Physical determinants of left ventricular isovolumic pressure decline: model prediction with in vivo validation

The rapid decline in pressure during isovolumic relaxation (IVR) is traditionally fit algebraically via two empiric indexes: tau, the time constant of IVR, or tau(L), a logistic time constant. Although these indexes are used for in vivo diastolic function characterization of the same physiological process, their characterization of IVR in the pressure phase plane is strikingly different, and no smooth and continuous transformation between them exists. To avoid the parametric discontinuity between tau and tau(L) and more fully characterize isovolumic relaxation in mechanistic terms, we modeled ventricular IVR kinematically, employing a traditional, lumped relaxation (resistive) and a novel elastic parameter. The model predicts IVR pressure as a function of time as the solution of d(2)P/dt(2) + (1/micro)dP/dt + E(k)P = 0, where micro (ms) is a relaxation rate (resistance) similar to tau or tau(L) and E(k) (1/s(2)) is an elastic (stiffness) parameter (per unit mass). Validation involved analysis of 310 beats (10 consecutive beats for 31 subjects). This model fit the IVR data as well as or better than tau or tau(L) in all cases (average root mean squared error for dP/dt vs. t: 29 mmHg/s for model and 35 and 65 mmHg/s for tau and tau(L), respectively). The solution naturally encompasses tau and tau(L) as parametric limits, and good correlation between tau and 1/microE(k) (tau = 1.15/microE(k) - 11.85; r(2) = 0.96) indicates that isovolumic pressure decline is determined jointly by elastic (E(k)) and resistive (1/mu) parameters. We conclude that pressure decline during IVR is incompletely characterized by resistance (i.e., tau and tau(L)) alone but is determined jointly by elastic (E(k)) and resistive (1/micro) mechanisms.     

The two modes of binding of Ru(phen)(2)dppz(2+) to DNA: thermodynamic evidence and kinetic studies

The binding of Ru(phen)(2)dppz(2+) (dppz=dipyrido[3,2-a:2',3'-c]phenazine) to DNA was investigated at pH 7.0 and 25 degrees C using stopped-flow and spectrophotometric methods. Equilibrium measurements show that two modes of binding, whose characteristics depend on the polymer to dye ratio (C(P)/C(D)), are operative. The binding mode occurring for values of C(P)/C(D) higher than 3 exhibits positive cooperativity, which is confirmed by kinetic experiments. The reaction parameters are K=2 x 10(3)M(-1), omega=550, n=1, k(r)=(1.9+/-0.5) x 10(7)M(-1)s(-1) and k(d)=(9.5+/-2.5)x10(3)s(-1) at I=0.012 M. The results are discussed in terms of prevailing surface interaction with DNA grooves accompanied by partial intercalation of the dppz residue. The other binding mode becomes operative for C(P)/C(D)<3 and the equilibria analysis shows this is an ordinary intercalation mode (K=1.3 x 10(6) M(-1), n=1.5 at I=0.012 M and K=2 x 10(5) M(-1), n=1.2 at I=0.21 M). Similar behaviour is displayed by double-stranded poly(A).     

Shear stress influences spatial variations in vascular Mn-SOD expression: implication for LDL nitration

Fluid shear stress modulates vascular production of endothelial superoxide anion (O2*-) and nitric oxide (*NO). Whether the characteristics of shear stress influence the spatial variations in mitochondrial manganese superoxide dismutase (Mn-SOD) expression in vasculatures is not well defined. We constructed a three-dimensional computational fluid dynamics model simulating spatial variations in shear stress at the arterial bifurcation. In parallel, explants of arterial bifurcations were sectioned from the human left main coronary bifurcation and right coronary arteries for immunohistolocalization of Mn-SOD expression. We demonstrated that Mn-SOD staining was prominent in the pulsatile shear stress (PSS)-exposed and atheroprotective regions, but it was nearly absent in the oscillatory shear stress (OSS)-exposed regions and lateral wall of arterial bifurcation. In cultured bovine aortic endothelial cells, PSS at mean shear stress (tau ave) of 23 dyn/cm2 upregulated Mn-SOD mRNA expression at a higher level than did OSS at tau ave = 0.02 dyn/cm2 +/- 3.0 dyn.cm(-2).s(-1) and at 1 Hz (PSS by 11.3 +/- 0.4-fold vs. OSS by 5.0 +/- 0.5-fold vs. static condition; P < 0.05, n = 4). By liquid chromatography and tandem mass spectrometry, it was found that PSS decreased the extent of low-density lipoprotein (LDL) nitration, whereas OSS increased nitration (P < 0.05, n = 4). In the presence of LDL, treatment with Mn-SOD small interfering RNA increased intracellular nitrotyrosine level (P < 0.5, n = 4), a fingerprint for nitrotyrosine formation. Our findings indicate that shear stress in the atheroprone versus atheroprotective regions regulates spatial variations in mitochondrial Mn-SOD expression with an implication for modulating LDL nitration.     

