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1.
The rate constant of the electron self-exchange reaction, which proceeds via the outer sphere mechanism, k11, of a redox couple, reflects the basic tendency of the reaction to participate in redox processes. Often k11 is derived from the rate constant of a redox reaction, k12, by applying the Marcus cross relation: k12 = (k11k22K12f12)1/2W12. This derivation is based on the assumption that the products of the cross reaction are formed in their ground states. However, the k11 values obtained by this method for several redox systems, e.g. for [Co(NH3)6]3+/2+, Cu2+/+(aq) and Eu3+/2+(aq), depend strongly on the redox reaction studied for its derivation. It is proposed that these discrepancies are due to the formation, in some of these reactions, of products in vibrationally excited states and/or as isomers of the final products. Thus, the lowest value of k11 obtained experimentally is either correct or an upper limit for the correct value if correct k22 are used. 相似文献
2.
Jun Soo Kim 《Biophysical journal》2010,98(6):951-958
The effect of macromolecular crowding on the binding of ligands to a receptor near membranes is studied using Brownian dynamics simulations. The receptor is modeled as a reactive patch on a hard surface and the ligands and crowding agents are modeled as spheres that interact via a steep repulsive interaction potential. When a ligand collides with the patch, it reacts with probability prxn. The association rate constant (k∞) can be decomposed into contributions from diffusion-limited (kD) and reaction-limited (kR) rates, i.e., 1/k∞ = 1/kD + 1/kR. The simulations show that kD is a nonmonotonic function of the volume fraction of crowding agents for receptors of small sizes. kR is always an increasing function of the volume fraction of crowding agents, and the association rate constant k∞ determined from both contributions has a qualitatively different dependence on the macromolecular crowding for high and low values of the reaction probability prxn. The simulation results are used to predict the velocity of the membrane protrusion driven by actin filament elongation. Based on the simple model where the protrusive force on the membrane is generated by the intercalation of actin monomers between the membrane and actin filament ends, we predict that crowding increases the local concentration of actin monomers near the filament ends and hence accelerates the membrane protrusion. 相似文献
3.
I.Kofi Adzamli Henry Ong Wah Kim A.Geoffrey Sykes George V. Buxton 《Journal of inorganic biochemistry》1982,16(4):311-317
The reaction of parsley 2Fe-2S ferredoxin in the normal oxidized state with eaq? generated by pulse radiolysis techniques has been studied at ~25°C, pH 7–8, I = 0.10 M (NaClO4). Rate constants ke (eaq? decay) and kp (protein absorbance change) are the same, second-order rate constant 9.7 × 109 M?1 sec?1. The reaction exhibits close to 100% efficiency. With 8Fe-8S ferredoxin from Clostridium pasteurianum under identical conditions it now appears that kp (although sometimes significantly smaller) is equal to ke. Varying efficiencies are also observed with this protein depending on the batch used. The reasons for such variable behavior are not fully understood. With oxidized and reduced forms of Chromatium v. high-potential iron-sulfur protein (HIPIP), ke and kp are essentially the same, but the highest efficiency observed is only ~50%. The prevailing pattern is therefore that rate constants ke and kp are generally in step for proteins having a single (or identical) active site(s). When the active site is buried as with HIPIP the efficiency of the reaction appears to decrease. 相似文献
4.
Saturation kinetics are observed in the inhibition of cobalt carboxypeptidase A by the chelating agent 1,10-phenanthroline. The association constant K1 for the formation of the enzyme-metal-ligand ternary complex and k2, the rate of breakup of the ternary complex, have been obtained. A mechanism is proposed to account for the pH profile of the reaction which, in conjunction with K1, permits the calculation of the individual rate constants k1, K?1, k2, k3. The magnitude of the rate constant k1 suggests that cobalt(II) in CoCPA is five-coordinate. Similar but less extensive studies on inhibition by 2,2′-bipyridyl and 8-hydroquinoline-5-sulfonic acid have also been carried out 相似文献
5.
