Effects of isosorbide dinitrate on portal hypertension in alcoholic cirrhosis

It has recently been reported that vasodilators lower portal pressure in patients with cirrhosis. This effect, however, is not definitively proven. The effect of isosorbide dinitrate (5 mg sublingually) on splanchnic and systemic hemodynamics was investigated in 13 patients with alcoholic cirrhosis and portal hypertension. The administration of isosorbide dinitrate reduced hepatic venous pressure gradient by 34% (P less than 0.001), mean arterial pressure by 30% (P less than 0.001), cardiac index by 17% (P less than 0.001) and systemic vascular resistance by 11% (P = 0.05). Hepatic blood flow was not affected by the treatment. Significant correlations were found between the decrease in hepatic venous pressure gradient and that of cardiac index (P less than 0.05) and mean arterial pressure (P less than 0.05). These data indicate that isosorbide dinitrate lowers portal pressure in patients with cirrhosis. Decrease in cardiac output, rise in splanchnic arterial vascular resistance and decrease in porto-hepatic resistance seem to participate in determining the effect.

Nadolol for prevention of variceal rebleeding in cirrhosis: a controlled clinical trial

Nadolol, a nonselective beta-blocker, has been shown to decrease portal pressure in patients with cirrhosis at the same degree as propranolol. No data are available, however, about its effect on rebleeding rate and mortality in patients undergoing prevention of rebleeding from esophageal varices. A prospective randomized clinical trial was performed in patients with cirrhosis who survived a documented episode of variceal hemorrhage. 12 patients received nadolol, 12 placebo. Patients with child's C grade, tense ascites, renal failure, contraindications to beta-blocker, or age greater than 70 were not included. After a follow-up of up to 145 weeks, 9 patients in the nadolol group and 4 in the placebo group survived free from rebleeding (log-rank test: chi 2 = 4.35, p less than 0.05). Survival was not statistically different in the two groups (1 death in the nadolol group, 3 in the placebo group). In conclusion, nadolol appears to represent an effective therapy in the prevention of variceal rebleeding in cirrhotic patients.

Inhibition of serine proteinases by tetra-p-amidinophenoxy-neo-pentane: thermodynamic and molecular modeling study

The inhibitory effect of the aromatic tetra-benzamidine derivative tetra-p-amidinophenoxy-neo-pentane (TAPP) on the catalytic properties of beta-trypsin (EC 3.4.21.4), alpha-thrombin (EC 3.4.21.5), factor Xa (EC 3.4.21.6), Lys77-plasmin (EC 3.4.21.7) and beta-kallikrein-B (EC 3.4.21.35) was investigated (between pH 2 and 8, I = 0.1 M; T = 37 +/- 0.5 degrees C), and analyzed in parallel with that of benzamidine, commonly taken as a molecular inhibitor model of serine proteinases. Over the whole pH range explored, TAPP and benzamidine show the same values of the dissociation inhibition constant (Ki) for beta-trypsin; at variance with the affinity of TAPP for alpha-thrombin, factor Xa, Lys77-plasmin and beta-kallikrein-B which is higher than that found for benzamidine association around neutrality, but tends to converge in the acidic pH limb. On lowering the pH from 5.5 to 3.0, values of Ki for TAPP binding to beta-trypsin as well as for benzamidine association to all the enzymes investigated decreased thus reflecting the pK-shift, upon inhibitor binding, of a single ionizing group. Over the same pH range, values of Ki for TAPP binding to alpha-thrombin, factor Xa, Lys77-plasmin and beta-kallikrein-B may be described as depending on the pK-shift, upon inhibitor association, of two equivalent proton-binding amino acid residues. Considering the X-ray three-dimensional structures and the computer-generated molecular models of serine proteinases: TAPP and :benzamidine adducts, the observed binding behaviour of TAPP and benzamidine to the enzymes considered has been related to the inferred stereochemistry of proteinase: inhibitor contact region(s).

Renal effects of nadolol in cirrhosis

The effects of nadolol on renal haemodynamics and function, and on the renin-angiotensin-aldosterone system and on renal prostaglandin production were studied in eighteen cirrhotics. After 1 month of treatment, nadolol had significantly decreased cardiac output by 25% without affecting arterial pressure, renal plasma flow or renal vascular resistance. Glomerular filtration rate, filtration fraction and the proportion of the cardiac output delivered to the kidneys were significantly increased. The renin-angiotensin-aldosterone system was suppressed and urinary PGE2 excretion was slightly increased. The latter effects were not correlated with those on renal haemodynamics and function.

