Binding of Zn2+ to the plasma protein inter-alpha-inhibitor

Inter-alpha-inhibitor (I alpha I) is a serum protein consisting of a chondr oitin-sulfate-containing protein of 25 kDa (bikunin) and two other polypept ides of 75-80 kDa (heavy chains 1 and 2). The physiological function of I a lpha I is unclear but recent results suggest that it is required for the fo rmation of the extracellular matrix of certain cell types and that it has a nti-inflammatory activity. It was previously reported that I alpha I isolat ed from serum contains bound Zn2+, but details of this binding are lacking. Using equilibrium dialysis, we have found that when the free Zn2+ concentr ation is raised from 0.3 to 50 mu mol/L, the number of bound ions increases from 0.1 to 7. The concentration of free Zn2+ in plasma is in the nanomola r range; our results therefore suggest that inter-a-inhibitor does not cont ain stoichiometric amounts of zinc ions under normal in vivo conditions. (C ) 1999 Elsevier Science B.V. All rights reserved.