Binding of bovine basic pancreatic trypsin inhibitor (Kunitz) as well as bovine and porcine pancreatic secretory trypsin inhibitor (Kazal) to human cathepsin G: a kinetic and thermodynamic study

Kinetic and equilibrium characterization of the interaction between bovine trypsin and I-ovalbumin

Serpins are a superfamily of structurally linked proteins with interesting functional properties. Most serpins act as proteinase suicide inhibitors and play a key role in a number of physiological processes. Structural flexibility properties make serpins extremely available to conformational transitions, often causing changes in protein function. Ovalbumin is a member of the serpin family that is not able to inhibit serine proteinases in its native form. In contrast, I-ovalbumin, the product of a heating transition, is a potent reversible serine proteinase inhibitor. In this paper, a detailed equilibrium and kinetic characterization of the interaction between the serpin ovalbumin and bovine trypsin, using a resonant mirror technique, is reported. This methodology revealed that the high affinity interaction between the two binding partners is characterized by high kinetic association constants and low kinetic dissociation constants. The modulation exerted by protons in solution, examined taking into account structural motifs characterizing the binding interface between the two macromolecules, suggests an interaction reminiscent of that observed for the binding of other serine proteases to their ligands.

Cartilage degradation and invasion by rheumatoid synovial fibroblasts is inhibited by gene transfer of a cell surface-targeted plasmin inhibitor

OBJECTIVE

Joint destruction in rheumatoid arthritis (RA) is a result of degradation and invasion of the articular cartilage by the pannus tissue. The present study was undertaken to examine the role of the plasminogen activation system in cartilage degradation and invasion by synovial fibroblasts and investigate a novel gene therapeutic approach using a cell surface-targeted plasmin inhibitor (ATF.BPTI).

METHODS

Adenoviral vectors were used for gene transfer. The effects of ATF.BPTI gene transfer on RA synovial fibroblast-dependent cartilage degradation were studied in vitro, and cartilage invasion was studied in vivo in the SCID mouse coimplantation model.

RESULTS

The results indicate that cartilage matrix degradation by rheumatoid synovial fibroblasts is plasmin mediated and depends on urokinase-type plasminogen activator for activation. Targeting plasmin inhibition to the cell surface of the fibroblasts by gene transfer of a cell surface-binding plasmin inhibitor resulted in a significant reduction of cartilage matrix degradation in vitro and of cartilage invasion in vivo. Compared with uninfected rheumatoid synovial fibroblasts, the mean +/-SEM cartilage degradation in vitro was reduced to 87.9+/-0.9% after LacZ gene transfer versus a reduction to 24.0+/-1.6% after ATF.BPTI gene transfer (P<0.0001). The mean +/- SEM in vivo cartilage invasion score was 3.1+/-0.4 in the control-transduced fibroblasts and 1.8+/-0.4 in the ATF.BPTI-transduced fibroblasts (P<0.05).

CONCLUSION

These results indicate a role of the plasminogen activation system in synovial fibroblast-dependent cartilage degradation and invasion in RA, and demonstrate an effective way to inhibit this by gene transfer of a cell surface-targeted plasmin inhibitor.

Specificity of human cathepsin G

A series of tetrapeptide p-nitroanilide substrates of the general formula: suc-Ala-Ala-Pro-Aaa-p-nitroanilide was used to map the S1 binding pocket of human cathepsin G. Based on the kcat/Km parameter, the following order of preference was found: Lys=Phe>Arg=Leu>Met>Nle=Nva>Ala>Asp. Thus, the enzyme exhibits clear dual and equal trypsin- and chymotrypsin-like specificities. Particularly deleterious were beta-branched side chains of Ile and Val. The P1 substrate preferences found for cathepsin G are distinctly different from many other serine proteinases, including fiddler crab collagenase and chymotrypsin. The kcat/Km values obtained for P1 Lys, Phe, Arg and Leu substrates correlate well with those determined for analogous P1 mutants of basic pancreatic trypsin inhibitor (BPTI) obtained through recombinant techniques. To characterise the subsite specificity of the enzyme, a series of Cucurbita maxima trypsin inhibitor I (CMTI I) mutants were used comprising P2-P3' and P12' positions. All the mutants obtained were inhibitors of cathepsin G with association constants in the range: 105-109 M-1. Some of the mutations destabilised complex formation. In particular, Met8-->Arg substitution at P3', which increased association constant for chymotrypsin 46-fold, led to a 7-fold decrease of binding with cathepsin G. In addition, mutation of Ile6 at position P1' either to Val or Asp was deleterious for cathepsin G. In two cases (Ala18-->Gly (P12') and Pro4-->Thr (P2)), about a 10-fold increase in association constants was observed.

Ionizable P1 residues in serine proteinase inhibitors undergo large pK shifts on complex formation

The burial of charged residues in proteins is rare as it is thermodynamically strongly disfavored. However, in "standard mechanism" protein inhibitors of serine proteinases, the P1 residue, which is highly exposed, becomes buried in the S1 specificity pocket of the enzyme. In many enzymes, such as Streptomyces griseus proteinase B (SGPB) the S1 pocket is hydrophobic. We measured the pH dependence of the association equilibrium constant for the interaction of SGPB with turkey ovomucoid third domain P1 mutants, Glu18 OMTKY3 and His18 OMTKY3. In order to eliminate the effects of other ionizable groups on the enzyme and the inhibitor, we divided these pH dependences by the pH dependence of the association equilibrium constant for the Gln18 OMTKY3 mutant. This yielded for Glu18, pKf (free inhibitor) of 4.46 +/- 0.05 and pKc (complex) of 8.74 +/- 0.06. For His18 the values are pKf 6.63 +/- 0.08 and pKc 4.31 +/- 0.07. At low pH values Glu18 variant is a relatively good inhibitor for SGPB. This may be biologically relevant.