In situ kinetic analysis of glyoxalase I and glyoxalase II in Saccharomyces cerevisiae.

The kinetics of glyoxalase I [(R)-S-lactoylglutathione methylglyoxal-lyase; EC 4.4.1.5] and glyoxalase II (S-2-hydroxyacylglutathione hydrolase; EC 3.1.2.6) from Saccharomyces cerevisiae was studied in situ, in digitonin permeabilized cells, using two different approaches: initial rate analysis and progress curves analysis. Initial rate analysis was performed by hyperbolic regression of initial rates using the program HYPERFIT. Glyoxalase I exhibited saturation kinetics on 0.05-2.5 mM hemithioacetal concentration range, with kinetic parameters Km 0.53 +/- 0.07 mM and V (3.18 +/- 0.16) x 10(-2) mM.min(-1). Glyoxalase II also showed saturation kinetics in the SD-lactoylglutathione concentration range of 0.15-3 mM and Km 0.32 +/- 0.13 mM and V (1.03 +/- 0.10) x 10(-3) mM.min(-1) were obtained. The kinetic parameters of both enzymes were also estimated by nonlinear regression of progress curves using the raw absorbance data and integrated differential rate equations with the program GEPASI. Several optimization methods were used to minimize the sum of squares of residuals. The best parameter fit for the glyoxalase I reaction was obtained with a single curve analysis, using the irreversible Michaelis-Menten model. The kinetic parameters obtained, Km 0.62 +/- 0.18 mM and V (2.86 +/- 0.01) x 10(-2) mM.min(-1), were in agreement with those obtained by initial rate analysis. The results obtained for glyoxalase II, using either the irreversible Michaelis-Menten model or a phenomenological reversible hyperbolic model, showed a high correlation of residuals with time and/or high values of standard deviation associated with Km. The possible causes for the discrepancy between data obtained from initial rate analysis and progress curve analysis, for glyoxalase II, are discussed.     

A new procedure to derive weighting factors for nonlinear regression analysis applied to enzyme kinetic data.

The experimental variance of enzymic steady-state kinetic experiments depends on velocity as approximated by a power function (Var(v) = K1 . valpha (Askel?f, P., Korsfeldt, M. and Mannervik, B. (1976) Eur. J. Biochem. 69, 61--67). The values of the constants (K1, alpha) can be estimated by making replicate measurements of velocity, and the inverse of the function can then be used as a weighting factor. In order to avoid measurement of a large number of replicates to establish the error structure of a kinetic data set, a different approach was tested. After a preliminary regression using a 'good model', which satisfies reasonable goodness-of-fit criteria, the residuals were taken to represent the experimental error. The neighbouring residuals were grouped together and the sum of their mean squared values was used as a measure of the variance in the neighbourhood of the corresponding measurements. The values of the constants obtained in this way agreed with those obtained by replicates.     

Rate constants for a mechanism including intermediates in the interconversion of ternary complexes by horse liver alcohol dehydrogenase

Transient kinetic data for partial reactions of alcohol dehydrogenase and simulations of progress curves have led to estimates of rate constants for the following mechanism, at pH 8.0 and 25 degrees C: E in equilibrium E-NAD+ in equilibrium *E-NAD+ in equilibrium E-NAD(+)-RCH2OH in equilibrium E-NAD+-RCH2O- in equilibrium *E-NADH-RCHO in equilibrium E-NADH-RCHO in equilibrium E-NADH in equilibrium E. Previous results show that the E-NAD+ complex isomerizes with a forward rate constant of 620 s-1 [Sekhar, V. C., & Plapp, B. V. (1988) Biochemistry 27, 5082-5088]. The enzyme-NAD(+)-alcohol complex has a pK value of 7.2 and loses a proton rapidly (greater than 1000 s-1). The transient oxidation of ethanol is 2-fold faster in D2O, and proton inventory results suggest that the transition state has a charge of -0.3 on the substrate oxygen. Rate constants for hydride ion transfer in the forward or reverse reactions were similar for short-chain aliphatic substrates (400-600 s-1). A small deuterium isotope effect for transient oxidation of longer chain alcohols is apparently due to the isomerization of the E-NAD+ complex. The transient reduction of aliphatic aldehydes showed no primary deuterium isotope effect; thus, an isomerization of the E-NADH-aldehyde complex is postulated, as isomerization of the E-NADH complex was too fast to be detected. The estimated microscopic rate constants show that the observed transient reactions are controlled by multiple steps.     

