1,721,062 research outputs found
The interaction between full and partial inhibitors acting on a single enzyme: a theoretical analysis
No full textA theoretical analysis has been made of multiple inhibition systems involving a full and a partial inhibitor. This analysis applies to single- and multisubstrate enzyme systems obeying Michaelis-Menten kinetics. It has been shown that a plot of the reciprocal of the enzyme velocity versus the concentration of the full inhibitor, at constant substrate concentration, is linear in either the presence or the absence of a fixed level of the partial inhibitor. If the slope of the plot is increased or unaltered in the presence of a fixed concentration of the partial inhibitor, the two inhibitors are mutually nonexclusive. If the slope of the plot is decreased, the two inhibitors may be either mutually exclusive or nonexclusive. When a decrease in slope is observed, mutual exclusivity can be distinguished from nonexclusivity by the use of secondary plots based on the effect of the partial inhibitor on the slope or the abscissal intercept of the primary plot. The rules proposed for distinguishing mutually exclusive from nonexclusive inhibitors hold irrespective of the type of inhibition (competitive, noncompetitive, uncompetitive, mixed), so that a knowledge of the kinetic nature of the inhibitors is not required. The results of such an analysis are also discussed in terms of summation, antagonism, and synergism between inhibitors. It has been pointed out that independent inhibitor binding does not necessarily result in independent inhibitor effects, and the conditions necessary for observation of independent inhibitory effects have been defined
Anion activation of dopamine beta-hydroxylase: a kinetic model.
No full text1. A theoretical analysis has been made of the mechanism of anion activation of dopamine beta-hydroxylase on the basis of accumulated experimental data. A model is presented that accounts for the numerous different effects of activating anions on the enzyme kinetics. This model has a general validity, since it holds for any of the kinetic mechanisms thus far proposed for dopamine beta-hydroxylase. 2. It has been shown that the results of this analysis have direct implications for the experimental conditions to be used in the study of the dopamine beta-hydroxylase reaction. 3. The present analysis has proved that, under appropriate assumptions, theoretical treatment of nonessential activation, so far limited to the single-substrate case, can be easily extended to steady-state multireactant enzymes
A simple method for discriminating between full and partial inhibitors
No full textA method is described which provides a more straightforward and reliable discrimination between full and partial inhibitors than do other available procedures. Its advantages are demonstrated by analysis of actual inhibition data. The proposed method proves in certain cases convenient also for the determination of inhibition constants
Mechanism of inhibition of sodium- and potassium-dependent adenosine triphosphatase by tricyclic antipsychotics
No full text
Fumarate is the cause of the apparent ping-pong kinetics of dopamine beta-hydroxylase.
No full textThe kinetic mechanism of dopamine beta-hydroxylase (dopamine beta-monooxygenase EC 1.14.17.1) was studied either in the absence or the presence of the nonessential activator fumarate. In the absence of fumarate, intersecting initial velocity patterns were obtained, consistent with a sequential mechanism. In the presence of saturating concentrations of fumarate, initial velocity patterns became parallel. Other activating anions, such as acetate and chloride, could replicate the effects of fumarate. Since previous initial rate studies of dopamine beta-hydroxylase have been performed in the presence of saturating concentrations of fumarate, the present results may explain why parallel initial velocity patterns, apparently consistent with a ping-pong mechanism, have been so far observed. As a plausible mechanism of the anion effect it is proposed that activating anions induce saturation of the enzyme with oxygen
Mg-dependent inhibition of beef heart soluble mitochondrial ATPase by tricyclic antipsychotics