Gabexate mesylate inhibition of serine proteases: thermodynamic and computer-graphics analysis

The inhibitory effect of gabexate mesylate, which is used therapeutically in the treatment of pancreatitis and disseminated intravascular coagulation, and as a regional anticoagulant agent for hemodialysis, has been measured on bovine factor Xa, bovine alpha-thrombin, human Lys77-plasmin, human urinary kallikrein, human urokinase, porcine pancreatic beta-kallikrein-B, and bovine beta-trypsin catalyzed hydrolysis of p-nitrophenyl esters of N-alpha-carbobenzoxy-L-arginine and N-alpha-carbobenzoxy-L-lysine. On the basis of enzyme:gabexate mesylate affinities, the serine proteases can be arranged as follows: human urinary kallikrein approximately porcine pancreatic beta-kallikrein-B much less than bovine beta-trypsin approximately bovine factor Xa approximately human Lys77-plasmin approximately human urokinase approximately bovine alpha-thrombin. The mode of binding of gabexate mesylate to the serine proteases conforms to the active-reactive site geometries observed in their complexes with natural and synthetic inhibitors. Differences in gabexate mesylate affinities for these proteases reflect structural differences at their primary specificity subsite, which have been investigated by comparative analysis of amino acid sequences and by computer-graphics techniques.

Binding of the bovine basic pancreatic trypsin inhibitor (Kunitz) to human Lys77-plasmin

The effect of pH and temperature on the association equilibrium constant (Ka) for the binding of the bovine basic pancreatic trypsin inhibitor (BPTI Kunitz inhibitor) to human Lys77-plasmin has been investigated. Ka values decrease with decreasing pH, reflecting the acid-pK and -midpoint shifts, upon BPTI binding, of a single ionizable group, between pH 5 and 9, and of a three-proton transition, between pH 3 and 5. At pH 8.0, values of thermodynamic parameters for BPTI binding to human Lys77-plasmin are: Ka = 1.2 X 10(9) M-1, delta G degree = -12.2 kcal/mol, and delta S degree = +49 entropy units (at 21 degrees C); and delta H degree = +2.3 kcal/mol (temperature independent between 5 degrees C and 45 degrees C; 1 kcal = 4184 J). BPTI binding properties of human Lys77-plasmin have been analysed in parallel with those of serine (pro)enzymes acting on cationic and non-cationic substrates. Considering the known molecular structures of homologous serine (pro)enzymes, or Kunitz and Kazal-type inhibitors and of their complexes, the observed binding behaviour of BPTI to human Lys77-plasmin was related to the inferred stereochemistry of the enzyme-inhibitor contact region.

Catalytic properties of bovine alpha-thrombin: a comparative steady-state and pre-steady-state study

Values of steady-state and pre-steady-state parameters for the bovine alpha-thrombin-catalyzed hydrolysis of ZArgONp and ZLysONp have been determined between pH 2.5 and 8 (I = 0.1 M) at 21 +/- 0.5 degree C. Kinetic properties of bovine alpha-thrombin have been analyzed in parallel with those of porcine pancreatic beta-kallikrein-B and bovine beta-trypsin, all acting on cationic substrates. The different primary specificity and catalytic behaviour of these three serine proteinases reflect subtle structural differences at their S1 subsite, especially at residue positions 190, 221 and 226 as well as in the 217-219 segment.

Primary specificity of ancrod, the coagulating serine proteinase from the Malayan pit viper (Agkistrodon rhodostoma) venom

Values of steady-state and pre-steady-state parameters for the hydrolysis of ZArgONp and ZLysONp catalysed by ancrod, the coagulating serine proteinase from the Malayan pit viper (Agkistrodon rhodostoma) venom, have been determined, between pH 2.5 and 8 (I = 0.1 M) at 21 +/- 0.5 degrees C, and analysed in parallel with those of bovine alpha-thrombin and porcine pancreatic beta-kallikrein-B. In addition to the well-known coagulating behaviour, ancrod also shows catalytic properties, in the hydrolysis of ZArgONp and ZLysONp, reminiscent of those of porcine pancreatic beta-kallikrein-B.

