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Biomedical subjects

S D Lewis

Publications and source records attributed to S D Lewis.

At least 55 records · Page 3Linked to original sources
PubMed1985

Regulation of formation of factor XIIIa by its fibrin substrates.

Thrombin-catalyzed release of activation peptide (AP) from plasma factor XIII was studied to characterize the regulation of this initial step in the activation of factor XIII zymogen (fibrin-stabilizing factor). High-performance liquid chromatography was used to monitor the kinetics of release of AP. Non-cross-linked polymeric fibrins I and II (polymerized des-A- and des-A,B-fibrinogens), physiological substrates of factor XIIIa, were shown to be potent promoters of thrombin-catalyzed release of activation peptide from factor XIII. These promoters are proposed to act by complexing factor XIII and reducing the apparent Km for thrombin-catalyzed release of AP. Since thrombin-catalyzed release of AP is inefficient in the absence of polymerized fibrin, this mode of regulation should minimize formation of factor XIIIa prior to the formation of its fibrin substrates. The promoting activity of polymeric fibrin was rapidly lost when catalytically competent factor XIIIa was allowed to form. This observation suggested the possibility that factor XIIIa catalyzed cross-linking of fibrin inactivates fibrin as a promoter for the thrombin-catalyzed release of AP from factor XIII. Consistent with this view, the thiol reagent S-methyl methanethiosulfonate inactivated factor XIIIa, blocked cross-linking of fibrin, and protected against loss of its promoter activity. This mode of feedback regulation of the activation process by catalytically active factor XIIIa may serve to ensure against continued generation of factor XIIIa after its fibrin substrates have been cross-linked.

Calcium↗
PubMed1985

Characterization of the kinetic pathway for liberation of fibrinopeptides during assembly of fibrin.

The time dependence of the release of fibrinopeptides from fibrinogen was studied as a function of the concentration of fibrinogen, thrombin, and Gly-Pro-Arg-Pro, an inhibitor of fibrin polymerization. The release of fibrinopeptides during fibrin assembly was shown to be a highly ordered process. Rate constants for individual steps in the formation of fibrin were evaluated at pH 7.4, 37 degrees C, gamma/2 = 0.15. The initial event, thrombin-catalyzed proteolysis at Arg-A alpha 16 to release fibrinopeptide A (kcat/Km = 1.09 X 10(7) M-1s-1) was followed by association of the resulting fibrin I monomers. Association of fibrin I was found to be a reversible process with rate constants of 1 X 10(6) M-1s-1 and 0.064 s-1 for association and dissociation, respectively. Assuming random polymerization of fibrin I monomer, the equilibrium constant for fibrin I association (1.56 X 10(7) M-1) indicates that greater than 80% of the fibrin I protofibrils should contain more than 10 monomeric units at 37 degrees C, pH 7.4, when the fibrin I concentration is 1.0 mg/ml. Association of fibrin I monomers was shown to result in a 6.5-fold increase in the susceptibility of Arg-B beta 14 to thrombin-mediated proteolysis. The 6.5-fold increase in the observed specificity constant from 6.5 X 10(5) M-1s-1 to 4.2 X 10(6) M-1s-1 upon association of fibrin I monomers and the rate constant for fibrin association indicates that most of the fibrinopeptide B is released after association of fibrin I monomers. The interaction between a pair of polymerization sites in fibrin I dimer was found to be weaker than the interaction of fibrin I with Gly-Pro-Arg-Pro and weaker than the interaction of fibrin I with fibrinogen.

Fibrin↗
PubMed1984

Relationship between the subunit structure of insulin receptor and its competence to bind insulin and undergo phosphorylation.

Insulin receptor partially purified from human placenta by chromatography on immobilized wheat germ agglutinin was subjected to affinity cross linking to determine the relationship between the subunit structure of the multiple forms of the insulin receptor and their competence to bind insulin and undergo autophosphorylation. It was demonstrated that, whereas the 340-kDa intact receptor undergoes autophosphorylation, the 290- and 320-kDa insulin binding forms of the receptor do not. Phosphorylation at tyrosyl residues in the intact receptor was verified using a new facile method for determination of phosphorylated amino acids. The competence of the phosphorylated 340-kDa protein to bind insulin was demonstrated using a double-probe labeling protocol wherein receptor phosphorylated with [gamma-32P]ATP was cross-linked with disuccinimidyl suberate (DSS) in the presence of N epsilon B29-biotinylinsulin. The observation that succinylavidin, by virtue of its interaction with biotinyl residues, decreased the electrophoretic mobility of receptor radiochemically labeled with 32P indicated that the phosphorylated 340-kDa protein was competent to bind insulin. This result is compelling evidence that the 340-kDa phosphorylated species is insulin receptor itself, rather than a closely associated contaminant. Treatment of the receptor with the crosslinking agent DSS produced (after reduction and denaturation) alpha-dimer, beta-dimer, and a smaller amount of tetramer. This observation is consistent with a symmetrical, tetrameric, alpha 2 beta 2 structure for insulin receptor from human placenta, and excludes previously proposed alternative structures containing one alpha and one beta chain.

