THROMBOSIS RESEARCH 39; 379-386, 1985 0049-3848185 $3.00 + .OO Printed in the USA. Copyright (cl 1985 Pergamon Press itd. All rights reserved. BRIEF COMMUNICATION THE FIBRINOGEN-FIBRIN COMPLEX: STOICHIOMETRY AND NEGATIVE COOPERATIVITY J.L. USERO, F.J. B"RGUILLO* , C. IZQUIERDO, A. DEL ARC0 and M.A. HERRAEZ Departamento de Qufmica Fisica, Facultad de Quimica, Universidad de Salamanca, Espafia. (Received 24.11.1984; Accepted in original form 21.3.1985 by Editor P.J. Gaffney) (Received in final form by Executive Editorial Office 14.5.1985) INTRODUCCION The existence of soluble complexes formed by monomers of fibrin and nati- ve fibrinogen has been described by numerous authors both "in vivo" (1) and "in vitro" (Z-4). Their formation fits into the general scheme of fibrinofor - mation according to the following equilibria: FIBRINOGEN (0) THROMBIN > FIBRIN MONOMER (F) @+F * ?? 0-F COMPLEX nF (F), CLOT However, and in spite of the importance of the formation of these com-- plexes, their nature remains practically unknown and the literature concer- ning them is scanty (5,6). In 1981, our team obtained a preliminary estimation of the formation constants of the complexes, assuming a stoichiometry of 1:l (7). The present work offers suggestive results about fibrin-fibrinogen interactions. MATERIALS AND METHODS Kabi grade L (AB Kabi, Stockholm) human fibrinogen was used in the study after treatment as has been described elsewhere (8). The fibrin monomers we re prepared using Fibrindex human thrombin (Ortho Diagnostic Inc), according to the method also described previously (8). In the case of fibrin monomers prepared from Batroxobin, the fibrinogen was incubated at 37% with Batroxobin (Reptilase, Pentapharm Switzerland) at a ratio of 1 unit of Batroxobin (B.U.) per 5 mg of fibrinogen over 5 min; after this time the clot was centrifuged and the same procedure was followed as for the monomers prepared from fibrinogen and thrombin. The determination of the fibrinogen-bound fibrin was carried out by spec- trophotometric measurement at 350 nm of the aggregation rate of the fibrin monomers in the absence and presence of fibrinogen. The existence of fibri - -.-____ __-_____ *Present address: Departamento de Fisicoquimica, Facultad de Farmacia, Uni- versidad de Salamanca, Espafia. Key words: Fibrinogen-fibrin complex, fibrinogen, fibrin monomer 379