Mechanism of lectin-cell interactions: thermodynamic and kinetic analysis of the binding of winged bean acidic lectin (WBA II) to human erythrocytes.

In order to identify the forces involved in the binding and to understand the mechanism involved, equilibrium and kinetic studies were performed on the binding of the winged bean acidic lectin to human erythrocytes. The magnitudes of delta S and delta H were positive and negative respectively, an observation differing markedly from the lectin-simple sugar interactions where delta S and delta H are generally negative. Analysis of the sign and magnitudes of these values indicate that ionic and hydrogen bonded interactions prevail over hydrophobic interactions resulting in net -ve delta H (-37.12 kJ.mol-1) and +ve delta S (14.4 J.mole-1 K-1 at 20 degrees C), thereby suggesting that this entropy driven reaction also reflects conformational changes in the lectin and/or the receptor. Presence of two kinds of receptors for WBA II on erythrocytes, as observed by equilibrium studies, is consistent with the biexponential dissociation rate constants (at 20 degrees C K1 = 1.67 x 10(-3) M-1 sec-1 and K2 = 11.1 x 10(-3) M-1 sec-1). These two rate constants differed by an order of magnitude accounting for the difference in the association constants of the two receptors of WBA II. However, the association process remains monoexponential suggesting no observable difference in the association rates of the lectin molecule with both the receptors, under the experimental conditions studied. The thermodynamic parameters calculated from kinetic data correlate well with those observed by equilibrium. A two-step binding mechanism is proposed based on the kinetic parameters for WBA II-receptor interaction.(ABSTRACT TRUNCATED AT 250 WORDS)     

Glucose-6-phosphate dehydrogenase from brewers' yeast. The effects of pH and temperature on the steady-state kinetic parameters of the two-chain protein species.

A systematic study has been made of the pH- and temperature-dependency of the steady-state kinetic parameters of the stabilized two-subunit enzyme species of glucose-6-phosphate dehydrogenase, in the absence of superimposed association-dissociation reactions. The Vmax(app) data obtained in several buffers between pH 5 and 10 and at 18-32 degrees C lead to the postulate that at least two sets of protonic equilibria may govern the catalysis (one near pH 5.7 AT 25 DEGREES C and another near pH 9.2); furthermore, two pathways for product formation (i.e., two Vmax's) appear to be required to explain the biphasic nature of the log Vmax(app) vs. pH curves, with Vmax(basic) greater than Vmax(acidic + neutral). Of the several buffers explored, either a uniform degree of interaction or a minimal degree of buffer species interaction could be assessed from the enthalpy changes associated with the derived values for ionization constants attributed to the protonic equilibria in the enzyme-substrates ternary complexes for the case of Tris-acetate-EDTA buffers, at constant ionic strength. With the selection of this buffer at 0.1 (T/2) and at 25 and 32 degrees C, a self-consistent kinetic mechanism has emerged which allows for the random binding of the two fully ionized substrates to the enzyme via two major pathways, and product formation by both E-A--B- and HE-A--B-. As before (Kuby et al. Arch. Biochem, Biophys. 165, 153-178, 1974), a quasi-equilibrium is presumed, with rate-limiting steps (k + 5 and k + 5') at the interconversion of the ternary complexes. Values for the two sets of protonic equilibria defined by this mechanism (viz., pKk, pKH2 for the first ionizations, and pKk', pKH' for the second) could then be estimated. From their numerical values (e.g., at 25 degrees C: pKK = 5.7 PKH2 = 5.2; and pKK' = 9.1, PKH' = 8.2) and from the values for delta H degrees ioniz (e.g., delta H degrees pKK APPROXIMATELY 5.1 KCAL/MOL; DELTA H degrees pKK' APPROXIMATELY 11 KCAL/MOL), A POSTULATE IS PRESENTED WHICH ATTRIBUTES THESE Acid dissociation constants to an imidazole and epsilon-amino group, respectively.     

Studies on the aminopeptidase activity of rat cathepsin H.