Adenosine Triphosphate↗
PubMed1984

A thrombin assay based upon the release of fibrinopeptide A from fibrinogen: definition of a new thrombin unit.

An assay for thrombin is presented wherein thrombin-catalyzed hydrolysis at Arg-A alpha-16 to release fibrinopeptide A (FPA) from fibrinogen is measured using high-performance liquid chromatography (HPLC). In this assay one thrombin unit (TU) is defined as that amount of thrombin that will release half of the FPA in one min from one ml of a solution of greater than 90% clottable normal human fibrinogen (less than or equal to 0.35 microM) at 37 degrees C, pH 7.4, /2 0.15. One TU is equivalent to approximately 0.1 NIH unit of thrombin and approximately 1 pmol of pure human thrombin. At 37 degrees C, pH 7.4, and plasma levels of fibrinogen of 3 mg/ml, one TU will catalyze the release of 3.6 nmol FPA min-1. Variability in fibrinogen samples which produce dramatic differences in clotting time assays with the same sample of thrombin, produce little or no variation in the catalytic assay for TU. The assay for TU obviates the need for maintenance of a thrombin reference standard.

Blood Coagulation Tests↗
PubMed1983

Promotion of thrombin-catalyzed activation of factor XIII by fibrinogen.

High-performance liquid chromatography was used to analyze the kinetics of the thrombin-catalyzed release of the activation peptide from the factor XIII zymogen (fibrin-stabilizing factor). The specificity constant (kcat/Km) for this reaction, measured at factor XIII concentrations much below Km, was (0.13-0.16) X 10(6) M-1 s-1 at pH 7.4, mu = 0.15, and 37 degrees C. Separate estimates, obtained from the dependence of the initial rates of release of the activation peptide on the concentration of factor XIII, gave values of 10 (+/- 3) s-1 for kcat and 84 (+/- 30) microM for Km, in terms of ab protomers of the zymogen. The thrombin-mediated release of the activation peptide was dramatically enhanced in the presence of fibrinogen. Furthermore, the time course of release, in relation to that of fibrinopeptide A, suggested that some des-A-fibrinogen species (e.g., alpha 2B beta 2 gamma 2) may be the true activator for promoting the cleavage of the Arg-36 peptide bonds in the a subunits of factor XIII. This observation suggests that generation of factor XIIIa and its substrate (fibrin) is coordinated so that thrombin-mediated zymogen activation proceeds efficiently only after the process of clotting has been initiated by the removal of fibrinopeptide A from fibrinogen.

Blood Coagulation↗
PubMed1983

Steady state kinetic parameters for the thrombin-catalyzed conversion of human fibrinogen to fibrin.

Steady state kinetic parameters were evaluated for the hydrolytic release of fibrinopeptides A and B (FPA and FPB) from human fibrinogen by human thrombin at pH 7.4, 37 degrees C, and gamma/2 0.15. At low concentrations of fibrinogen (less than 0.4 microM), the release of FPA from A alpha-chains was first order with respect to both the concentration of fibrinogen A alpha-chains and thrombin. The second order rate constant yielded a value of 11.6 (+/- 0.3) X 10(6) M-1 S-1 for the specificity constant (kcat/Km) for this process. Values of 84 (+/- 4) S-1 and 7.2 (+/- 0.9) microM were evaluated for kcat and Km for the thrombin-catalyzed release of FPA from normal human fibrinogen. The amino acid replacement ArgA alpha 16 leads to His present in fibrinogen Petoskey was shown to result in a 160-fold decrease in the specificity constant for hydrolysis at A alpha 16 and concomitant release of FPA. A kinetic analysis for determination of the sequentiality of release of fibrinopeptides was presented. It indicated that at least 97% of FPB was released after FPA. The specificity constant for release of FPB from intact fibrinogen (if it occurs) was less than 3% of that for release of FPA and less than 10% of that for release of FPB from des-A fibrinogen. The specificity constant for the release of FPB from des-A fibrinogen was 4.2 (+/- 0.2) X 10(6) M-1 S-1. The polymerization inhibitors EDTA and Gly-Pro-Arg-Pro inhibited release of FPB but not FPA. These observations are consistent with the generally accepted view that the predominant pathway for the conversion of normal human fibrinogen to fibrin is one wherein FPA is released, des-A fibrinogen polymerizes, and then FPB is released.

