No abstract

Human alpha-thrombin is inhibited by the circulating protease inhibitors alpha1-antitrypsin, antithrombin III, and alpha2-macroglobulin. Kinetic analyses of the inhibitor thrombin interactions were carried out utilizing either fibrinogen or the synthetic substrate Bz-Phe-Val-Arg-p-nitroanilide as substrates to determine residual thrombin activity. These studies demonstrated that the inhibition of thrombin by alpha1-antitrypsin, antithrombin III, and alpha2-macroglobulin followed second-order kinetics. The rate constants for the inhibition of thrombin by alpha1-antitrypsin, antithrombin III, and alpha2-macroglobulin are 6.51 +/- 0.38 x 10(3), 3.36 +/- 0.34 x 10(5), and 2.93 +/- 0.02 x 10(4) M-1 min-1, respectively. Comparison of the second-order rate constants and the normal plasma levels of the three inhibitors demonstrates that, under the in vitro conditions utilized, antithrombin III is five times and alpha2-macroglobulin is one-third as effective as alpha1-antitrypsin in the inhibition of thrombin.

Diketopiperazine has been used as a model compound to investigate hydrogen bond interactions between the peptide bond and urea. The solubility has been measured in aqueous urea solutions as a function of temperature and urea concentration.The heats of solution obtained from this temperature relation serve to evaluate the hydrogen bond interaction of diketopiperazine with urea. The fraction of peptide-urea interactions is estimated by simple equilibrium considerations. The heat of association of urea with a cis peptide bond can then be evaluated from experimental data as -3.4 f 0.5 kcal. An equilibrium constant of 0.096 is found for the association of urea with the peptide bond in aqueous solution. The strong hydrogen bond interaction between urea and peptides is further illustrated by the discovery of a 2 urea: 1 diketopiperazine solid complex which was 0bservt.d to form from 8 M urea solutions below 30'.It is obEerved that the solubility follows a simple exponential dependence upon l / T .

No abstract

No abstract