In view of conflicting evidence for a major hepatic role in the synthesis of circulating plasminogen, the precursor of the fibrinolytic enzyme plasmin, we carried out the present study with a sensitive assay in order to measure the accumulation of small quantities of plasminogen in a recycling rat liver perfusion system. We have purified plasminogen from Sprague-Dawley rat plasma and have raised a monospecific antiserum against it in rabbits. Isolated rat liver perfusions were performed with an oxygenated recycling perfusate consisting of a perfluorotributylamine/Pluronic F-68 emulsion (Fluosol 43) These data indicate that theliver is a major site of plasminogen production.Plasminogen, a protein of Mr t85,000, is present in all mammalian plasmas and is a precursor of the proteolytic (fibrinolytic) enzyme plasmin. Activation of the precursor initiates the process of dissolution of intravascular fibrin deposits (1).The site of synthesis of plasminogen has not been established with certainty, and several organs and tissues have been proposed as possible sources. Earlier immunofluorescence studies demonstrated plasminogen in eosinophilic granulocytes and led to the suggestion that the protein was produced in developing eosinophils of the bone marrow (2). Although the kidney has been implicated as a site of synthesis (3), plasma concentrations and de novo biosynthesis of plasminogen are maintained in anephric rats (4). In contrast, the decreased titer of plasma plasminogen in patients with advanced hepatic cirrhosis (5) suggests that the liver may be a major site of production in man. This conclusion is supported by the recent observation that, after hepatic homotransplantation in man, the plasminogen phenotype of the host converted to that of the donor (6).Although a previous study failed to identify release of plasminogen by the isolated perfused rat liver (7), the search for small quantities of the protein in the perfusate may have been hindered by the lack of a sensitive assay technique at that time.In the present paper we describe a specific and sensitive radioimmunoassay for rat plasminogen that is capable of detecting increasing titers of plasminogen in the plasma-free perfluorochemical perfusing the isolated rat liver. Inhibition of the increase in titer and of incorporation of a labeled leucine by cycloheximide confirmed that the liver is a site of de novo biosynthesis of plasminogen. Furthermore, the rate of release of plasminogen is compatible with the conclusion that the liver is the predominant site of synthesis in vivo. The findings of our study have been presented in abstract form (8). Since this work was completed, evidence has appeared for the capacity of rat hepatocytes in culture to synthesize plasminogen during a period of 48 hr (9).MATERIALS AND METHODS Materials. Rat plasminogen was isolated from pooled, citrated plasma of Sprague-Dawley rats (Pel-Freez) by lysineSepharose affinity chromatography (10) and Sephadex G-150 column chromatography. The purified preparation showed a sharp si...
