrane-associated

Intact isolated rat hepatocytes show a small amount of specific ¹²⁵I-labeled hyaluronic acid (HA) binding. However, in the presence of digitonin, a very large increase in the specific binding of ¹²⁵I-HA is observed. Chondroitin sulfate, heparin and dextran sulfate were as effective as unlabeled HA in competing for ¹²⁵I-HA binding to permeabilized hepatocytes, indicating that the binding sites may have a general specificity for glycosaminoglycans. After rat hepatocytes had been homogenized in a hypotonic buffer, more than 98% of the ¹²⁵I-HA binding activity could be pelleted by centrifugation at 100 000 × g for 1 h. Mild alkaline treatment of hepatocyte membranes did not release ¹²⁵I-HA binding activity, suggesting that the HA binding site is an integral membrane molecule. Furthermore, trypsin treatment of deoxycholate-extracted membranes destroyed the binding activity, as assessed by a dot-blot assay. This suggests that a protein component in the membrane is necessary for ¹²⁵I-HA binding activity. Rat fibrinogen could be a possible candidate for the HA binding activity because HA binds specifically to human fibrinogen (LeBoeuf et al. (1986) J. Biol. Chem. 261, 12 586). Also, fibrinogen can be found in a quasi-crystalline form in rat hepatocytes and could be pelleted with the membranes. Rat fibrinogen was not responsible for the ¹²⁵I-HA binding activity, since (1) purified rat fibrinogen did not bind to ¹²⁵I-HA, and (2) immunoprecipitation of rat fibrinogen from hepatocyte extracts did not decrease the ¹²⁵I-HA binding of these extracts. We conclude that the internal HA binding sites are membrane- or cytoskeleton-associated proteins and are neither cytosolic proteins nor fibrinogen.