Clinica Chimica Acta 288 (1999) 37–46 www.elsevier.com / locate / clinchim 2 1 Binding of Zn to the plasma protein inter-a-inhibitor * Anna M. Blom, Maria Thuveson, Witold Kilarski, Erik Fries Department of Medical Biochemistry and Microbiology, Uppsala University, Biomedical Center, Box 582, S-751 23 Uppsala, Sweden Received 29 January 1999; received in revised form 18 June 1999; accepted 24 June 1999 Abstract Inter-a-inhibitor (IaI) is a serum protein consisting of a chondroitin-sulfate-containing protein of 25 kDa (bikunin) and two other polypeptides of 75–80 kDa (heavy chains 1 and 2). The physiological function of IaI is unclear but recent results suggest that it is required for the formation of the extracellular matrix of certain cell types and that it has anti-inflammatory activity. 21 It was previously reported that IaI isolated from serum contains bound Zn , but details of this 21 binding are lacking. Using equilibrium dialysis, we have found that when the free Zn concentration is raised from 0.3 to 50 mmol / L, the number of bound ions increases from 0.1 to 7. 21 The concentration of free Zn in plasma is in the nanomolar range; our results therefore suggest that inter-a-inhibitor does not contain stoichiometric amounts of zinc ions under normal in vivo conditions.  1999 Elsevier Science B.V. All rights reserved. 21 Keywords: Inter-a-inhibitor; Zn 1. Introduction Inter-a-inhibitor (IaI) is a proteinase inhibitor that occurs at a concentration of 0.3–0.5 g/l in human plasma. IaI consists of three polypeptides: bikunin, *Corresponding author. Tel.: 1 46-18-471-4199; fax: 1 46-18-471-4975. E-mail address: erik.fries@medkem.uu.se (E. Fries) 0009-8981 / 99 / $ – see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S0009-8981(99)00135-7