A specific disulfide bond associated with the activity of human urokinase. Its topological identification and reductive cleavage followed by kinetic changes in enzymatic reaction and affinity labeling

Biochemical properties of recombinant mutants of nonglycosylated single chain urokinase-type plasminogen activator

The role of glycosylation on the enzymatic properties of single chain urokinase-type plasminogen activator (scu-PA) was investigated by site-specific mutagenesis of the glycosylated Asn-302 residu to Gln. In addition, the role of the NH2-terminal polypeptide chain and of the Cys-148 to Cys-279 interchain disulphide bond on the activity of non-glycosylated scu-PA was investigated. Therefore, variants of recombinant scu-PA (rscu-PA) were produced by transfecting Chinese hamster ovary cells with cDNA encoding rscu-PA N302Q (rscu-PA with Asn-302 to Gln mutation), rscu-PA C279A,N302Q (rscu-PA with Cys-279 to Ala and Asn-302 to Gln mutations) or rscu-PA del(N2-F157)C279A,N302Q (rscu-PA C279A,N302Q with deletion of Asn-2 through Phe-157). These mutants were purified to homogeneity from conditioned cell culture medium and were obtained essentially as single chain molecules with specific activities on fibrin plates of (mean +/- S.E.; n = 6) 45,000 +/- 5000. IU/mg, 19,000 +/- 800 IU/mg and < or = 100 IU/mg for rscu-PA N302Q, rscu-PA C279A,N302Q and rscu-PA del(N2-F157)C279A,N302Q, respectively, as compared to 64,000 +/- 2600 IU/mg for wild-type rscu-PA obtained in the same expression system. Plasmin quantitatively converts rscu-PA N302Q and rscu-PA C279A,N302Q to amidolytically active two-chain derivatives with a specific activity of 56,000 IU/mg and 32,000 IU/mg, respectively, as compared to 75,000 IU/mg for wild-type rscu-PA. Plasminogen activation as a function of time was comparable for rscu-PA N302Q and wild-type rscu-PA, and somewhat slower for rscu-PA C279A,N302Q. In a human plasma milieu in vitro, consisting of a 125I-fibrin labeled plasma clot submerged in plasma, 50 percent clot lysis in 2 h required 2.2 micrograms/ml rscu-PA N302Q and 6.0 micrograms/ml rscu-PA C279A,N302Q, as compared to 3.2 micrograms/ml wild-type rscu-PA. In contrast, rscu-PA del(N2-F157)C279A,N302Q was not converted to an amidolytically active two chain derivative by plasmin, and did not induce significant plasminogen activation in purified systems or clot lysis in a human plasma milieu. Following bolus injections in hamsters, the initial half-lives (1.8-2.6 min) and the plasma clearances (0.6-1.5 ml min-1) were comparable for wild-type rscu-PA and for the three rscu-PA mutants. These results suggest that the fibrinolytic activity in a plasma milieu in vitro and the in vivo turnover of rscu-PA are not markedly affected by the absence of carbohydrate.(ABSTRACT TRUNCATED AT 400 WORDS)

The fibrinolytic system in man

The fibrinolytic system comprises a proenzyme, plasminogen, which can be activated to the active enzyme plasmin, that will degrade fibrin by different types of plasminogen activators. Inhibition of fibrinolysis may occur at the level of plasmin or at the level of the activators. Fibrinolysis in human blood seems to be regulated by specific molecular interactions between these components. In plasma, normally no systemic plasminogen activation occurs. When fibrin is formed, small amounts of plasminogen activator and plasminogen adsorb to the fibrin, and plasmin is generated in situ. The formed plasmin, which remains transiently complexed to fibrin, is only slowly inactivated by alpha 2-antiplasmin, while plasmin, which is released from digested fibrin, is rapidly and irreversibly neutralized. The fibrinolytic process, thus, seems to be triggered by and confined to fibrin. Thrombus formation may occur as the result of insufficient activation of the fibrinolytic system and (or) the presence of excess inhibitors, while excessive activation and/or deficiency of inhibitors might cause excessive plasmin formation and a bleeding tendency. Evidence obtained in animal models suggests that tissue-type plasminogen activator, obtained by recombinant DNA technology, may constitute a specific clot-selective thrombolytic agent with higher specific activity and fewer side effects than those currently in use.

Conformational changes in human urokinase induced by a specific reduction of disulfide bond in Cys-194-Cys-222 associated with exhibition of enzymatic activity

The disulfide bond in Cys-194-Cys-222 of the isolated B chain (UK X B) of human urinary urokinase was selectively carboxamidomethylated (R X CAM-UK X B) after specific reduction with dithiothreitol in the presence of the competitive inhibitor N-alpha-benzoyl-L-argininamide (BzArgNH2) and 0.3 M guanidine with inactivation. There were no differences between UK X B and R X CAM-UK X B in CD spectra of 200-250 nm and infrared spectra in the amide I and II regions, whereas two out of five negative CD bands of 250-310 nm were slightly reduced in R X CAM-UK X B. R X CAM-UK X B was more susceptible than UK X B to guanidine denaturation, H-2H exchange in peptide protons and tyrosine-residue ionization. These results indicate that Cys-194-Cys-222 contributes to conformational stabilization against perturbants, but not to static conformation in the resting state, except for a slight disruption in the amino-acid side-chains. BzArgNH2 and 0.3 M guanidine, corresponding to the threshold point toward denaturing transition, enhanced more markedly all the five CD bands of 250-310 nm in UK X B than in R X CAM-UK X B, but did not alter CD spectra of 200-250 nm. The deletion of either BzArgNH2 or guanidine did not induce the inactivation or the cleavage of Cys-194-Cys-222 upon the reduction of UK X B, nor did it induce a marked CD enhancement in the near-ultraviolet upon the addition to UK X B. The results suggest that the BzArgNH2-UK X B interaction enhances the solvent accessibility to the disulfide bond with aid of critical guanidine-denaturation.