Vascular endothelial growth factor (VEGF) is a potent and specific mitogen for endothelial cells. VEGF is synthesized and secreted by many differentiated cells in response to a variety of stimuli including hypoxia. VEGF is expressed in a variety of tissues as multiple homodimeric forms (121, 165, 189, and 206 amino acids/monomer) resulting from alternative RNA splicing. VEGF121 is a soluble mitogen that does not bind heparin; the longer forms of VEGF bind heparin with progressively higher affinity. The higher molecular weight forms of VEGF can be cleaved by plasmin to release a diffusible form(s) of VEGF. We characterized the proteolysis of VEGF by plasmin and other proteases. Thrombin, elastase, and collagenase did not cleave VEGF, whereas trypsin generated a series of smaller fragments. The isolated plasmin fragments of VEGF were compared with respect to heparin binding, interaction with soluble VEGF receptors, and ability to promote endothelial cell mitogenesis. Plasmin yields two fragments of VEGF as indicated by reverse phase high performance liquid chromatography and SDS-polyacrylamide gel electrophoresis: an amino-terminal homodimeric protein containing receptor binding determinants and a carboxyl-terminal polypeptide which bound heparin. Amino-terminal sequencing of the carboxyl-terminal peptide identified the plasmin cleavage site as Arg110-Ala111. A heterodimeric form of VEGF165/110, was isolated from partial plasmin digests of VEGF165. The carboxyl-terminal polypeptide (111-165) displayed no affinity for soluble kinase domain region (KDR) or Fms-like tyrosine kinase (FLT-1) receptors. The various isoforms of VEGF (165, 165/110, and 121) bound soluble kinase domain region receptor with similar affinity (approximately 30 pM). In contrast, soluble FLT-1 receptor differentiated VEGF isoforms (165, 165/110, 110, and 121) with apparent affinities of 10, 30, 120, and 200 pM, respectively. Endothelial cell mitogenic potencies of VEGF110 and VEGF121 were decreased more than 100-fold compared to that of VEGF165. The present findings indicate that removal of the carboxyl-terminal domain, whether it is due to alternative splicing of mRNA or to proteolysis, is associated with a significant loss in bioactivity.