Effect of nadolol on liver haemodynamics and function in patients with cirrhosis

Beta-adrenoceptor blockers used in the medical management of portal hypertension decrease liver blood flow. The sporadic onset of hepatic encephalopathy during propranolol treatment was ascribed to this decrease. The aim of the present study was to evaluate the effect of chronic treatment with nadolol on liver blood flow and liver function. Nadolol, a non-cardioselective beta-adrenoceptor blocker, has been reported to be as powerful as propranolol in decreasing portal pressure. Before and after 1 month of treatment with nadolol at a dose reducing heart rate by 25%, in 15 cirrhotic patients with portal hypertension, the following parameters were determined: hepatic venous pressure gradient, hepatic blood flow, galactose eliminating capacity, aminopyrine metabolic activity, ICG clearance and intrinsic hepatic clearance. Hepatic venous pressure gradient and hepatic blood flow were decreased by nadolol. However liver function was not affected by the drug. We conclude that, despite a lowered hepatic blood flow, liver function is not affected by 1 month of nadolol treatment.

Aromatic tetra-amidines: antiproteolytic and antiesterolytic activities towards serine proteinases involved in blood coagulation and clot lysis

The inhibitory effect of 1,3-di-(p-amidinophenoxy)-2,2-bis-(p-amidinophenoxymethyl)propane (TAPP-H), TAPP-halo derivatives (with Cl, Br or I) and benzamidine on the Ancrod, bovine Factor Xa, and human plasmin catalyzed hydrolysis of esters and anilides of amino acids was investigated, at pH 8.1 and 37 degrees, and compared with that shown from these compounds on bovine thrombin and porcine pancreatic kallikrein catalysis. The inhibitory effect of TAPP-H and TAPP-halo derivatives on the proteinases considered, involved, to different extents, in blood coagulation and clot lysis, is higher by at least 10-fold, than that of benzamidine, which binds at the primary specificity subsite (S1) of serine endopeptidases and is commonly taken as a molecular inhibitor model. The high inhibitory effect of aromatic tetra-amidines has been interpreted taking into account an additional productive binding for a second benzamidine or halo-benzamidine moiety to the enzyme surface. Moreover, the data reported here allowed us to clarify the inhibition mechanism (in vitro) of TAPP-H on blood coagulation, induced by the "cancer coagulation factor" produced by the Walker carcinoma in Wistar rats and on the fibrinogen-to-fibrin conversion, and to identify some serine proteinases which act as targets for aromatic tetra-amidines.

Trypsin activation. Effect of the Ile-Val dipeptide concentration on Kazal inhibitor binding to bovine trypsinogen

The effect of Ile-Val concentration (up to 2.0 M) on the thermodynamic parameters for the binding of the porcine pancreatic secretory trypsin inhibitor (Kazal inhibitor) to trypsinogen has been investigated at pH 5.5 between 7 degrees C and 42 degrees C. Thermodynamic parameters for Kazal inhibitor binding to the Ile-Val:zymogen adduct are more favorable than those observed for inhibitor association to the free proenzyme, but less so than those reported for beta-trypsin:Kazal inhibitor adduct formation (even under saturating dipeptide concentrations), suggesting that the effector dipeptide does not induce a complete rigidification of the proenzyme's activation domain. Considering the dependence of the association equilibrium constant for Kazal inhibitor binding to trypsinogen from Ile-Val concentration, thermodynamic parameters for the effector dipeptide binding to the free proenzyme and to its binary complex with Kazal inhibitor have been obtained. Differences in affinity for Ile-Val binding to the free zymogen and its binary complexes with inhibitors and substrates are indicative of the presence of different activation levels of the proenzyme, none of them exactly coincident with that of beta-trypsin. Such different discrete states should correspond to those involved in the zymogen-to-active-enzyme transition which should not be considered as an all-or-nothing process, but as a multistep event.

