Proteins of the inter-alpha-trypsin inhibitor family stabilize the cumulus extracellular matrix through their direct binding with hyaluronic acid.

We have previously identified a glycoprotein of the inter-alpha-trypsin inhibitor family of proteins as a serum factor responsible for the stabilization of the expanding cumulus mass. In this study, the mechanism of interaction of this cumulus extracellular matrix stabilizing factor (cESF) with hyaluronic acid (HA) has been explored. It was found that the pH optimum for binding of cESF and HA is 7 and that binding is sensitive to ionic strength. The dissociation constant is about 1.9 x 10(-8) M in 10 mM sodium phosphate buffer (pH 7.2). Circular dichroism studies show that cESF contains about 24% alpha-helical and 42% beta-sheet structure. Gross conformational changes in cESF, however, are not detected in the presence of HA. We also found that modification of lysine residues of cESF with citraconic anhydride greatly reduced its binding with HA and completely abolished its cumulus stabilizing activity, and deblocking lysine residues restored its capacity to bind with HA and its cumulus matrix stabilizing activity. This evidence supports the hypotheses that cESF stabilizes the expanding cumulus extracellular matrix by directly binding with HA and that cESF may serve as a structural protein to organize the formation of the cumulus extracellular matrix. Our evidence also supports the view that binding of cESF and HA is through a stereo-specific charge interaction. Putative binding sites of cESF that interact with HA are postulated.