No full textThe effect of fluphenazine and related phenothiazine and thioxanthene derivatives on beef heart soluble mitochondrial ATPase (EC 3.6.1.3) was studied under a precise control of the Mg2+-ATP equilibrium. These drugs were shown to be reversible, noncompetitive inhibitors with respect to the substrate (the Mg . ATP complex). The inhibition was found to be dependent on the concentration of free magnesium ions, although free Mg2+ was not essential for the interaction of the inhibitors with the enzymatic protein. Bicarbonate anions, which are known to antagonize the effect of free Mg2+ on the enzyme kinetics, also antagonized the drug-induced inhibition. Concentrations giving 50% inhibition of enzyme activity were in the micromolar range. Inhibitory potencies increased when the pH of the reaction mixture was lowered from 8.2 to 6.9. Cleland [The Enzymes (P. D. Boyer, ed.), Vol. II. Academic Press, New York, 1--65 (1970)] analysis of the inhibition, by means of slope and intercept replots, indicated that the inhibition was the result of the interaction with more than one drug molecule. All drugs tested afforded complete protection against the cold-induced inactivation of soluble mitochondrial ATPase. These results point to a specific mode of inhibition that mimics, in some respects, the action of the natural inhibitor protein of mitochondrial ATPase
A new plot for multiple enzyme inhibition
No full textA new graphical method is described for analyzing the results of multiple inhibition experiments. It is applicable to either single- or multi-substrate enzyme systems obeying Michaelis-Menten kinetics and is valid irrespective of the type of inhibition (competitive, noncompetitive, uncompetitive, mixed). According to this method, mutually exclusive inhibitor binding gives rise to lines that converge on the vertical axis, whereas mutually nonexclusive inhibitors yield lines that intersect to the left of the vertical axis. It has been pointed out that the inhibitor interaction factor can be determined directly from multiple inhibition experiments only if at least one of the inhibitors is noncompetitive. When this is the case, the present plot provides a very simple way of determining the inhibitor interaction factor from the coordinates of the intersection poin
Disposition kinetics of phospholipid liposomes.
No full textThis article reviews the disposition of intravenously injected phospholipid liposomes and discusses the problems related to its kinetic modeling. The processes responsible for the plasma clearance of liposomes are examined in detail and it is shown that mechanisms other than reversible distribution to the extravascular space are, as a rule, responsible for the biphasic plasma clearance patterns that are typically observed following bolus intravenous injection of liposomes. Accordingly, a one-compartment open model is generally sufficient to describe the disposition kinetics of phospholipid vesicles. Two factors may be responsible for the observation of a biphasic decline of plasma liposome concentration. The first factor is the presence of different liposomal species with different kinetic behaviors. Kinetically distinct vesicles are present in preparations of liposomes that are heterogeneous in size, since the larger vesicles are cleared at a faster rate than the smaller ones. Different liposomal species may also originate in the plasma as a result of: i) fusion between phospholipid vesicles with generation of larger liposomal structures; and ii) interaction with high-density lipoproteins (HDL) with consequent production of either liposomes that have acquired apoproteins or lipoprotein particles enriched in phospholipids. Both these species are cleared by specific mechanisms at rates different from that of the original vesicle. The second factor is a time-dependent decrease in clearance due to progressive saturation of the retention capacity of the cells that take up liposomes. A convex concentration-time decay curve has also been reported. This decay pattern is consistent with a concentration (dose)-dependent elimination. As this observation relates to only one type of liposome (small unilamellar vesicles composed of sphingomyelin and cholesterol), its relevance to the disposition of liposomes of different size and composition remains to be established
Reversal by phospholipids of the oligomycin induced inhibition of membrane associated adenosintriphosphatases.
No full textThe inhibitory effect of oligomycin on sodium, potassium-stimulated ATPase and on mitochondrial ATPase was removed by the addition of purified phospholipids to the incubation medium. Ouabain effect was not modified. Phospholipids with isoelectric ionic portion (particularly lecithins) were found more active than phosphatidylethanolamines and bovine brain phosphatidylserine. Differences in the activity of lecithins from various sources were also seen. This indicates that the composition of hydrophobic fatty acid chains influences the reversal of oligomycin effect. The antagonism by the external added phospholipids may reflect the affinity of oligomycin with the phospholipids of the enzyme preparation. © 1970
- …