Intrachain reactions of supercoiled DNA simulated by Brownian dynamics

We considered an irreversible biochemical intrachain reaction of supercoiled DNA as a random event that occurs, with certain probability, at the instant of collision between two reactive groups bound to distant DNA sites. Using the Brownian dynamics technique, we modeled this process for a supercoiled DNA molecule of 2.5 kb length in dilute aqueous solution at an NaCl concentration of 0.1 M. We calculated the mean reaction time tau(Sigma) as a function of the intrinsic second-order rate constant k(I), the reaction radius R, and the contour separation S of the reactive groups. At the diffusion-controlled limit (k(I) --> infinity), the kinetics of reaction are determined by the mean time tau(F) of the first collision. The dependence of tau(F) on R is close to inversely proportional, implying that the main contribution to the productive collisions is made by bending of the superhelix axis. At sufficiently small k(I), the mean reaction time can be satisfactory approximated by tau(Sigma) = tau(F)(app) + 1/(k(I)c(L)), where c(L) is the local concentration of one reactive group around the other, and tau is an adjustable parameter, which we called the apparent time of the first collision. The value of tau depends on R very weakly and is approximately equal to the mean time of the first collision caused by mutual reptation of two DNA strands forming the superhelix. The quasi-one-dimensional reptation process provides the majority of productive collisions at small k(I) values.     

Kinetic and structural insight into the mechanism of BphD, a C-C bond hydrolase from the biphenyl degradation pathway

Kinetic and structural analyses of 2-hydroxy-6-oxo-6-phenylhexa-2,4-dienoic acid (HOPDA) hydrolase from Burkholderia xenovorans LB400 (BphD(LB400)) provide insight into the catalytic mechanism of this unusual serine hydrolase. Single turnover stopped-flow analysis at 25 degrees C showed that the enzyme rapidly (1/tau(1) approximately 500 s(-1)) transforms HOPDA (lambda(max) = 434 nm) into a species with electronic absorption maxima at 473 and 492 nm. The absorbance of this enzyme-bound species (E:S) decayed in a biphasic manner (1/tau(2) = 54 s(-1), 1/tau(3) = 6 s(-1) approximately k(cat)) with simultaneous biphasic appearance (48 and 8 s(-1)) of an absorbance band at 270 nm characteristic of one of the products, 2-hydroxypenta-2,4-dienoic acid (HPD). Increasing solution viscosity with glycerol slowed 1/tau(1) and 1/tau(2) but affected neither 1/tau(3) nor k(cat), suggesting that 1/tau(2) may reflect diffusive HPD dissociation, and 1/tau(3) represents an intramolecular event. Product inhibition studies suggested that the other product, benzoate, is released after HPD. Contrary to studies in a related hydrolase, we found no evidence that ketonized HOPDA is partially released prior to hydrolysis, and, therefore, postulate that the biphasic kinetics reflect one of two mechanisms, pending assignment of E:S (lambda(max) = 492 nm). The crystal structures of the wild type, the S112C variant, and S112C incubated with HOPDA were each determined to 1.6 A resolution. The latter reveals interactions between conserved active site residues and the dienoate moiety of the substrate. Most notably, the catalytic residue His265 is hydrogen-bonded to the 2-hydroxy/oxo substituent of HOPDA, consistent with a role in catalyzing ketonization. The data are more consistent with an acyl-enzyme mechanism than with the formation of a gem-diol intermediate.     