《Bioorganic chemistry》1987,15(2):167-182
The kinetics of the Ni(II)-catalyzed ester hydrolysis of O-acetyl-2-pyridine-carboxaldoxime, O-acetyl-2-acetylpyridineketoxime, and O-acetyl-6-carboxy-2-pyridine-carboxaldoxime are measured and the values of various kcat parameters are calculated for reaction paths involving one metal ion (kcatW and kcatOH) and two metal ions kcatA and kcatB). Examination of the kinetic data reveals that the kcatW and kcatOH paths for the Ni(II)-catalyzed reactions involve the same mechanism as those for the previously reported Cu(II)-catalyzed reactions. For the kcatA and kcatB paths, the mechanism involving binuclear Ni(II) ions is preferred by analogy with the previously reported Zn(II)-catalyzed reactions. Comparison of kcatOH values for the Cu(II)- and Ni(II)-catalyzed hydrolysis of 1–3 indicates that markedly different steric effects are exerted by the substituents of 2 and 3 on the catalytic behavior of the two metal ions. This is explained in terms of differences in the fit of the metal ions in the metal complexes of 1–3. Present results demonstrate that slight changes in the geometry around the central metal atom can affect the catalytic outcome significantly. The implications of the present results on metal substitution in metalloenzymes are also discussed. 相似文献
6.
Phthalic anhydride has been detected spectrophotometrically in the hydrolysis of phthalamic acid and N-phenylphthalamic acid in solutions which were made up to 5 M with sodium perchlorate. In solutions of lower ionic strength the variation of kobs with acid concentration follows the equation, kobs=(k1 + k2 [H3O+])/(1 + Ka/[H3O+]), and the values of k1 and k2 are both enhanced. The p values for the variation of k1 and k2 with the aryl group for the hydrolysis of N-arylphthalamic acids are ?1.23 and ?0.94. A mechanism involving nucleophilic catalysis by the carboxyl group in which breakdown of the tetrahedral intermediate is rate-limiting was proposed. 相似文献
7.
Charge-pulse relaxation experiments of valinomycin-mediated Rb+ transport have been carried out in order to study the influence of membrane structure on carrier kinetics. From the experimental data the rate constants of association (kR) and dissociation (kD) of the ion-carrier complex as well as the rate constants of translocation of the complex (kMS) and of the free carrier (kS) could be obtained. The composition of the planar bilayer membrane was varied in a wide range. In a first series of experiments, membranes made from glycerolmonooleate dissolved in different n-alkanes (n-decane to n-hexadecane), as well as solvent-free membranes made from the same lipid by the Montal-Mueller technique were studied. The translocation rate constants kS and kMS were found to differ by less than a factor of two in the membranes of different solvent content. Much larger changes of the rate constants were observed if the structure of the fatty acid residue was varied. For instance, an increase in the number of double bonds in the C20 fatty acid from one to four resulted in an increase of kS by a factor of seven and in an increase of kMS by a factor of twenty-four. The stability constant K = kR/kD of the ion-carrier complex as well as the translocation rate constants kS and kMS were found to depend strongly on the nature of the polar headgroup of the lipid. The incorporation of cholesterol into glycerolmonooleate membranes reduced kR, kMS and kS up to seven-fold. 相似文献
8.
Manfred J. Sippl 《Journal of molecular biology》1982,156(2):359-388
The development of prediction schemes and the search for evolutionary relationships amongst proteins require reliable methods for the comparison of polypeptide structures. It is shown that methods which attempt to describe structural similarities by a single value generally do not yield reliable estimates of the relatedness of two conformations. A new method is reported, called the Dk procedure, which yields a spectrum of deviations between two structures. Each particular Dk value is a measure of the similarity of the diagonals of the distance matrices of the compared conformations, k being the distance of the diagonals relative to the main diagonal.The method has the following features. (1) Dk is independent of chain length; (2) the method yields the relatedness of two conformations in terms of different structural levels; (3) Dk is a high-speed algorithm; (4) the Dk deviations of random structures from any particular conformation are predictable.The following applications are reported. (1) The success of both secondary and tertiary structure predictions are measured in terms of a reliable quality index. (2) The route of a conformation during simulation studies is followed on different structural levels, which exhibit the characteristics of the simulation. (3) The significance of hypotheses on protein folding subject to prediction schemes can be established. (4) A priori information (fixing pieces of secondary structure derived from X-ray investigations during prediction) is extractable from the predicted structures. (5) The evolutionary relatedness of two nucleotide binding proteins is established.The simplicity and speed of the Dk procedure allow its implementation even on minicomputers. 相似文献
9.