Activating effect of the Ile-Val dipeptide on the catalytic properties of bovine trypsinogen

The dependence of pre-steady-state and steady-state kinetics for the trypsinogen-catalyzed hydrolysis of ZArgONp on the concentration (up to 2.0 M) of the Ile-Val effector dipeptide has been investigated at pH 8.0 and 21 +/- 0.5 degrees C. Kinetic parameters for the hydrolysis of ZArgONp catalyzed by the Ile-Val:zymogen adduct are more favorable than those observed for the free proenzyme but lower than those reported for beta-trypsin; these data indicate that the effector dipeptide induces only a partial activation of the zymogen even under saturating Ile-Val concentrations. From the dependence of kinetic parameters of proenzyme catalysis on the effector dipeptide concentration, values of the equilibrium constants for binding of Ile-Val to the free trypsinogen, to the reversible zymogen-ZArgONp complex and to the proenzyme-ZArg acyl intermediate have been obtained. Thermodynamics of binding of Ile-Val to trypsinogen, in the presence and absence of substrates and inhibitors, are indicative of the presence of different activation levels of the proenzyme, none of which is superimposable on that of beta-trypsin. On this basis, it is suggested that some of these different states could correspond to those involved in the zymogen-to-active-enzyme transition, which should be considered as a multistep process, rather than an all-or-nothing event.

Crystallization of an odorant-binding protein from cow nasal mucosa

The first odorant-binding protein isolated from mammalian nasal mucosa is a dimer of subunits of identical molecular weight (19,000) that specifically binds bell pepper odorants, "green" smelling compounds. The protein can be purified in milligram quantities from tissue extractions by sequential use of a silica based anion exchange column and Mono-P chromatofocussing column. In the presence of the binding compound 2-isobutyl-3-methoxypyrazine and of the organic solvent 2-methyl-2,4-pentanediol (17%, v/v), the protein crystallizes in the monoclinic space group P2(1), with unit cell constants a = 54.3 A, b = 66.7 A, c = 41.5 A, beta = 97.2 degrees. From consideration of the crystal packing densities compatible with its unit cell, it can be concluded that two subunits of 19,000 Mr each are present in the asymmetric unit. The diffraction pattern on still photographs of this crystal form of the protein extends to 2.5 A resolution and allows for a detailed crystallographic investigation.

Catalytic properties of Ancrod, the thrombin-like proteinase from the Malayan pit viper (Agkistrodon rhodostoma) venom

Kinetics for the hydrolysis of p-nitrophenyl esters of N-alpha-carbobenzoxy-L-amino acids catalyzed by Ancrod were determined between pH 5 and 10 (I = 0.1 M) at 21 +/- 0.5 degrees C; the results are consistent with the minimum three-step mechanism: (formula: see text) For all substrates examined, the pH profiles of kcat and/or kcat/Km reflect the ionization of two groups with pKa values ranging between 6.9 and 7.2, and 9.3 and 9.6 (probably, the histidine residue involved in the catalytic triad and the N-terminus, respectively); at variance, values of Km are pH-independent. Moreover, the formation of the E X S complexes may be regarded as a pseudo-equilibrium process, and the acylation step (k + 2) is always rate-limiting in catalysis. Among p-nitrophenyl esters examined, ZArgONp shows the most favourable kinetic parameters and may be the substrate of choice for Ancrod, in that it allows the determination of the enzyme concentration as low as 1 X 10(-9) M (approximately equal to 0.1 Ancrod units/ml), at the optimum pH value (approximately equal to 8). The catalytic behaviour of Ancrod is compared to that of serine proteinases acting on cationic and non-cationic substrates; differences in kinetics, which refer to a lower enzyme:substrate affinity, may be related to a higher rigidity, lower hydrophobicity and/or adverse steric hindrance of the S1 subsite of Ancrod.

Crystal structure of ferric Aplysia limacina myoglobin at 2 X 0 A resolution

The three-dimensional structure of ferric myoglobin from the mollusc Aplysia limacina has been refined at 2 X 0 A resolution. The crystallographic R factor, calculated at this stage, is 0 X 194. Despite its high content of apolar residues (both aromatic and aliphatic), Aplysia limacina myoglobin, which contains only one histidine residue (at the proximal position), has a structure that conforms to the common eight-helices globin fold observed in other phyla.

Blood flow of experimental liver metastases in rat as evaluated by the locally injected 133-Xenon washout

The investigations on blood flow in liver metastases are interesting from both a pathophysiological and a therapeutic point of view. Available data, however, are few and not definitive, as these studies are complex and difficult to perform. In a group of 25 Sprague-Dawley rats, in which liver metastases of Walker-256 carcinoma had been implanted, the blood flows in a metastasis and in the normal liver surrounding it were determined by means of the locally injected 133-Xenon washout. Thirteen sham-operated rats were a control group. Blood flow in the metastases was decreased as a group compared to that in the normal liver surrounding metastases and to that in liver of sham-operated rats. Small metastases showed normal or increased blood flow, large ones decreased or stagnant blood flow. Moreover, a significant inverse correlation was found between blood flow and diameter of metastasis. It is concluded that liver metastases of Walker-256 carcinoma show a decrease in blood flow which is related to the size of metastasis.