Edetic Acid↗
PubMed1982

A kinetic method for characterization of heterogenous fibrinogen and its application to fibrinogen Grand Rapids, a congenital dysfibrinogenemia.

A kinetic analysis was developed to determine the steady state kinetic parameter Kcat/KM for the thrombin-catalyzed release of FPA from abnormal and normal fibrinogen in mixtures of the two. Such mixtures are likely to comprise the fibrinogen of individuals with congenital dysfibrinogenemia. The analysis was used to characterize fibrinogen Grand Rapids a new congenital dysfibrinogenemia. It indicated that fibrinogen from affected individuals was composed of normal and abnormal fibrinogen in roughly equal amounts, and that the value of kcat/KM for the thrombin-catalyzed release of FPA from the fibrinogen variant was 77-fold lower than that for the release of FPA from the normal fibrinogen. In separate studies, fibrinogen Grand Rapids was found to exhibit a reduced clottability. Additionally, affected individuals appeared to have plasma fibrinogen concentrations which were about one-third the normal value.

Blood Coagulation Disorders↗
PubMed1981

Determination of a low pK for histidine-159 in the S-methylthio derivative of papain by proton nuclear magnetic resonance spectroscopy.

Proton NMR spectroscopy was used to study the ionization behavior of His-159 in a derivative of papain (papain-S-SCH3). In this catalytically inactive derivative of papain, the active-site thiol group of Cys-25 is S-methyl-thiolated so that it cannot form a thiolate anion. The pH dependence of the chemical shift of the C epsilon 1 H resonance of His-159 indicated a pK of 3.45 +/- 0.07 at 45 degrees C in 2H2O with no added ions other than those required for titration. In acetate buffers at an ionic strength of 0.05, the pK increased to 3.87 +/- 0.12. Conversion of papain-S-SCH3 to active papain at pH* 4.17 (at 45 degrees C and an ionic strength of 0.05) caused the position of the C epsilon 1 H resonance to change from a position indicative of partial protonation of His-159 to a position indicative of full protonation, consistent with the existence of an imidazolium-thiolate ion-pair interaction between His-159 and Cys-25 in the active enzyme.

Histidine↗
PubMed1981

Effect of cysteine-25 on the ionization of histidine-159 in papain as determined by proton nuclear magnetic resonance spectroscopy. Evidence for a his-159--Cys-25 ion pair and its possible role in catalysis.

Papain was succinylated in order to increase its solubility above pH 8 so that proton NMR spectroscopy could be used to study the ionization of His-159 at the active site of the enzyme. The pH dependence of NMR spectra of catalytically active succinyl-papain and the methylthio derivative of the active-site cysteinyl residue of succinyl-papain (succinyl-papain-S-SCH3) were determined between pH 6 and 10. The pH dependence of the C epsilon 1 H resonance of His-159 in catalytically active succinyl-papain indicates that His-159 has a pK of about 8.6 in the catalytically active form of the enzyme. The position of this resonance in succinyl-papain-S-SCH3 indicates that when the active-site cysteinyl residue is methylthiolated, His-159 is completely deprotonated between pH 6 and 10. This result is taken as evidence for an imidazolium--thiolate ion-pair interaction between His-159 and Cys-25 wherein neutralization of the charge on the thiolate anion by methylthiolation would be expected to cause a marked decrease in the pK of His-159. A possible catalytic role for the ion pair in the acylation step in papain-catalyzed reactions is proposed wherein attack of a substrate by the imidazolium--thiolate ion pair is accompanied by an increase in the acidity of the imidazolium group that facilitates expulsion of the leaving group of the substrate.

Binding Sites↗
PubMed1981

Perturbations in the free energy and enthalpy of ionization of histidine-159 at the active site of papain as determined by fluorescence spectroscopy.

Fluorometric titrations of papain, succinyl-papain, and the corresponding methylthio derivatives of Cys-25 (papain-S-SCH3 and succinyl-papain-S-SCH3) were determined. Removal of the methylthio group from Cys-25 resulted in an increase of approximately 4 pK units in the fluorometrically determined pK value. The correspondence between the ionization behavior as determined by proton NMR and fluorometric titrations indicated that fluorescence titrations reflect the ionization behavior of His-159 in both the active enzyme and the methylthio derivative. The ionic strength dependence of the pK was analyzed in terms of simple electrostatic theory and was shown to be consistent with the charge on the protein. The temperature dependence of the pK values of His-159 indicated an increase in the heat of ionization from about 0 to 8 kcal/mol upon removal of the methylthio blocking group from Cys-25. Measurements of the effect of solvent on the pK's and heats of ionization of simple model compounds indicated that the observed shift in enthalpy of ionization of His-159 upon removal of th methylthio group from Cys-25 is not unreasonable in light of the accompanying perturbation is more than 4 pK units in the pK of His-159. The perturbations in enthalpies and free energies are attributed to formation of an ion pair. The ionization behavior of His-159 in thiol-blocked derivatives of papain is consistent with the involvement of His-159 in the deacylation step in papain catalysis.