Despite methylmercury (MeHg) production in boreal wetlands being a research focus for decades, little is known about factors in control of methylation and demethylation rates and the effect of wetland type. This is the first study reporting potential Hg methylation (k m ) and MeHg demethylation rate constants (k d ) in boreal wetland soils. Seven wetlands situated in northern and southern Sweden were characterized by climatic parameters, nutrient status (e.g. type of vegetation, pH, C/N ratio, specific UV-absorption), iron and sulfur biogeochemistry. Based on nutrient status, the wetlands were divided into three groups; (I) three northern, nutrient poor fens, (II) a nutrient gradient ranging from an ombrotrophic bog to a fen with intermediate nutrient status, and (III) southern, more nutrient rich sites including two mesotrophic wetlands and one alder (Alnus) forest swamp. The k m /k d ratio in general followed %MeHg in soil and both measures were highest at the fen site with intermediate nutrient status. Northern nutrient poor fens and the ombrotrophic bog showed intermediate values of %MeHg and k m /k d . The two mesotrophic wetlands showed the lowest %MeHg and k m /k d , whereas the alder swamp had high k m and k d , resulting in an intermediate k m /k d and %MeHg. Molybdate addition experiments suggest that net MeHg production was mainly caused by the activity of sulfate reducing bacteria. A comparison with other studies, show that k m and %MeHg in boreal freshwater wetlands in general are higher than in other environments. Our results support previous suggestions that the highest MeHg net production in boreal landscapes is to be found in fens with an intermediate nutrient status. 相似文献
10.
The reversibility of adenosine triphosphate cleavage by myosin 总被引:12,自引:12,他引:0
For the simplest kinetic model the reverse rate constants (k−1 and k−2) associated with ATP binding and cleavage on purified heavy meromyosin and heavy meromyosin subfragment 1 from rabbit skeletal muscle in the presence of 5mm-MgCl2, 50mm-KCl and 20mm-Tris–HCl buffer at pH8.0 and 22°C are: k−1<0.02s−1 and k−1=16s−1. Apparently, higher values of k−1 and k−2 are found with less-purified protein preparations. The values of k−1 and k−2 satisfy conditions required by previous 18O-incorporation studies of H218O into the Pi moiety on ATP hydrolysis and suggest that the cleavage step does involve hydrolysis of ATP or formation of an adduct between ATP and water. The equilibrium constant for the cleavage step at the myosin active site is 9. If the cycle of events during muscle contraction is described by the model proposed by Lymn & Taylor (1971), the fact that there is only a small negative standard free-energy change for the cleavage step is advantageous for efficient chemical to mechanical energy exchange during muscle contraction. 相似文献
11.
Yuan-Yuan Hu Da-Yong Fan Pasquale Losciale Wah Soon Chow Wang-Feng Zhang 《Photosynthesis research》2013,117(1-3):517-528
Using radioactively labelled amino acids to investigate repair of photoinactivated photosystem II (PS II) gives only a relative rate of repair, while using chlorophyll fluorescence parameters yields a repair rate coefficient for an undefined, variable location within the leaf tissue. Here, we report on a whole-tissue determination of the rate coefficient of photoinactivation k i , and that of repair k r in cotton leaf discs. The method assays functional PS II via a P700 kinetics area associated with PS I, as induced by a single-turnover, saturating flash superimposed on continuous background far-red light. The P700 kinetics area, directly proportional to the oxygen yield per single-turnover, saturating flash, was used to obtain both k i and k r . The value of k i , directly proportional to irradiance, was slightly higher when CO2 diffusion into the abaxial surface (richer in stomata) was blocked by contact with water. The value of k r , sizable in darkness, changed in the light depending on which surface was blocked by contact with water. When the abaxial surface was blocked, k r first peaked at moderate irradiance and then decreased at high irradiance. When the adaxial surface was blocked, k r first increased at low irradiance, then plateaued, before increasing markedly at high irradiance. At the highest irradiance, k r differed by an order of magnitude between the two orientations, attributable to different extents of oxidative stress affecting repair (Nishiyama et al., EMBO J 20: 5587–5594, 2001). The method is a whole-tissue, convenient determination of the rate coefficient of photoinactivation k i and that of repair k r . 相似文献
12.
The initial rates and steady-state values of proton uptake by broken chloroplasts have been measured as functions of light intensity at various concentrations of chlorophyll, pyocyanine, supporting electrolyte, buffer, as well as pH and temperature. Kinetic analysis of the data shows that the rate of decay of proton gradient due to backward leakage depends on light intensity. Under steady illumination, the decay constant kL is equal to kD + mR0, where R0 is the initial rate of proton uptake which is a function of light intensity, kD is the decay constant in the dark and m is a parameter which is independent of light intensity. Treatment of chloroplasts with lysolecithin, neutral detergent, 2,4-dinitrophenol, or valinomycin in the presence of K+ increases kD without affecting m. Treatment with N,N′-dicyclohexylcarbodiimide or adenylyl imidodiphosphate under appropriate conditions decreases m without affecting kD. Treatment with glutaraldehyde makes kL independent of light intensity and hence m = 0. These results suggest that the light-dependent part (mR0) of kL is due to leakage of protons through the coupling factor (CF1-CF0) complex which can open or close depending on light intensity and that the light-independent part (kD) of the decay constant kL is due to proton leakage elsewhere. 相似文献
13.