Effect of ajmaline on transient outward current in rat ventricular myocytes

The mechanism of ajmaline-induced inhibition of the transient outward current (I(to)) has been investigated in right ventricular myocytes of rat using the whole cell patch clamp technique. Ajmaline decreased the amplitude and the time integral of I(to) in a concentration-dependent, but frequency- and use-independent manner. In contrast to the single exponential time course of I(to)-inactivation in control conditions (tau(i) = 37.1 +/- 2.7 ms), the apparent inactivation was fitted by a sum of two exponentials under the effect of ajmaline with concentration-dependent fast and slow components (tau(f) = 11.7 +/- 0.8 ms, tau(s) = 57.6 +/- 2.7 ms at 10 micromol/l) suggesting block development primarily in the open channel state. An improved expression enabling to calculate the association and dissociation rate constants from the concentration dependence of tau(f) and tau(s) was derived and resulted in k(on) = 4.57 x 10(6) +/- 0.32 x 10(6) mol(-1).l.s(-1) and k(off) = 20.12 +/- 5.99 s(-1). The value of K(d) = 4.4 micromol/l calculated as k(off) / k(on) was considerably lower than IC(50) = 25.9 +/- 2.9 micromol/l evaluated from the concentration dependence of the integrals of I(to). Simulations on a simple model combining Hodgkin-Huxley type gating kinetics and drug-channel interaction entirely in open channel state agreed well with the experimental data including the difference between the K(d) and IC(50). According to the model, the fraction of blocked channels increases upon depolarization and declines if depolarization is prolonged. The repolarizing step induces recovery from block with time constant of 52 ms. We conclude that in the rat right ventricular myocytes, ajmaline is an open channel blocker with fast recovery from the block at resting voltage.     

Lateral mobility and specific binding to GABA(A) receptors on hippocampal neurons monitored by fluorescence correlation spectroscopy

The binding behavior of a fluorescently labeled muscimol derivative to the GABA(A) receptor was analyzed at rat hippocampal neurons by fluorescence correlation spectroscopy. After muscimol had been labeled with the fluorophore Alexa Fluor 532, specific binding constants for binding of the dye-labeled ligand (Mu-Alexa) to the GABA(A) receptor were determined. We found a high specific binding affinity of Mu-Alexa with a K(D) value of 3.4 +/- 0.5 nM and a rate constant of ligand-receptor dissociation (k(diss)) of (5.37 +/- 0.95) x 10(-2) s(-1). A rate constant of ligand-receptor association (k(ass)) of (1.57 +/- 0.28) x 10(7) L mol(-1) s(-1) was calculated. The following diffusion coefficients were observed: D(free) = 233 +/- 20 microm(2)/s (n = 66) for free diffusing Mu-Alexa, D(bound1) = 2.8 +/- 0.9 microm(2)/s (n = 64) for the lateral mobility, and D(bound2) = 0.14 +/- 0.05 microm(2)/s (n = 56) for the hindered mobility of the GABA(A) receptor-ligand complex in the cell membrane. Saturation of Mu-Alexa binding was observed at a concentration of 50 nM. A maximum number of binding sites [B(max) = 18.4 +/- -0.4 nM (n = 5)] was found. Similar K(i) values of 4.5 +/- 1.0 nM for nonlabeled muscimol and 8.8 +/- 1.8 nM for Mu-Alexa were found by RRAs using [(3)H]muscimol as a radioligand. A concentration-dependent increase in the level of specific Mu-Alexa binding was demonstrated by the positive cooperative activity of co-incubated midazolam, which was selectively found in GABA(A) receptor-ligand complexes with hindered mobility.     

Diffusion-limited component of reactions catalyzed by Bacillus cereus beta-lactamase I

The Bacillus cereus beta-lactamase I catalyzes the hydrolysis of a wide variety of penicillins and cephalosporins with values of k(cat)/K(m) varying over several orders of magnitude. The values of this parameter for the most reactive of these compounds, benzylpenicillin, I, and furylacryloyl-penicillin, II (k(cat)/K(m) = 2.43 x 10(7) M(-1) s(-1) and 2.35 x 10(7) M(-1) s(-1), respectively, at pH 7.0 in potassium phosphate buffer containing 0.17 M KCl, I(c) = 0.63, 25 degrees C) are decreased markedly by increasing viscosity in sucrose- or glycerol-containing buffers. The relative sensitivities to viscosity of k(cat)/K(m) values for I and for cephaloridine, III, were found to be virtually unchanged at pH 3.8 from those observed at pH 7.0. The differential effects of viscosity on the reactive vs. the sluggish [e.g., cephalothin (IV), k(cat)/K(m) = 1 x 10(4) M(-1) s(-1)] substrates support the contention that the rates of reaction of the former with the enzyme are in part diffusion controlled. Quantitative analysis gives values for the association rate constants, k(1), of 7.6 x 10(7) M(-1) s(-1), 4 x 10(7) M(-1) s(-1), and 1.1 x 10(7) M(-1) s(-1) for I, II, and III, respectively. As both reactive and sluggish substrates associate with the active site of the enzyme with relatively similar rate constants, the variation in k(cat)/K(m) values is primarily due to the variation in the partition ratios k(-1)/k(2), for the ES complex, which are 2.3, 0.77, and 30 for I, II, and III, respectively. The preceding analysis is based on direct application of the Stokes-Einstein diffusion law to enzyme kinetics. The range of applicability of this law to the diffusion of substrate size molecules and the mechanics of diffusion of ionic species through viscous solutions of sucrose vs. polymers are explored.     

