Synthesis and properties of cyclic peptides containing the activation site of plasminogen

The activation of plasminogen results from proteolytic cleavage of the Arg560-Val561 bond by plasminogen activators (Sottrup-Jensen et al. PNAS (1975) 72, 2577). This region of the zymogen occurs in a small disulfide loop that must restrict the conformation around this bond. The nonapeptide sequence NH2-Cys-Pro-Gly-Arg-Val-Val-Gly-Gly-Cys-NH2 of plasminogen containing the activator sensitive arginyl valine bond was synthesized by carbodiimide coupling of Boc-Cys-Pro-Gly-OH(S-4-methylbenzyl) to NH2-Arg(NO2)-Val-Val-Gly-Gly-Cys-NH2(S-4-methylbenzyl), followed by HF treatment and K3Fe(CN)6 oxidation to form a disulfide bond. Purified peptide was not a substrate for urokinase (UK) or plasminogen activator (PA) but possessed a slightly inhibitory activity towards PA. Addition of a lysine to the N-terminus of the nonapeptide yielded a decapeptide sequence of plasminogen that was a better substrate for UK but not for PA. The decapeptide inhibits PA slightly but not UK. These results suggest that active site geometry for PA must be more restrictive than that of UK and that other regions may be involved in the productive interactions with the activators inducing a better fit of the cyclic peptide loop.