Aromatic tetra-amidines: synthesis of halo-derivatives and their antiproteolytic activity

Mono-halo derivatives of 1,3-di-(p-amidinophenoxy)-2,2-bis-(p-amidinophenoxymethyl)p ropane (TAPP-H) have been synthesized and their inhibitory effect on the bovine trypsin, bovine thrombin and porcine pancreatic kallikrein catalyzed hydrolysis of p-nitroanilides of amino acids was investigated, at pH 8.1 and 37 degrees, in parallel with that of TAPP-H and benzamidine. The addition of a halogen (Cl, Br or I) at position 2 of each benzamidine moiety of TAPP-H is accompanied by an increase of the inhibitory effect. This is especially evident in the case of porcine pancreatic kallikrein inhibition by TAPP-Cl. The structural basis of the different inhibitory effect of TAPP-H, TAPP-halo derivatives and benzamidine on the catalyzed hydrolysis of the proteinases examined are discussed and the role of Asp-189 in bovine trypsin, Asp-189 and Glu-149 or Asp-192 in bovine thrombin and Asp-189 and Asp-A148 or Glu-150 in porcine pancreatic kallikrein is focused.

Crystallization of hen egg-white avidin in a tetragonal form

Hen egg-white avidin has been crystallized at pH 5.7 from ammonium sulfate solutions. The crystals belong to the tetragonal space group P4(2)2(1)2, with unit cell edges a = b = 79.6 A, c = 84.3 A. Assuming a molecular weight of 15,600 per avidin monomer, this crystal form is compatible with the presence of a dimer in the asymmetric unit, and is suitable for a crystallographic structural investigation at high resolution.

Benzamidine as a spectroscopic probe for the primary specificity subsite of trypsin-like serine proteinases. A case for BPTI binding to bovine beta-trypsin

Formation and dissociation of the benzamidine:beta-trypsin adduct is accompanied by reversible spectral changes in the ultraviolet region (between 230 and 300 nm). The pH-independent difference extinction coefficient of the adduct (benzamidine:beta-trypsin complex minus the free proteinase) is 1.75 mM-1 cm-1 at 248 nm. This signal can be used in studies of inhibitor and substrate binding by rapid kinetic techniques. Therefore, following the spectral changes associated with the displacement of benzamidine from the primary specificity subsite, the kinetics of the beta-trypsin:BPTI complex formation were investigated between pH 2.9 and 7.6 (I = 0.1 M) at 21 +/- 0.5 degree C. Under all the experimental conditions the beta-trypsin:BPTI complex formation, examined by benzamidine displacement experiments, may be described in terms of a simple competition event. On the other hand, the very same reaction followed by displacement of another spectroscopic probe, proflavine, appears to involve the ternary proflavine:beta-trypsin:BPTI adduct (7). The difference between the kinetic processes of beta-trypsin:BPTI complex formation, observed by using benzamidine and proflavine as reaction indicators, suggests that the two dye molecules bind at non-coincident regions of the proteinase active center. The advantages in using benzamidine as a sensitive probe specific for the S1 subsite of the recognition center of trypsin-like proteinases, as compared to proflavine, are emphasized.

Different primary specificity of porcine pancreatic beta-kallikrein-B and bovine beta-trypsin. A comparative steady-state and pre-steady-state study

The values of pre-steady-state and steady-state parameters for the beta-trypsin catalyzed hydrolysis of Z-Arg-ONp and Z-Lys-ONp are superimposable between pH 2.4 and 8. At variance, the kinetic parameters for the beta-kallikrein-B catalyzed hydrolysis of Z-Arg-ONp are more favourable than those observed for Z-Lys-ONp and depend on different pKa values. The different primary specificity and the catalytic behaviour of beta-trypsin and beta-kallikrein-B reflect structural differences at their S1 subsite, especially at level of the 226 residue as well as the 217-220 segment.