Nonfixed relationship of the Michaelis constant and maximum velocity with their corresponding rate constants

The Michaelis constant (K(m)) and V(mas) (E0k(cat)) values for two mutant sets of enzymes were studied from the viewpoint of their definition in a rapid equilibrium reaction model and in a steady state reaction model. The "AMP set enzyme" had a mutation at the AMP-binding site (Y95F, V67I, and V67I/L76V), and the "ATP set enzyme" had a mutation at a possible ATP-binding region (Y32F, Y34F, and Y32A/Y34A). Reaction rate constants obtained using steady state model analysis explained discrepancies found by the rapid equilibrium model analysis. (i) The unchanged number of bound AMPs for Y95F and the wild type despite the markedly increased K(m) values for AMP of the AMP set of enzymes was explained by alteration of the rate constants of the AMP step (k(+2), k(-2)) to retain the ratio k(+2)/k(-2). (ii) A 100 times weakened selectivity of ATP for Y34F in contrast to no marked changes in K(m) values for both ATP and AMP for the ATP set of enzymes was explained by the alteration of the rate constants of the ATP steps. A similar alteration of the K(m) and k(cat) values of these enzymes resulted from distinctive alterations of their rate constants. The pattern of alteration was highly suggestive. The most interesting finding was that the rate constants that decided the K(m) and k(cat) values were replaced by the mutation, and the simple relationships between K(m), k(cat), and the rate constants of K(m)1 = k(+1)/k(-1) and k(cat) = k(f) were not valid. The nature of the K(m) and k(cat) alterations was discussed.     

Moving beyond noninformative priors: why and how to choose weakly informative priors in Bayesian analyses

Throughout the last two decades, Bayesian statistical methods have proliferated throughout ecology and evolution. Numerous previous references established both philosophical and computational guidelines for implementing Bayesian methods. However, protocols for incorporating prior information, the defining characteristic of Bayesian philosophy, are nearly nonexistent in the ecological literature. Here, I hope to encourage the use of weakly informative priors in ecology and evolution by providing a ‘consumer's guide’ to weakly informative priors. The first section outlines three reasons why ecologists should abandon noninformative priors: 1) common flat priors are not always noninformative, 2) noninformative priors provide the same result as simpler frequentist methods, and 3) noninformative priors suffer from the same high type I and type M error rates as frequentist methods. The second section provides a guide for implementing informative priors, wherein I detail convenient ‘reference’ prior distributions for common statistical models (i.e. regression, ANOVA, hierarchical models). I then use simulations to visually demonstrate how informative priors influence posterior parameter estimates. With the guidelines provided here, I hope to encourage the use of weakly informative priors for Bayesian analyses in ecology. Ecologists can and should debate the appropriate form of prior information, but should consider weakly informative priors as the new ‘default’ prior for any Bayesian model.     

Reaction mechanism for mammalian pyruvate dehydrogenase using natural lipoyl domain substrates