《Inorganica chimica acta》1988,150(1):81-100
The (NH3)5CoOC(NH2)23+ ion is consumed in water according to the rate law k(obs.) = k1 + k2[OH−], where k1 = 4.0 × 10−5 s−1 and k2 = 14.2 M−1 s−1 (0–0.1 M [OH−];μ = 1.1 M, NaClO4, 25 °C). A hitherto unrecognized intramolecular O- to N- linkage isomerization reaction has been detected. In strongly acid solution only aquation to (NH3)5CoOH23+ is observed, but in 0.1–1.0 M [OH−], 7% of the directly formed products is the urea-N complex (NH3)5CoNHCONH22+ which has been isolated. In the neutral pH region a much greater proportion (25%) of the products is the urea-N species. These results are interpreted in terms of an urea-O to urea-N linkage isomerization reaction competing with hydrolysis for both spontaneous (k1) and base-catalyzed (k2) pathways; the rearrangement is not observed in strongly acidic solution (pH ⩽ 1) because the protonated N-bonded isomer (pK′a ≈ 3) is unstable with respect to the O-bonded form. The appearance of the isomerization pathway as the pH is raised in the 0–6 region is commensurate with a rate increase which cannot be attributed to a contribution from the base catalysis term k2[OH−]. It is argued that this observation establishes, for the spontaneous pathway, that hydrolysis and linkage isomerization are separate reaction pathways — there is no common intermediate. The product distribution and rate data lead to the complete rate law, k(obs.) = k1 + k2[OH−] = (ks + kON) + (kOH + k′ON) [OH−] for the reactions of the O-bonded isomers, where ks, kOH are the specific rates for hydrolysis, and kON, k′ON are the specific rates for O- to N-linkage isomerization, by spontaneous and base-catalyzed pathways respectively; kON = 1.3 × 10−5 s−1 and k′ON = 1.1 M−1 s−1 (μ = 1.0 M, NaClO4, 25 °C). The O- to N- linkage isomerization has been observed also for complexes of N-methylurea, N,N-dimethylurea and N-phenylurea, but not for the N,N′-dimethylurea species. There is an approximately statistical relationship among the data for −NH2 capture (versus H2O), while −NHR and −NR2 do not compete with water as nucleophiles for Co(III) in either the spontaneous or base-catalyzed hydrolysis processes. For each urea-O complex, O- to N-isomerization is a more significant parallel reaction in the spontaneous as opposed to the base-catalyzed pathway. This is interpreted as being indicative of more associative character in the spontaneous route to products, a conclusion supported by other evidence. Some activation parameter data have been recorded and the effect of the N-substitution on the rates of solvolysis (H2O, Me2SO) is discussed. The urea-N complexes have been isolated as their deprotonated forms, [(NH3)5CoNHCONRR′](ClO4)2·xH2O (R,R′ = H, CH3). They are kinetically inert in neutral to basic solution but in acid they protonate (H2O, pK′a 2–3; μ = 1.0 M, 25 °C) and then isomerize rapidly back to their O-bonded forms. Some solvolysis accompanies this N- to O-rearrangement in H2O and Me2SO. Specific rates and activation parameters are reported. The kinetic data follow a rate law of the form kNO(obs.) = (k + kNO)[H+]/(K′a + [H+]) and the active species in the reaction is the protonated form; k, kNO are the specific rates for hydrolysis and isomerization, respectively. Proton NMR data establish that the site of protonation (in Me2SO) is the cobalt-bound nitrogen atom. For the unsubstituted urea species (NH3)5CoNH2CONH23+, diastereotopic exo-NH2 protons arising from restricted rotation about the CN bond are observed. The relevance to the mechanism of the linkage isomerization process is considered. 13C and 1H NMR and electronic absorption spectral data are presented, and distinctions between linkage isomers and the solution structures (electronic and conformational) are discussed. The urea-N/urea-O complex equilibrium is governed by the relation K′NO(obs.) = K′NO[H+]/[H+](K′a), where K′NO is the equilibrium constant = [(NH35Co(urea-O)3+]/[(NH3)5Co(urea-N)3+]. Values for K′NO(=kNO/kON = 260 and pK′a ≈ 3 for the NH2CONH2 system are consistent with the stability of the N-isomer in feebly acidic to basic solution (e.g. pH 6, K′NO(obs.) = 2.6 × 10−2) and instability in acid solution (e.g. pH 1, K′NO(obs.) = 240). The equilibrium data for this and other urea complexes of (NH3)5Co(III) are contrasted with the result for the analogous Rh(III)NH2CONH2 system K′NO ≈ 1). 相似文献
14.