Three synthetic substrates H-Arg-NH-Mec, Bz-Arg-NH-Mec and H-Cit-NH-Mec (Bz, Benzoyl; NH-Mec, 4-methylcoumaryl-7-amide; Cit, citrulline) were used to characterize specificity requirements for the P1-S1 interaction of cathepsin H from rat liver. From rapid equilibrium kinetic studies it was shown that Km, kcat and the specificity constants kcat/Km are quite similar for substrates with a free alpha-amino group. In contrast, a 25-fold decrease of kcat/Km was observed for the N-terminal-blocked substrate Bz-Arg-NH-Mec. The activation energies for H-Arg-NH-Mec and Bz-Arg-NH-Mec were determined to be 37 kJ/mol and 55 kJ/mol, respectively, and the incremental binding energy delta delta Gb of the charged alpha-amino group was estimated to -8.1 kJ/mol at pH 6.8. The shown preference of cathepsin H for the unblocked substrates H-Arg-NH-Mec and H-Cit-NH-Mec was further investigated by inspection of the pH dependence of kcat/Km. The curves of the two substrates with a charged alpha-amino group showed identical bell-shaped profiles which both exhibit pKa1 and pKa2 values of 5.5 and 7.4, respectively, at 30 degrees C. The residue with a pKa1 of 5.5 in the acid limb of the activity profile of H-Arg-NH-Mec was identified by its ionization enthalpy delta Hion = 21 kJ/mol as a beta-carboxylate or gamma-carboxylate of the enzyme, whereas the residue with a pKa2 of 7.4 was assigned to the free alpha-amino group of the substrate with a delta Hion of 59 kJ/mol. Bz-Arg-NH-Mec showed a different pH-activity profile with a pKa1 of 5.4 and a pKa2 of 6.6 at 30 degrees C. Cathepsin H exhibits no preference for a basic P1 side chain as has been shown by the similar kinetics of H-Arg-NH-Mec and the uncharged, isosteric substrate H-Cit-NH-Mec. In summary, specific interactions of an anionic cathepsin H active site residue with the charged alpha-amino group of substrates caused transition state stabilization which proves the enzyme to act preferentially as an aminopeptidase.     

Heavy metal ions inhibition of jack bean urease: potential for rapid contaminant probing

The kinetics of heavy metal ions inhibition of jack bean urease was studied by progress curve analysis in a reaction system without enzyme-inhibitor preincubation. The inhibition was found to be biphasic with an initial, small inhibitory phase changing over the time course of 5-10 min into a final linear steady state with a lower velocity. This time-dependent pattern was best described by mechanism B of slow-binding inhibition, involving the rapid formation of an EI complex that subsequently undergoes slow conversion to a more stable EI* complex. The kinetic parameters of the process, the inhibition constants Ki and Ki* and the forward k5 and reverse k6 rate constants for the conversion, were evaluated from the reaction progress curves by nonlinear regression treatment. Based on the values of the overall inhibition constant Ki*, the heavy metal ions were found to inhibit urease in the following decreasing order: Hg2+ > Cu2+ > Zn2+ > Cd2+ > Ni2+ > Pb2+ > Co2+ > Fe3+ > As3+. With the Ki* values as low as 1.9 nM for Hg2+ and 7.1 nM for Cu2+, 100-1000 times lower than those of the other ions, urease may be utilized as a bioindicator of the trace levels of these ions in environmental monitoring, bioprocess control or pharmaceutical analysis.     

A model of photosystem II for the analysis of a fast fluorescence rise in plant leaves

The polyphasic patterns of fluorescence induction rise in pea leaves in vivo and after the treatment with ionophores have been studied using a plant efficiency analyzer. To analyze in detail photosystem II (PS II) electron transfer processes, an extended PS II model was applied, which included the sums of exponential functions to specify explicitly the light-driven formation of the transmembrane electric potential (delta psi(t)) as well as pH in the lumen (pHL(t)) and stroma (pHs(t)). PS II model parameters and numerical coefficients in delta psi(t), and pHs(t) were evaluated to fit fluorescence induction data for different experimental conditions: leaf in vivo or after ionophore treatment at low or high light intensity. The model imitated changes in the pattern of fluorescence induction rise due to the elimination of transmembrane potential in the presence of ionophores, when delta psi = 0 and pHL(t), pHS(t) altered to small extent relative to control values in vivo, with maximum delta psi(t) approximately 90 MB and delta psi(t) approximately 40 MB, for the stationary state at deltapH aproximately equal to 1.8. As the light intensity was increased from 300 to 1200 micromol x m(-2) x s(-1), the heat dissipation rate constants increased threefold for nonradiative recombination of P680+Phe- and by approximately 30% for P680+Q(A)-. The parameters delta psi, pH(S) and pH(L) were analyzed as factors of PS II redox state populations and fluorescence yield. The kinetic mechanism of qE quenching is discussed, which is related with light induced pH(L) lumen acidification, when Q(A)- and P680+ recombination probability increases to regulate the QA reduction.     