Binding of the bovine basic pancreatic trypsin inhibitor (Kunitz) to human urinary kallikrein and to porcine pancreatic beta-kallikreins A and B

The effect of pH and temperature on the association equilibrium constant (Ka) for bovine basic pancreatic trypsin inhibitor (BPTI, Kunitz inhibitor) binding to human urinary kallikrein and porcine pancreatic beta-kallikreins A and B has been investigated. Ka values decrease with decreasing pH, reflecting the acid-midpoint and pK shifts, upon BPTI binding, of a three-proton co-operative transition, between pH 3 and 5, and of a single ionizable group, between pH 5 and 9. At pH 8, the values of delta H degree (between 7 degrees C and 42 degrees C) and delta S degree (at 21 degrees C) for BPTI binding to the glandular kallikreins considered were determined. In particular, the delta H degree values have been found to be independent of temperature and the following values have been obtained by van't Hoff plots: +1.8 kcal/mol, +2.3 kcal/mol and +2.4 kcal/mol (1 kcal = 4184 J) for the inhibitor binding to human urinary kallikrein and porcine pancreatic beta-kallikreins A and B, respectively. Considering the known molecular structures of free porcine pancreatic beta-kallikrein A and BPTI, and of their complex, the stereochemistry of the enzyme : inhibitor contact regions was analysed for the three serine proteinases, in relation to their respective types of behaviour.

The pH dependence of pre-steady-state and steady-state kinetics for the porcine pancreatic β-kallikrein-B-catalyzed hydrolysis of p-nitrophenyl ester

Pre-steady-state and steady-state kinetics of the porcine pancreatic beta-kallikrein-B (EC 3.4.21.35) catalyzed hydrolysis of ZArgONp have been determined between pH 2.4 and 8. The results are consistent with a minimum three-step mechanism involving an acyl-enzyme intermediate: (see formula). The formation of the E X S complex may be regarded as a pseudoequilibrium process; the minimum values for k+1 are 5.9 X 10(6) M-1 X s-1 (pH 5.5) and 9.4 X 10(5) M-1 X s-1 (pH 2.4) and that for k-1 is 600 s-1. The value of k-1/k+1 (= Ks) changes from 102 microM at pH greater than or equal to 5.5 to 638 microM at pH less than 2.4. The pH dependence of k+2 conforms to two ionizing groups, in the E X S complex, with pKa values of 3.4 +/- 0.1 and 7.05 +/- 0.10. The pH profile of k+2/Ks (= kcat/Km) reflects the ionization of two groups, in the free enzyme, with pKa values of 4.2 +/- 0.1 and 7.05 +/- 0.10. The pH dependence of k+3 implicates two ionizing groups in the deacylation step with pKa values of 4.6 +/- 0.1 and 7.0 +/- 0.1. At acid pH values (pH 2.4-4.4), k+3 is rate-limiting in catalysis, whereas for pH values higher than 4.4, k+2 becomes rate-limiting. The observed neutral and acid ionizations probably reflect the acid-base equilibrium of His-57 and Asp-189 involved in the central site of beta-kallikrein-B. The structural basis for the specificity and catalytic behaviour of this proteinase are discussed and a role for Ser-226 is pinpointed.

Catalytic and ligand binding properties of bovine trypsinogen and its complex with the effector dipeptide Ile-Val. A comparative study

Steady-state and pre-steady-state kinetic data for the trypsinogen catalyzed hydrolysis of a series of synthetic substrates (i.e. p-nitrophenyl esters of N-alpha-carbobenzoxy-L-amino acids) have been obtained as a function of pH (3.4-8). Moreover, the effect of ethylamine on the hydrolysis of a neutral substrate and benzamidine binding have been extensively studied. In order to obtain direct information on the transition of trypsinogen to a beta-trypsin-like structure, the role of the effector dipeptide Ile-Val on the catalytic and ligand binding properties of the zymogen has been investigated. Kinetic and thermodynamic data for beta-trypsin and alpha-chymotrypsin are also reported for the purpose of an homogeneous comparison of the various (pro)enzymes. Under all the experimental conditions, kinetic data for (pro)enzyme catalysis are consistent with the minimum three-step mechanism: (formula; see text) involving the acyl intermediate E X P. In the presence of Ile-Val dipeptide, trypsinogen assumes catalytic and ligand binding properties that are reminiscent of activated beta-trypsin. This is at variance with free trypsinogen, which shows a alpha-chymotrypsin-like behavior. The large differences in the results of kinetic and thermodynamic measurements for free trypsinogen, as compared to its binary adduct with Ile-Val, can be ascribed to the substantial differences in the two molecular species, which include the spatial orientation of Asp189.