Synthesis of peptide p-nitroanilides mimicking fibrinogen- and hirudin-binding to thrombin. Design of slow reacting thrombin substrates

The design and synthesis of 20 peptide p-nitroanilides is described. The nitroanilides are used as thrombin substrates that share uncommon properties. These substrates are tested for their applicability to measure thrombin generation in activated plasma. This technique requires substrates that must slowly but selectively be hydrolyzed by thrombin. To ensure selectivity, thrombin's natural substrate and its most potent inhibitor were used as lead compounds. Eighteen peptide p-nitroanilides were synthesized using human fibrinogen A alpha [7-16] decapeptide as lead structure since this fragment constitutes the minimal sequence which binds to thrombin with high affinity. Two other chromogenic substrates were designed using 5-amino-2-nitrobenzoic acid as the chromophore. A peptide giving subsite interactions with the fibrinogen recognition exosite was coupled to the carboxylic function. As the peptide segment, the carboxyl terminal part of hirudin (residues 50-65) was chosen to ensure highly specific thrombin recognition. From the 20 nitroanilides synthesized, we indeed obtained a number of compounds which can be used as substrate in the thrombin generation assay.

Peptide synthesis catalyzed by an antibody containing a binding site for variable amino acids

Monoclonal antibodies, induced with a phosphonate diester hapten, catalyzed the coupling of p-nitrophenyl esters of N-acetyl valine, leucine, and phenylalanine with tryptophan amide to form the corresponding dipeptides. All possible stereoisomeric combinations of the ester and amide substrates were coupled at comparable rates. The antibodies did not catalyze the hydrolysis of the dipeptide product nor hydrolysis or racemization of the activated esters. The yields of the dipeptides ranged from 44 to 94 percent. The antibodies were capable of multiple turnovers at rates that exceeded the rate of spontaneous ester hydrolysis. This achievement suggests routes toward creating a small number of antibody catalysts for polypeptide syntheses.

Immunochemical studies of adrenocorticotropin using tritium labeled hormone

A radioimmunoassay using specifically tritiated corticotropin (3H-ACTH) of high specific radioactivity (46 Ci/mmol) and full biological activity has been developed. The assay is sensitive to ACTH in the range 200 pg-10 ng. 50% displacement of 3H-ACTH is produced by less than a twofold excess of unlabeled hormone. Immunochemical studies with various ACTH analogs showed that the antiserum recognized the intact molecule better than any of other fragments.

Conformational studies of corticotropin1-32 and constitutive peptides by circular dichroism

Circular dichroism spectra on corticotropin1-32 and its constitutive N-, and C-terminal peptides are determined in water and trifluoroethanol under several conditions in the aromatic and peptide spectral regions. Furthermore, the effects of pH and varied mixtures of water-trifluoroethanol are examined on the corticotropin1-32 molecule. The results show that the N- and C-terminal series have a different behaviour in both aqueous and organic media. Corticotropin and the former peptides display "random" spectra in water, and alpha-helix type spectra in trifluoroethanol, while the latter have "random" spectra in both solvents. In the holopeptide corticotropin, the side chain-side chain effects, as reflected by the titration curves obtained from variations in the aromatic region, support the idea of an helical organization of part of the backbone even in aqueous solution. When going from water to trifluoroethanol corticotropin1-32 undergoes a conformational change which leads to an alpha-helix, following a linear pathway. These results, together with other observations, indicate the possible role of the conformation of corticotropin molecules in their biological life.