Comparative study of antithrombin III-protease complex metabolism by fibroblasts and vascular endothelial cells

¹²⁵I-labeled human antithrombin III (¹²⁵I-AT III)-protease complexes are specifically bound to both cultured human skin fibroblast (HSF) cells and adult bovine aortic endothelial (ABAE) cells; however, there is a significant difference in the rate and degree of metabolism of the complexes by these two cell types. HSF cells appear to internalize the complexes at a rate of about 2.5 pmole/1×10⁶ cells/h and subsequently degrade them at a rate of 0.6 pmole/1×10⁶ cells/h. ABAE cells internalize and degrade the complexes at rates approximately 100 and 30 times lower, respectively. Neither cell type interacts with free ¹²⁵I-AT III but only with its combined form with either thrombin or trypsin. These data indicate the major role of HSF cells in the removal of AT III protease complexes from extravascular spaces in the body, in contrast to the inert vascular surface with regard to AT III.protease complexes provided by the vascular endothelium.