Plasminogen carbohydrate side chains in receptor binding and enzyme activation: A study of C6 glioma cells and primary cultures of rat hepatocytes

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Abstract
The human [Glu1]‐plasminogen carbohydrate isozymes, plasminogen type I (Pg 1) and plasminogen type II (Pg 2), were separated by chromatography and studied in cell binding experiments at 4°C with primary cultures of rat hepatocytes and rat C6 glioma cells. In both cell systems, Pg 1 and Pg 2 bound to an equivalent number of receptors, apparently representing the same population of surface molecules The affinity for Pg 2 was slightly higher. With hepatocytes, the KD for Pg 1 was 3.2 ± 0.2 μM, and the KD for Pg 2 was 1.9 ± 0.1 μM, as determined from Scatchard transformations of the binding isotherms. The Bmax was approximately the same for both isozymes. With C6 cells, the KD for Pg 1 was 2.2 ± 0.1 μM vs. 1.5 ± 0.2 μM for Pg 2. Again, the Bmax was similar with both isozymes. 125I‐Pg 1 and 125I ‐Pg 2 were displaced from specific binding sites by either nonradiolabeled isozyme. The KI for Pg 2 was slightly lower than the KI for Pg 1 with hepatocytes (0.9 vs. 1.3 μM) and with C6 cells (0.6 vs. 1.1 μM). No displacement was detected with miniplasminogen at concentrations up to 5.0 μM. Activation of Pg 1 and Pg 2 by recombinant two‐chain tissue‐plasminogen activator (rt‐PA) was enhanced by hepatocyte cultures. The enhancing effect was greater with Pg 2. Hepatocyte cultures did not affect the activation of miniplasminogen by rt‐PA or the activation of plasminogen by streptokinase. Unlike the hepatocytes, C6 cells did not enhance the activation of plasminogen by rt‐PA or streptokinase; however, plasmin generated in the presence of C6 cells reacted less readily with α2 ‐antiplasmin.