The pyruvate dehydrogenase (E1) component of the pyruvate dehydrogenase complex (PDC) catalyzes a two-step reaction. Recombinant production of substrate amounts of the lipoyl domains of the dihydrolipoyl transacetylase (E2) component of the mammalian PDC allowed kinetic characterization of the rapid physiological reaction catalyzed by E1. Using either the N-terminal (L1) or the internal (L2) lipoyl domain of E2 as a substrate, analyses of steady state kinetic data support a ping pong mechanism. Using standard E1 preparations, Michaelis constants (Km) were 52 +/- 14 microM for L1 and 24.8 +/- 3.8 microM for pyruvate and k(cat) was 26.3 s(-1). With less common, higher activity preparations of E1, the Km values were > or =160 microM for L1 and > or =35 microM for pyruvate and k(cat) was > or =70 s(-1). Similar results were found with the L2 domain. The best synthetic lipoylated-peptide (L2 residues 163-177) was a much poorer substrate (Km > or =15 mM, k(cat) approximately equals 5 s(-1); k(cat)/Km decreased >1,500-fold) than L1 or L2, but a far better substrate in the E1 reaction than free lipoamide (k(cat)/Km increased >500-fold). Each lipoate source was an effective substrate in the dihydrolipoyl dehydrogenase (E3) reaction, but E3 had a lower Km for the L2 domain than for lipoamide or the lipoylated peptides. In contrast to measurements with slow E1 model reactions that use artificial acceptors, we confirmed that the natural E1 reaction, using lipoyl domain acceptors, was completely inhibited (>99%) by phosphorylation of E1 and the phosphorylation strongly inhibited the reverse of the second step catalyzed by E1. The mechanisms by which phosphorylation interferes with E1 activity is interpreted based on accrued results and the location of phosphorylation sites mapped onto the 3-D structure of related alpha-keto acid dehydrogenases.     

Domain flexibility in ligand-free and inhibitor-bound Escherichia coli adenylate kinase based on a mode-coupling analysis of 15N spin relaxation

Adenylate kinase from Escherichia coli (AKeco), consisting of a 23.6-kDa polypeptide chain folded into domains CORE, AMPbd, and LID catalyzes the reaction AMP + ATP <--> 2ADP. The domains AMPbd and LID execute large-amplitude movements during catalysis. Backbone dynamics of ligand-free and AP(5)A-inhibitor-bound AKeco is studied with slowly relaxing local structure (SRLS) (15)N relaxation, an approach particularly suited when the global (tau(m)) and the local (tau) motions are likely to be coupled. For AKeco tau(m) = 15.1 ns, whereas for AKeco*AP(5)A tau(m) = 11.6 ns. The CORE domain of AKeco features an average squared order parameter, , of 0.84 and correlation times tau(f) = 5-130 ps. Most of the AKeco*AP(5)A backbone features = 0.90 and tau(f) = 33-193 ps. These data are indicative of relative rigidity. Domains AMPbd and LID of AKeco, and loops beta(1)/alpha(1), alpha(2)/alpha(3), alpha(4)/beta(3), alpha(5)/beta(4), and beta(8)/alpha(7) of AKeco*AP(5)A, feature a novel type of protein flexibility consisting of nanosecond peptide plane reorientation about the C(i-1)(alpha)-C(i)(alpha) axis, with correlation time tau(perpendicular) = 5.6-11.3 ns. The other microdynamic parameters underlying this dynamic model include S(2) = 0.13-0.5, tau(parallel) on the ps time scale, and a diffusion tilt beta(MD) ranging from 12 to 21 degrees. For the ligand-free enzyme the tau(perpendicular) mode was shown to represent segmental domain motion, accompanied by conformational exchange contributions R(ex) < or = 4.4 s(-1). Loop alpha(4)/beta(3) and alpha(5)/beta(4) dynamics in AKeco*AP(5)A is related to the "energetic counter-balancing of substrate binding" effect apparently driving kinase catalysis. The other flexible AKeco*AP(5)A loops may relate to domain motion toward product release.     

Passage times for polymer translocation pulled through a narrow pore

We study the passage times of a translocating polymer of length N in three dimensions, while it is pulled through a narrow pore with a constant force F applied to one end of the polymer. At small to moderate forces, satisfying the condition FN(nu)/k(B)T less, similar 1, where nu approximately 0.588 is the Flory exponent for the polymer, we find that tau(N), the mean time the polymer takes to leave the pore, scales as N(2+nu) independent of F, in agreement with our earlier result for F = 0. At strong forces, i.e., for, FN(nu)/k(B)T > 1, the behavior of the passage time crosses over to tau(N) approximately N(2)/F. We show here that these behaviors stem from the polymer dynamics at the immediate vicinity of the pore-in particular, the memory effects in the polymer chain tension imbalance across the pore.     