Effects of Replacement of External Sodium Chloride with Sucrose on Membrane Currents of the Squid Giant Axon 总被引:3,自引:0,他引:3
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It was observed that a reduction of the sodium chloride concentration in the external solution bathing a squid giant axon by replacement with sucrose resulted in marked decreases in the peak inward and steady-state outward currents through the axon membrane following a step decrease in membrane potential. These effects are quantitatively acounted for by the increase in series resistance resulting from the decreased conductivity of the sea water and the assumption that the sodium current obeys a relation of the form I = k1C1 - k2C2 where C1, C2 are internal and external ion activities and k1, k2 are independent of concentration. It is concluded that the potassium ion current is independent of the sodium concentration. That the inward current is carried by sodium ions has been confirmed. The electrical potential (or barrier height) profile in the membrane which drives sodium ions appears to be independent of sodium ion concentration or current. A specific effect of the sucrose on hyperpolarizing currents was observed and noted but not investigated in detail. 相似文献
15.
《生物化学与生物物理学报:生物膜》1986,862(1):199-204
The endocytotic rate constant, ke, originally described for the quantification growth factor by fibroblasts (Wiley, H.S. and Cunningham, D.D.(1982) J. Biol. Chem. 257, 4222–4229) has been adapted to measure receptor-mediated endocytosis of asialoglycoproteins by hepatocytes. A ke value of 0.21 min−1 was obtained for the internalisation of β-d-galactosyl bovine serum albumin by freshly isolated hepatocytes. The addition of ethaniol to the incubation medium had a biphasic effect on e. The value of ke was increased by up to 30% by low concentrations of ethanol, whereas higher concentrations progressively decreased ke and in 500 mM ethanol the ke value was 0.1 min−1. The amount of ligand bound to the cell surface was independent of the extracellular concentration of ethanol and the changes in ke were exclusively due to changes in the amount of internalised ligand. There was a progressive decrease in the value of ke in hepatocytes prepared from rats that were maintained on an ethanol-impregnated liquid diet for up to 20 days. The decrease was already apparent by day 2 when blood alcohol levels were only 50 mg%, indicating that the effects of chronic alcoholism on endocytosis are manifested at an early stage. 相似文献
16.
Emmanuel T. Rakitzis 《Biophysical chemistry》1983,18(2):133-137
A mathematical treatment of protein modification reactions is presented, and it is shown thai in these cases protein modification is described by a summation of exponential functions of reaction time, the number of exponentials being equal to the number of modified protein species. It is shown that in cases of protein modification cooperativity, there is a strict dependence of the coefficients of the multiexponential modification equation on the constants of the same equation. The conditions necessary for a reduction of a multiexponential protein modification equation to one of a summation of two exponentials only are examined. The possible formulae for the coefficients of a two-exponential-summation equation, used to describe the modification of protein models with two, three or four modifiable residues (as well as some aspects of models with five and six modifiable residues) per protein molecule are derived. It is seen that the number of such coefficients is severely limited. The most frequently obtained formula for the lower stoichiomelric coefficient of a 'wo-exponential-summation equation is Aka/(ka-kb). where kb and kb are the constants of the two exponentials of the equation, and A is a constant. The value most frequently arrived at for A is (n?1)/n, where n is the number of modifiable residues per protein molecule, while values such as 1/n, or a/n (where a is an integer, and also where a < n) are also possible. In most of the cooperative protein modification models worked out, ka is identical with kn, viz., ka is identical with the rate constant for the first stoichiometric protein modification. 相似文献
17.