Benzoates↗
PubMed1980

Determination of interactive thiol ionizations in bovine serum albumin, glutathione, and other thiols by potentiometric difference titration.

A potentiometric difference titration (PDT) method is used to study the ionization behavior of the thiol group in bovine serum albumin and in the following less complex compounds: glutathione, cysteine, 2-mercaptoethanol, 3-mercaptopropionic acid, 2-mercaptoethylamine, cis-2-mercaptocyclobutylamine, 2-aminothiophenol, and 5-mercapto-2-nitrobenzoic acid. In the PDT method the pH dependence of the amount of protons released in the reaction RSH + CH3SO2SCH3 leads to RSSCH3 + CH3SO2- + H+ is measured in order to obtain the pH dependence of the molar proton content of the thiol (hu) relative to the molar proton content of its methylthio derivative (hm). The pH dependence of hu--hm reflects the ionization behavior of the thiol group and of other groups whose ionization is thermodynamically linked to that of the thiol group. Data presented here indicate that the ionization behavior of the single thiol group in albumin is strikingly different in the native and the urea-denatured proteins. Three ionizable groups appear to affect ionization of the thiol in the native protein whereas only one group appears to affect ionization of the thiol in the urea-denatured protein. Furthermore, the measured PDT curves are consistent with an abnormally high acidity (pK less than 5) for the thiol in native albumin and a normal acidity for the thiol in the urea-denatured protein. Comparisons of microscopic ionization constants determined for cysteine by using the PDT method with those determined by other methods indicate that the PDT method should be useful in characterizing the ionization behavior of thiol groups in proteins and other polyprotic substances.

Animals↗
PubMed1979

Carbon dioxide versus electronic urethrocystometry for the detection of detrusor dyssynergia.

The reliability of rapid-fill carbon dioxide urethrocystometry in detecting detrusor dyssynergia was compared with the more physiologic method of electronic urethrocystometry. Fifty female patients who had urinary incontinence were studied. All patients with abnormal electronic studies had abnormal studies with carbon dioxide. Ten patients with normal electronic studies had an abnormal response to carbon dioxide and 40 per cent of these patients responded well to anticholinergic agents, indicating that carbon dioxide may be more sensitive in detecting mild forms of detrusor dysfunction that are medically treatable. Complete urodynamic assessment is recommended in conjunction with carbon dioxide urethrocystometry.

Carbon Dioxide↗
PubMed1978

Dependence of the catalytic activity of papain on the ionization of two acidic groups.

The pH dependence of kcat/Km for the papain-catalyzed hydrolysis of ethyl hippurate, N-alpha-benzoyl-L-citrulline methyl ester, and the p-nitroanilide, amide, and ethyl ester derivatives of N-alpha-benzoyl-L-arginine was determined below pH 6.4. The value of kcat/Km was observed to be modulated by two acid ionizations rather than a single ionization as previously believed. For the five substrates studied, the average pK values for the two ionizations are 3.78 +/- 0.2 and 3.95 +/- 0.1 at T/2 0.3, 25 degrees C. The observation that similar pK values were obtained with different substrates was taken as evidence that the kinetically determined pK values are close in value to true macroscopic ionization constants for ionization of groups on the free enzyme.

Hydrogen-Ion Concentration↗
PubMed1976

Potentiometric determination of ionizations at the active site of papain.

The ionization behavior of groups at the active site of papain was determined from the pH dependence of the difference of proton content of papain and the methylthio derivative of the thiol group at the active site of papain (papain-S-SCH3). This difference in proton content was determined directly by two independent methods. One method involved potentiometric measurements of the protons released and demethylthiolation of papain-S-SCH3 with dithiothreitol, as a function of pH. The other method involved analogous measurements of the protons released on methylthiolation of papain with methyl methanethiosulfonate. The methylthio pH-difference titrations generated by these measurements indicate that ionization of the thiol group at the active site of papain is linked to the ionization of His-159. The pK of the thiol group changes from 3.3 to 7.6 on deprotonation of His-159 at 29 degrees C/20.05. Similarly, the pK of His-159 shifts from 4.3 to 8.5 when the active site thiol group is deprotonated. The microscopic ionization constants determined in this work for Cys-25 and His-159 indicate that equilibrium constant for transfer of the proton from Cys-25 to His-159 is 8--12, and that in the physiological pH range the active site thiol group exists mainly as a thiol anion.

Binding Sites↗