Re-engineering cytochrome P450 2B11dH for enhanced metabolism of several substrates including the anti-cancer prodrugs cyclophosphamide and ifosfamide

Based on recent directed evolution of P450 2B1, six P450 2B11 mutants at three positions were created in an N-terminal modified construct termed P450 2B11dH and characterized for enzyme catalysis using five substrates. Mutant I209A demonstrated a 3.2-fold enhanced k(cat)/K(m) for 7-ethoxy-4-trifluoromethylcourmarin O-deethylation, largely due to a dramatic decrease in K(m) (0.72 microM vs. 18 microM). I209A also demonstrated enhanced selectivity for testosterone 16beta-hydroxylation over 16alpha-hydroxylation. In contrast, V183L showed a 4-fold increased k(cat) for 7-benzyloxyresorufin debenzylation and a 4.7-fold increased k(cat)/K(m) for testosterone 16alpha-hydroxylation. V183L also displayed a 1.7-fold higher k(cat)/K(m) than P450 2B11dH with the anti-cancer prodrugs cyclophosphamide and ifosfamide, resulting from a approximately 4-fold decrease in K(m). Introduction of the V183L mutation into full-length P450 2B11 did not enhance the k(cat)/K(m). Overall, the re-engineered P450 2B11dH enzymes exhibited enhanced catalytic efficiency with several substrates including the anti-cancer prodrugs.     

Catalytic reaction pathway for the mitogen-activated protein kinase ERK2

The structural, functional, and regulatory properties of the mitogen-activated protein kinases (MAP kinases) have long attracted considerable attention owing to the critical role that these enzymes play in signal transduction. While several MAP kinase X-ray crystal structures currently exist, there is by comparison little mechanistic information available to correlate the structural data with the known biochemical properties of these molecules. We have employed steady-state kinetic and solvent viscosometric techniques to characterize the catalytic reaction pathway of the MAP kinase ERK2 with respect to the phosphorylation of a protein substrate, myelin basic protein (MBP), and a synthetic peptide substrate, ERKtide. A minor viscosity effect on k(cat) with respect to the phosphorylation of MBP was observed (k(cat) = 10 +/- 2 s(-1), k(cat)(eta) = 0.18 +/- 0.05), indicating that substrate processing occurs via slow phosphoryl group transfer (12 +/- 4 s(-1)) followed by the faster release of products (56 +/- 4 s(-1)). At an MBP concentration extrapolated to infinity, no significant viscosity effect on k(cat)/K(m(ATP)) was observed (k(cat)/K(m(ATP)) = 0.2 +/- 0.1 microM(-1) s(-1), k(cat)/K(m(ATP))(eta) = -0.08 +/- 0.04), consistent with rapid-equilibrium binding of the nucleotide. In contrast, at saturating ATP, a full viscosity effect on k(cat)/K(m) for MBP was apparent (k(cat)/K(m(MBP)) = 2.4 +/- 1 microM(-1) s(-1), k(cat)/K(m(MBP))(eta) = 1.0 +/- 0.1), while no viscosity effect was observed on k(cat)/K(m) for the phosphorylation of ERKtide (k(cat)/K(m(ERKtide)) = (4 +/- 2) x 10(-3) microM(-1) s(-1), k(cat)/K(m(ERKtide))(eta) = -0.02 +/- 0.02). This is consistent with the diffusion-limited binding of MBP, in contrast to the rapid-equilibrium binding of ERKtide, to form the ternary Michaelis complex. Calculated values for binding constants show that the estimated value for K(d(MBP)) (/= 1.5 mM). The dramatically higher catalytic efficiency of MBP in comparison to that of ERKtide ( approximately 600-fold difference) is largely attributable to the slow dissociation rate of MBP (/=56 s(-1)), from the ERK2 active site.     

Hydrolysis of biological peptides by human angiotensin-converting enzyme-related carboxypeptidase