Protrusions are deformations that form at the surface of living cells during biological activities such as cell migration. Using combined optical tweezers and fluorescent microscopy, we quantified the mechanical properties of protrusions in adherent human embryonic kidney cells in response to application of an external force at the cell surface. The mechanical properties of protrusions were analyzed by obtaining the associated force-length plots during protrusion formation, and force relaxation at constant length. Protrusion mechanics were interpretable by a standard linear solid (Kelvin) model, consisting of two stiffness parameters, k
0 and k
1 (with k
0>k
1), and a viscous coefficient. While both stiffness parameters contribute to the time-dependant mechanical behavior of the protrusions, k
0 and k
1 in particular dominated the early and late stages of the protrusion formation and elongation process, respectively. Lowering the membrane cholesterol content by 25% increased the k
0 stiffness by 74%, and shortened the protrusion length by almost half. Enhancement of membrane cholesterol content by nearly two-fold increased the protrusion length by 30%, and decreased the k
0 stiffness by nearly two-and-half-fold as compared with control cells. Cytoskeleton integrity was found to make a major contribution to protrusion mechanics as evidenced by the effects of F-actin disruption on the resulting mechanical parameters. Viscoelastic behavior of protrusions was further characterized by hysteresis and force relaxation after formation. The results of this study elucidate the coordination of plasma membrane composition and cytoskeleton during protrusion formation. 相似文献
18.
Paolo Ascenzi Alessandra di Masi Marco Mattu Massimo Coletta 《Biochemical and biophysical research communications》2010,393(2):196-3424
Cyanide binds to ferric heme-proteins with a very high affinity, reflecting the very low dissociation rate constant (koff). Since no techniques are available to estimate koff, we report herewith a method to determine koff based on the irreversible reductive nitrosylation reaction to trap ferric myoglobin (Mb(III)). The koff value for cyanide dissociation from ferric cyanide horse heart myoglobin (Mb(III)-cyanide) was determined at pH 9.2 and 20.0 °C. Mixing Mb(III)-cyanide and NO solutions brings about absorption spectral changes reflecting the disappearance of Mb(III)-cyanide with the concomitant formation of ferrous nitrosylated Mb. Since kinetics of reductive nitrosylation of Mb(III) is much faster than Mb(III)-cyanide dissociation, the koff value, representing the rate-limiting step, can be directly determined. The koff value obtained experimentally matches very well to that calculated from values of the second-order rate constant (kon) and of the dissociation equilibrium constant (K) for cyanide binding to Mb(III) (koff = kon × K). 相似文献
19.
The concentration-dependence of the diffusion and sedimentation coefficients (kD and ks, respectively) of a protein can be used to determine the second virial coefficient (B2), a parameter valuable in predicting protein-protein interactions. Accurate measurement of B2 under physiologically and pharmaceutically relevant conditions, however, requires independent measurement of kD and ks via orthogonal techniques. We demonstrate this by utilizing sedimentation velocity (SV) and dynamic light scattering (DLS) to analyze solutions of hen-egg white lysozyme (HEWL) and a monoclonal antibody (mAb1) in different salt solutions. The accuracy of the SV-DLS method was established by comparing measured and literature B2 values for HEWL. In contrast to the assumptions necessary for determining kD and ks via SV alone, kD and ks were of comparable magnitudes, and solution conditions were noted for both HEWL and mAb1 under which 1), kD and ks assumed opposite signs; and 2), kD ≥ ks. Further, we demonstrate the utility of kD and ks as qualitative predictors of protein aggregation through agitation and accelerated stability studies. Aggregation of mAb1 correlated well with B2, kD, and ks, thus establishing the potential for kD to serve as a high-throughput predictor of protein aggregation. 相似文献
20.
Cerebral microvessels contain a beta 2-adrenergic receptor 总被引:1,自引:0,他引:1
J A Nathanson 《Life sciences》1980,26(21):1793-1799
Cerebral microvessels isolated from cat forebrain contain a specific β-adrenergic-sensitive adenylate cyclase. Among various compounds tested, the most potent activator of enzyme activity is isoproterenol (ka = 1.4 × 10?7M), followed in order by epinephrine (ka= 1.5 × 10?6M), norepinephrine (ka= 1.4 × 10?5M) and phenylephrine (ka> 3 × 10?4M). Isoproterenol-stimulated enzyme activity is blocked by propranolol (ki= 2.4 × 10?9M, IPS 339 (ki= 4 × 10?9M), H35/25 (ki = 1.2 × 10?7M), atenolol (ki= 5.9 × 10?6M) and practolol (ki= 1.8 × 10?5M). These agonist and antagonist properties are quite similar to those demonstrated by β2-adrenergic receptors and β2-stimulated adenylate cyclase present in other tissues and indicate that the majority of adenylate cyclase-associated adrenergic receptors in cerebral microvessels are β2. The findings are relevant to physiological studies of cerebral blood flow and vascular permeability. 相似文献