Thrombin affinity mapping of its receptor tethered ligand.

Peptides were synthesized corresponding to those of the tethered ligand following the thrombin activation cleavage of the cloned human thrombin receptor. To determine affinities of synthetic peptides, the hydrolysis of a tripeptide chromogenic substrate by both human alpha-thrombin (with high fibrinogen clotting activity) and gamma-thrombin (essentially lacking this activity) was examined. For the longest peptide (22 residues), alpha- and gamma-thrombins gave similar but significantly different inhibition constants (i.e. 16 and 27 microM, respectively). However, the first 14-residue peptide did not behave significantly differently with either form of thrombin, nor did peptides containing the agonist ligand (e.g. SFLLRNP), suggesting that these peptides interacted with common regions in both thrombin forms. In contrast, peptide 8-22 (following the agonist peptide) was an activator of alpha- and an inhibitor of gamma-thrombin. Peptide 11-20 bracketed the enhancement activity with alpha-thrombin, while neither peptide 10-14 nor 15-22 displayed this activity. Such enhancement is believed to result from interactions with the fibrinogen recognition exosite (exosite I), which is present in alpha- but missing or disrupted in gamma-thrombin.

Thrombin-specific inhibition by and slow cleavage of hirulog-1.

Hirulog-1 [D-Phe-Pro-Arg-Pro-[Gly]4-desulphohirudin-(53-64) (HV1)] was designed to bind by its first four and last 12 residues to the alpha-thrombin catalytic site and anion-binding exosite for fibrin(ogen) recognition respectively, with a [Gly]4 bridge and an Arg-Pro bond at the scissional position. Human alpha-, gamma- and zeta-thrombins, as well as bovine trypsin, readily hydrolyse Spectrozyme-TH (D-hexahydrotyrosyl-Ala-Arg p-nitroanilide) at pH 7.4 and approx. 23 degrees C. Both alpha- and zeta-thrombins, which have high fibrinogen-clotting activities (greater than 3000 kunits/g), were inhibited with this substrate by hirulog-1 [Ki = 2.56 +/- 0.35 nM (n = 3) and 1.84 +/- 0.15 nM (n = 3) respectively] and slowly cleaved the inhibitor [k = 0.326 +/- 0.082 min-1 (n = 12) and 0.362 +/- 0.056 min-1 (n = 18) respectively], whereas gamma-thrombin, which has essentially no clotting activity (approx. 4 kunits/g), and trypsin were not inhibited with greater than 1000-fold molar excess of hirulog-1. Similar inhibition parameters were also obtained for hirulog-1 incubated with alpha-thrombin or zeta-thrombin at approx. 23 degrees C and by measuring thrombin activity with fibrinogen in the clotting assay at 37 degrees C. Cleavage of the Arg-3-Pro-4 bond in hirulog-1 by either alpha- or zeta-thrombin was shown by identical cleavage products of either thrombin on h.p.l.c. and by sequence analysis of the alpha-thrombin products. These data demonstrate that hirulog-1 is a specific inhibitor of thrombin forms with high fibrinogen-procoagulant activities and that its Arg-3-Pro-4 bond is slowly cleaved by these thrombin forms.

Human alpha- to zeta-thrombin cleavage occurs with neutrophil cathepsin G or chymotrypsin while fibrinogen clotting activity is retained.

Human neutrophil cathepsin G or bovine chymotrypsin proteolytically cleaved human alpha-thrombin at the B-chain Trp148-Thr149 bond generating a new form, zeta-thrombin. While incubation of alpha-thrombin with cathepsin G at pH 7.4 and 37 degrees C resulted in a partial loss of fibrinogen clotting activity, 86 +/- 13% of the clotting activity and 99 +/- 16% of the active sites titratable with p-nitrophenyl p-guanidinobenzoate were retained upon controlled passage of alpha-thrombin through chymotrypsin-Sepharose 4B at pH 6.2 or 7.4 and 24 degrees C (n = 15). Kinetic parameters for H-D-hexahydrotyrosyl-Ala-Arg p-nitroanilide were Km = 1.52 +/- 0.60 vs 1.32 +/- 0.18 microM and kcat = 51.9 +/- 2.9 vs 35.8 +/- 6.4 s-1 with alpha-thrombin vs chymotrypsin-prepared zeta-thrombin (n = 4 vs 3), respectively (I = 0.15 M, pH 7.4, and 24 degrees C). Some 95% of the clotting activity was lost when zeta-thrombin was passed through trypsin-Sepharose 4B under conditions for converting alpha- to nonclotting beta- and subsequently gamma-thrombin. The resulting gamma-like thrombins eluted bimodally with 260 and 310 mM NaCl when applied to Amberlite CG-50 resin [cross-linked poly(methylacrylic acid)] developed with a linear salt gradient in 50 mM Tris at pH 7.4 and 24 degrees C. These elution peaks correspond to 240, 330, and 350 mM NaCl for gamma-, alpha-, and zeta-thrombin, respectfully, implying that the anion-binding exosite is partially destroyed in gamma-like thrombins but is intact in zeta-thrombin.(ABSTRACT TRUNCATED AT 250 WORDS)

Thrombin inhibition with dipeptidyl argininals.

Human alpha- and gamma-thrombins (with high and essentially no fibrinogen clotting activities, respectively) were inhibited in chromogenic substrate assays by the dipeptidyl argininals: antipain less than leupeptin less than H-D-Phe-Pro-argininal approximately Boc-D-Phe-Pro-argininal. In clotting assays with alpha-thrombin, I50 values were slightly higher than Ki values from chromogenic substrate assays, except for a somewhat lower I50 for antipain. Our data cautions the use of argininal proteinase inhibitors in the assessment of thrombin functions, and the high potency of H-D-Phe-Pro-argininal and its derivative suggest pharmaceutical applications.

Human alpha- and gamma-thrombin specificity with tripeptide p-nitroanalide substrates under physiologically relevant conditions.

Kinetic parameters [the Michaelis-Menten (Km), catalytic (kcat), and specificity (kcat/Km) constants] were determined for human alpha- and gamma-thrombins with the chromogenic substrate S-2238 (H-D-Phe-Pip-Arg-p-nitroanilide), Chromozym-TH (Tos-Gly-Pro-Arg-p-nitroanilide), and Spectrozyme-TH (H-D-HHT-Ala-Arg-p-nitroanilide) under physiologically relevant conditions (0.15 M NaCl buffered with 10 mM HEPES and 10 mM Tris-HCL, pH 7.4 at 37 degrees C). No major differences were found between alpha-thrombin with high fibrinogen clotting activities (greater than 3,500 killo clotting units/g) and gamma-thrombin with essentially no clotting activities (less than 10 kCU/g), although the Km values and in most cases kcat values for alpha-thrombin were slightly lower than for the gamma-thrombin. At 37 degrees C, relative to 23 degrees C, Km values increased 2-fold for S-2238, approximately 1.5-fold for Chromozym-TH, and remained essentially the same for Spectrozyme-TH (e.g., reciprocal substrate binding), whereas the kcat values increased for all 3 substrates (e.g., enzyme turnover). This caused kcat/Km values to decrease slightly for S-2238, remain the same for Chromozym-TH, and increase for Spectrozyme-TH (e.g., enzyme efficiency). Since spontaneous hydrolysis was not limiting at 37 degrees C, assays employing these substrates may be satisfactorily performed under physiologically relevant conditions. Under these conditions, kcat/Km ratios for the 3 substrates are similar to that for the A alpha cleavage in fibrinogen by alpha-thrombin.