Kinetics of dissociation of parvalbumin complexes with calcium and magnesium ions

Dissociation kinetics of parvalbumin complexes with calcium and magnesium ions were studied by means of stopped-flow method employing intrinsic protein fluorescence registration. In the temperature range from 10 to 30 degrees C the kinetic curves of Ca2+ and Mg2+ dissociation are best fitted with a sum of two exponential terms, each term is ascribed to a dissociation process in one of two bindings sites of parvalbumin. Dissociation rate constants in this temperature range increase from 0.03 to 0.8 s-1 and from 0.18 to 5 s-1 for Ca2+, and from 0.9 to 4.5 s-1 and from 4 to 33 s-1 for Mg2+. Parvalbumin equilibrium binding constants of Ca2+ and Mg2+ were also measured in the same temperature range. It makes possible to estimate the rate constants of association of Ca2+ and Mg2+. In the case of Ca2+ the rate of association approaches the diffusion controlled limit.     

Analysis of the Escherichia coli ribosome-ribosomal subunit equilibrium using pressure-induced dissociation

Hydrostatic pressure can be used to perturb the ribosome-ribosomal subunit equilibrium. We have used glutaraldehyde fixation and subsequent sucrose gradient analysis to determine the equilibrium concentrations of Escherichia coli 70 S, 50 S, and 30 S particles at pressures from 1 to 1400 atm. This method is shown to be sufficiently rapid and free of interfering ribosomal aggregation artifacts when performed at Mg2+ concentrations below 8 mM. We show directly that the E. coli ribosome is in equilibrium with its subunits and that the pressure-sensitive reaction is appropriately described by the expression: In Kp = ln K0 + (P delta V/RT), where Kp and K0 are the equilibrium constants at pressure P and 1 atm, respectively, and delta V is the change in molecular volume that occurs during the reaction. The method provides values for K0 under different conditions, and the effects of Mg2+ ion can be readily ascertained. K0 and delta V were also estimated by a method of fitting computer-generated sucrose gradient profiles to experimental profiles. Determination of delta H0, delta S0, and delta V0 at 5 mM Mg2+ are presented. The results are discussed in the context of previous thermodynamic studies of the E. coli ribosome.     

Nigericin-mediated H+, K+ and Na+ transports across vesicular membrane: T-jump studies.

The decay of delta pH across vesicular membranes by nigericin-mediated H+ and metal ion (M+) transports has been studied at 25 degrees C after creating delta pH by temperature jump (T-jump). In these experiments K+ or Na+ were chosen as M+ for the compensating flux. Theoretical expressions derived to analyse these data suggest a method for estimating the intrinsic rate constants for the translocation of nig-H (k1) and for the translocation of nig-M (k2) across membrane, from the pH dependence of the delta pH decay. The following could be inferred from the analysis of data. (a) At pH approximately 7.5 and 250 mM ion concentrations, nigericin-mediated H+ and M+ transport rates are lower in a medium of K+ than in a medium of Na+, although ionophore selectivity of nigericin towards K+ is 25-45-times higher than that towards Na+. However, at lower [M+] (approximately 50 mM) the transport rates are higher in a medium of K+ than in a medium of Na+. Such behaviours can be understood with the help of parameters determined in this work. (b) The intrinsic rate constants k1 and k2 associated with the translocations of nig-H and nig-K or nig-Na across membrane are similar in magnitude. (c) At pH approximately 7.5 translocation of nig-H is the dominant rate-limiting step in a medium containing K+. In contrast with this, at this pH, translocation of nig-M is the dominant rate-limiting step when metal ion is Na+. (d)k1 approximately k2 approximately 6.10(3) s-1 could be estimated at 25 degrees C in vesicles prepared from soyabean phospholipid, and lipid mixtures of 80% phosphatidylcholine (PC) + 20% phosphatidylethanolamine and 92% PC + 8% phosphatidic acid. (e) The apparent dissociation constants of nig-M in vesicles were estimated to be approximately 1.5.10(-3) M for K+ and 6.4.10(-2) M for Na+ (at 50 mM ion concentrations) using approximately 10(-8.45) M for the apparent dissociation constant of nig-H.     