Human angiotensin-converting enzyme-related carboxypeptidase (ACE2) is a zinc metalloprotease whose closest homolog is angiotensin I-converting enzyme. To begin to elucidate the physiological role of ACE2, ACE2 was purified, and its catalytic activity was characterized. ACE2 proteolytic activity has a pH optimum of 6.5 and is enhanced by monovalent anions, which is consistent with the activity of ACE. ACE2 activity is increased approximately 10-fold by Cl(-) and F(-) but is unaffected by Br(-). ACE2 was screened for hydrolytic activity against a panel of 126 biological peptides, using liquid chromatography-mass spectrometry detection. Eleven of the peptides were hydrolyzed by ACE2, and in each case, the proteolytic activity resulted in removal of the C-terminal residue only. ACE2 hydrolyzes three of the peptides with high catalytic efficiency: angiotensin II () (k(cat)/K(m) = 1.9 x 10(6) m(-1) s(-1)), apelin-13 (k(cat)/K(m) = 2.1 x 10(6) m(-1) s(-1)), and dynorphin A 1-13 (k(cat)/K(m) = 3.1 x 10(6) m(-1) s(-1)). The ACE2 catalytic efficiency is 400-fold higher with angiotensin II () as a substrate than with angiotensin I (). ACE2 also efficiently hydrolyzes des-Arg(9)-bradykinin (k(cat)/K(m) = 1.3 x 10(5) m(-1) s(-1)), but it does not hydrolyze bradykinin. An alignment of the ACE2 peptide substrates reveals a consensus sequence of: Pro-X((1-3 residues))-Pro-Hydrophobic, where hydrolysis occurs between proline and the hydrophobic amino acid.     

Binding of dioxygen to non-metal sites in proteins: exploration of the importance of binding site size versus hydrophobicity in the copper amine oxidase from Hansenula polymorpha

Copper amine oxidases (CAOs) contain 2,4,5-trihydroxyphenylalanyl quinone (TPQ) and a copper ion in their active sites, catalyzing amine oxidation to aldehyde and ammonia concomitant with the reduction of molecular oxygen to hydrogen peroxide. Kinetic studies on the CAO from bovine serum (BSAO) [Su and Klinman (1999) Biochemistry 37, 12513-12525] and the recent reports on the cobalt substituted form of the enzyme from Hansenula polymorpha (HPAO) [Mills and Klinman (2000) J. Am. Chem. Soc. 122, 9897-9904, and Mills et al. (2002) Biochemistry, 41, 10577-10584] support pre-binding of molecular oxygen prior to a rate-limiting electron transfer from the reduced form of TPQ (p-aminohydroquinone form) to dioxygen. Although there is significant sequence homology between BSAO and HPAO, k(cat)/K(m)(O2) for BSAO under the optimal condition is one order of magnitude lower than that for HPAO. From a comparison of amino acid sequences for BSAO and HPAO, together with the X-ray crystal structure of HPAO, a plausible dioxygen pre-binding site has been identified that involves Y407, L425, and M634 in HPAO; the latter two residues are altered in BSAO to A490 and T695. To determine which of these residues plays a greater role in dioxygen chemistry, k(cat)/K(m)(O2) was determined in HPAO for the M634 --> T and L425 --> A mutants. The L425 --> A mutation does not alter k(cat)/K(m)(O2) to a large extent, whereas the M634 --> T decreased k(cat)/K(m)(O2) by one order of a magnitude, creating a catalyst that is similar to BSAO. A series of mutants at M634 (to F, L, and Q) were, therefore, prepared in HPAO and characterized with regard to k(cat)/K(m)(O2) as a function of pH. Structure reactivity correlations show a linear relationship of rate with side chain volume, rather than hydrophobicity, indicating that dioxygen reactivity increases with the bulk of the residue at position 634. This site also shows specificity for O2, in relation to the co-gas N2, since substitution of the inert gas N2 by either Ar or He has no effect on measured rates. In particular, He gas is expected to have little affinity for protein at 1 atmospheric pressure, implying little or no binding by N2 as well.     

Are the dynamic response characteristics of brachial artery flow-mediated dilation sensitive to the magnitude of increase in shear stimulus?

The purpose of this study was to determine the dynamic characteristics of brachial artery dilation in response to step increases in shear stress [flow-mediated dilation (FMD)]. Brachial artery diameter (BAD) and mean blood velocity (MBV) (Doppler ultrasound) were obtained in 15 healthy subjects. Step increases in MBV at two shear stimulus magnitudes were investigated: large (L; maximal MBV attainable), and small (S; MBV at 50% of the large step). Increase in shear rate (estimate of shear stress: MBV/BAD) was 76.8 +/- 15.6 s(-1) for L and 41.4 +/- 8.7 s(-1) for S. The peak %FMD was 14.5 +/- 3.8% for L and 5.7 +/- 2.1% for S (P < 0.001). Both the L (all subjects) and the S step trials (12 of 15 subjects) elicited a biphasic diameter response with a fast initial phase (phase I) followed by a slower final phase. Relative contribution of phase I to total FMD when two phases occurred was not sensitive to shear rate magnitude (r(2) = 0.003, slope P = 0.775). Parameters quantifying the dynamics of the FMD response [time delay (TD), time constant (tau)] were also not sensitive to shear rate magnitude for both phases (phase I: TD r(2) = 0.03, slope P = 0.376, tau r(2) = 0.04, slope P = 0.261; final phase: TD r(2) = 0.07, slope P = 0.169, tau r(2) = 0.07, slope P = 0.996). These data support the existence of two distinct mechanisms, or sets of mechanisms, in the human conduit artery FMD response that are proportionally sensitive to shear stimulus magnitude and whose dynamic response is not sensitive to shear stimulus magnitude.     