Chloride as allosteric effector of yeast aminopeptidase I

Activation of yeast aminopeptidase I by chloride was studied by kinetic methods. Several effects contributed to overall activity enhancement: At low concentrations of Zn2+ (an essential component of aminopeptidase I) chloride increased the amounts of active enzyme by reducing the cooperativity of metal binding. In addition, substrate turnover was enhanced due to increased kcat and a moderate decrease of Km. At high concentrations of Zn2+ substrate saturation curves were sigmoidal. Under these conditions chloride activated by restoring Michaelis-Menten kinetics of substrate turnover. At the same time, reconstitution of active enzyme from apoprotein and Zn2+ was substantially accelerated and its inactivation due to loss of Zn2+ was retarded. Co2+-Substituted aminopeptidase I, although catalytically active, was much less sensitive to chloride activation. Apparent binding constants for chloride, as estimated from its effects on metal binding and catalysis, respectively, were different. This suggests that two independent activation mechanisms may be operative. Both appear to be mediated by conformational changes of the enzyme protein.     

Estimation of kinetic parameters by progress curve analysis for the synthesis of (R)-mandelonitrile by Prunus amygdalus hydroxynitrile lyase

Consistent sets of kinetic parameters were estimated for the synthesis of (R)-mandelonitrile, catalyzed by Prunus amygdalus hydroxynitrile lyase, at 5 and 25 degrees C and pH 5.5 by progress curve analysis. The rate constants and equilibrium constants of the nonenzymatic reaction were determined separately to reduce the number of parameters to be estimated simultaneously. At a lower temperature the equilibrium is much more favorable and the formation of rac-mandelonitrile by the nonenzymatic reaction is suppressed. The estimated kinetic parameters were used to identify that the rate determining step in the catalytic cycle is the release of (R)-mandelonitrile from the ternary complex.     

Effect of solvent, pressure and temperature on reaction rates of the multiheme hydroxylamine oxidoreductase. Evidence for conformational change

Hydroxylamine oxidoreductase (HAO) of the ammonia-oxidizing bacterium Nitrosomonas catalyzes the oxidation: NH2OH + H2O----HNO2 + 2e- + 2 H+. The heme-like chromophore P460 is part of a site which binds substrate, extracts electrons and then passes them to the many c hemes of the enzyme. Reduction of the c hemes by hydroxylamine is biphasic with apparent first-order rate constants k1 and k2. CO binds to ferrous P460 with apparent first-order rate constants, k1,CO. In this work we have measured the binding of CO to ferrous P460 of hydroxylamine oxidoreductase and the reduction by substrate of some of the 24 c hemes of the ferric enzyme. These reactions have been studied in water and 40% ethylene glycol, at temperatures ranging from -15 degrees C to 20.7 degrees C and at hydrostatic pressures ranging over 0.1-80 MPa. From the measurements, thermodynamic parameters delta V+ (activation volume), delta G+, delta H+, and delta S+ have been calculated. CO binding. Binding of CO to ferrous P460 was similar to the binding of CO to ferrous horseradish peroxidase. The change of solvent had only a limited effect on delta V+ (-30 ml.mol-1), delta G+, delta H+ or delta S+ and did not cause an inflection in the Arrhenius plot or downward displacement of the linear relationship between ln k1,CO and P at a critical temperature. Binding was exothermic at high temperatures. The response of the binding of CO to solvent, temperature and pressure suggested that the CO binding site had little access to solvent and was not susceptible to change in protein conformation. Fast phase of reduction of c hemes. Changing the solvent from water to 40% ethylene glycol resulted in a decrease from 90% to 50% in the relative number of c hemes reduced during the fast phase, an increase in activation volume from -3.6 ml.mol-1 to 57 ml.mol-1 and changes in other thermodynamic parameters. The activation volume increased with decreasing temperature. The Arrhenius plot had a downward inflection at about 0 degrees C and, in water or ethylene glycol, the linear dependence of ln k1 on P was displaced downwards as the temperature changed from 3.5 degrees C to -15 degrees C. Slow phase of reduction of c hemes. Changing the solvent from water to 40% ethylene glycol resulted in an increase in the relative number of c hemes reduced during the slow phase from 10% to 50%. The activation volume, which was not measurable in water because of the low absorbance change, was -30 ml.mol-1 in ethylene glycol. The activation volume increased with increasing temperature.(ABSTRACT TRUNCATED AT 400 WORDS)     

Kinetic intermediates in prothrombin activation. Bovine prethrombin 1 conversion to thrombin by factor X

Two pathways are possible during the proteolytic formation of alpha-thrombin (alpha-IIa) from prothrombin (II) or prethrombin 1 (P1). One of the pathways, with prethrombin 2 or prethrombin 2 associated with fragment 2 (P2F2) as intermediates, has long been known to exist when activation is catalyzed by Factor Xa (Xa) alone. The second pathway, with meizothrombin or meizothrombin (des fragment 1) (MzIIa(-F1)) as intermediate, has been shown to exist when Factor Va and phospholipids are present with Xa. Until now, MzIIa(-F1) has not been detected in reactions catalyzed by Xa alone. In this study, we demonstrate that P1 activation by Xa alone occurs via both pathways, and we provide rate constants and kinetic equations for calculating the relative contributions of each of the pathways to the formation of alpha-IIa by Xa. Investigation of the initial rates of proteolytic cleavage of P2F2 and P1 by Xa alone indicated first-order dependence on substrate concentration with no evidence of saturation of Xa with either substrate at concentrations as high as 200 microM. Apparent second-order rate constants (kc/Km) of 113 +/- 9 M-1 s-1 for the formation of thrombin from P2F2 and 1,410 +/- 19 M-1 s-1 for the disappearance of P1 were determined at pH 7.5, 25 degrees C, 10 mM CaCl2, 0.15 M ionic strength. A two-step sequential first-order pathway employing these rate constants for thrombin activity production from P1 via P2F2 could not, however, account for the quantity of thrombin that was produced during the early stages of P1 activation. Addition of a parallel first-order reaction to produce thrombin activity from P1 independently of P2F2, tentatively identified as the formation of MzIIa(-F1), yielded progress curves in quantitative agreement with the experimental data. kc/Km for the parallel reaction was estimated to be 98 +/- 10 M-1 s-1. Independent determination of the second-order rate constant for the cleavage of isolated MzIIa (-F1), 15,000 +/- 420 M-1 s-1, indicated that MzIIa(-F1) could meet the kinetic requirements for an intermediate in the parallel activation pathway. The transient formation of MzIIa (-F1), as well as the generation of alpha-IIa, was directly demonstrated during activation of P1 by active site-affinity labeling of the reaction products with a biotin derivative of D-Phe-Pro-Arg chloromethyl ketone and visualization by semiquantitative Western blotting.(ABSTRACT TRUNCATED AT 400 WORDS)     

Allosteric regulation of rat testis mitochondrial aldehyde dehydrogenase by capronaldehyde and magnesium ion.

1. The influence of Mg2+ on the kinetic behaviour of mitochondrial aldehyde dehydrogenase from rat testis has been investigated using capronaldehyde as substrate. 2. The kinetic data, obtained by numerical analysis of the progress curves of aldehyde oxidation, were fitted to a modified version of the Monod-Wyman-Changeux model and the fitting procedure resulted in a good correspondence between theoretical and experimental reaction rates over a wide range of capronaldehyde and Mg2+ concentrations. 3. According to the model, the tetrameric enzyme is in equilibrium between two conformational states R and T which display comparable affinities for capronaldehyde (the dissociation constants are 0.17 and 0.3 microM, respectively), but different catalytic power (VT = 2VR). The T state can bind with lower affinity a second molecule of aldehyde (K = 2.5 microM). 4. Mg2+ stabilizes the T state (the dissociation constants for the R and T states are 2.2 and 0.12 mM, respectively) and acts as a strong activator of the R state, but as a weak inhibitor of the T state. In the absence of substrates and Mg2+, the R<-->T equilibrium favors the R state ([T]/[R] = 0.16). 5. The model is able to predict the kinetic behaviour also when the NAD+ concentrations are not saturating and when inhibitory effects by NADH are taken into account.     