Characterization of a new radioiodinated probe for the alpha2C adrenoceptor in the mouse brain

[125I]17alpha-hydroxy-20alpha-yohimban-16beta-(N-4-p6 hydroxyphenethyl)carboxamide or [125I]rauwolscine-OHPC, a new radioiodinated probe derived from rauwolscine was synthesized and its binding characteristics investigated on sections of the mouse caudate putamen. [125I]rauwolscine-OHPC binding was saturable and revealed interaction with a single class of binding sites (KD= 0.171 nM, Bmax = 3082 pCi/mg of tissue). The kinetically derived affinity was in close agreement with the affinity evaluated by saturation experiments: k(-1)/k(+1)(0.0403 min(-1)/114 10(6) M(-1) min(-1))=0.35 nM. Competition studies revealed interaction with one single class of binding sites for each of the twelve compounds tested. The rank of potency suggested an interaction with alpha2 adrenoceptors (atipamezole > or = RX 821002 > yohimbine > (-)epinephrine). Moreover, the good affinity of [125I] rauwolscine-OHPC binding sites for spiroxatrine, yohimbine, WB 4101, the relatively good affinity for prazosin (Ki =37.4 nM) and the affinity ratio prazosin/oxymetazoline (37.4/43.4=0.86) were consistent with an alpha2C selective labelling of [125I]rauwolscine-OHPC. The distribution of [125I]rauwolscine-OHPC binding sites in mouse brain was characterized by autoradiography. The density of binding sites was high in the islands of Calleja, accumbens nucleus, caudate putamen and olfactory tubercles, moderate in the hippocampus, amygdala and anterodorsal nucleus of the thalamus. These findings demonstrated that [125I]rauwolscine-OHPC is a useful radioiodinated probe to label alpha2C adrenoceptors in mouse brain.     

Enzymatic reaction of silent substrates: kinetic theory and application to the serine protease chymotrypsin

Investigating the selectivity that an enzyme expresses toward its substrates can be technically challenging if reaction of these substrates is not accompanied by a conveniently monitored change in some physicochemical property. In this paper, we describe a simple method for determining steady-state kinetic parameters for enzymatic turnover of such "silent" substrates. According to this method, silent substrate S is allowed to compete for enzymic reaction with signal-generating substrate S*, whose conversion to product can be conveniently monitored. Full reaction progress curves are collected under conditions of [S*](o) < K(m)* and [S](o) >or= 3K(m). Progress curves collected under these conditions are characterized by an initial lag phase of duration tau that is followed by the pseudo-first-order reaction of S. Steady-state kinetic parameters for the silent substrate can be obtained by one of two methods. One method combines least-squares fitting with numerical integration of appropriate rate equations to analyze the progress curves, while the other method relies on direct graphical analysis in which K(m) is the value of [S](o) that reduces the control velocity by a factor of 2 and V(max) is shown to simply equal the ratio [S](o)/tau. We use these methods to analyze the alpha-chymotrypsin-catalyzed hydrolysis of silent substrate Suc-Ala-Phe-AlaNH(2) with signal generator Suc-Ala-Phe-pNA. From the curve-fitting method, k(c) = 0.9 +/- 0.2 s(-1) and K(m) = 0.4 +/- 0.1 mM, while by direct graphical analysis, k(c) = 1.1 +/- 0.1 s(-1) and K(m) = 0.51 +/- 0.03 mM. As validation of this new method, we show agreement of these values with those determined independently by HPLC analysis of the hydrolysis of Suc-Ala-Phe-AlaNH(2) by alpha-CT, where k(c) = 1.1 +/- 0.1 s(-1) and K(m) = 0.5 +/- 0.1 mM.