A kinetic study of cyclic adenosine 3':5'-monophosphate binding and mode of activation of protein kinase from Drosophila melanogaster embryos.

Cyclic AMP-dependent protein kinase and its regulatory subunit were isolated from Drosophila melanogaster embryos. The profiles of cyclic AMP binding by these proteins were significantly different. In order to explain such a difference and to find the mode of enzyme activation by cyclic AMP, a kinetic study of cyclic AMP binding was carried out. First, the association rate constant k1 and dissociation rate constant k-1 in the cyclic AMP-regulatory subunit interaction at 0 degrees C were estimated to be 2.3 X 10(6)M-1s-1 and 1.1 X 10(-3)s-1, respectively. Secondly, the three possible modes of enzyme activation by cyclic AMP were mathematically considered and could be described by a unique formula: r=APt + BQt (A + B=1) in which the parameters A, B, P, and Q are equivalent to rate constants in the sense that the rate constants are simply expressed by these parameters. Thirdly, the values of the parameters and subsequently the values of rate constants involved in the possible mechanisms were evaluated using a curve-fitting technique and compared with experimental observation. It was then found that the following mechanism was the only one which fitted the experimental observations. Namely, RC + L k3 equilibrium k-3 LRC k4 equilibrium k-4 RL + C where R, C, and L represent the regulatory and catalytic subunits and cyclic AMP as a ligand. Thus, our results indicate that in the presence of cyclic AMP the active enzyme (C) is released from a ternary intermediate which is the primary product of the cyclic AMP-holoenzyme interaction. The estimated values of the rate constants are: k3=3.5 X 10(6)M-1s-1;k-3=7.3 X 10(-1)s-1;and k4=3.8 X 10(-2)s. These estimates indicate that the reaction LRC leads to RL + C is relatively slow and limits the rate of the overall reaction. By comparing k-3 and k4, it is apparent that a large part of newly formed ternary intermediate reverts to the holoenzyme.     

A study of the kinetics of the muscarinic effect on phosphatidylinositol and phosphatidic acid metabolism in rat brain synaptosomes.

The uptake of [32P]phosphate into phosphatidylinositol and phosphatidate was measured in synaptosomes incubated in Krebs-Ringer bicarbonate buffer, pH7.4. The apparent dissociation constants for acetylcholine and carbamoylcholine was estimated from the increase in 32P uptake caused by these agents. These apparent constants were similar for both phosphatidylinositol and phosphatidate and were 2.7 +/- 0.5 MICROmeter for acetylcholine and 12 +/- 2 micrometer for carbamoylcholine when Ca2+ concentration was 0.75 mM. Under the same conditions the inhibition of the carbamoylcholine-induced increase in 32P uptake, caused by atropine, is consistent with atropine being a competitive inhibitor, with an apparent inhibition constant of 0.35 +/- 0.05 micrometer. The apparent constants were dependent on the Ca2+ concentration, and were greater in 2.54 mM-Ca2+. The former values for the kinetic constants are similar to the muscarinic-receptor dissociation constant, which indicates that the binding of the agonist to the receptor may be rate-limiting in this series of reactions when the Ca2+ concentration is 0.75 mM.     

Comparison of the binding of Ca2+ and Mn2+ to bovine alpha-lactalbumin and equine lysozyme

The enthalpy change of the binding of Ca2+ and Mn2+ to equine lysozyme was measured at 25 degrees C and pH 7.5 by batch microcalorimetry: delta H degrees Ca2+ = -76 +/- 5 kJ mol-1, delta H degrees Mn2+ = -21 +/- 10 kJ mol-1. Binding constants, log KCa2+ = 6.5 +/- 0.2 and log KMn2+ = 4.1 +/- 0.5, were calculated from the calorimetric data. Therefore, delta S degrees Ca2+ = -131 +/- 20 JK-1 mol-1 and delta S degrees Mn2+ = 8 +/- 44 JK-1 mol-1. Removal of Ca2+ induces small but significant changes in the circular dichroism spectrum, indicating the existence of a partially unfolded apo-conformation, comparable with, but different from, the apo-conformation of bovine alpha-lactalbumin.