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Biomedical subjects

A Levitzki

Publications and source records attributed to A Levitzki.

At least 181 records · Page 10Linked to original sources
PubMed1980

Temperature dependence of beta receptor, adenosine receptor, and sodium fluoride stimulated adenylate cyclase from turkey erythrocytes.

The individual temperature dependencies of the process which control the activity of turkey erythrocyte adenylate cyclase have been determined. The temperature dependence of the fraction of activable cyclase units experiences a thermal transition at 24 degrees C for all three modes of enzyme activation: l-epinephrine, adenosine, and NaF. This thermal transition probably reflects the phase transition in the inner monolayer of the membrane which influences the behavior of the GTP regulatory unit which is involved in all three modes of enzyme activation. The "rate constant" of enzyme activation by adenosine reflects two thermal transitions, at 24 and at 35 degrees C; the apparent rate constant of cyclase activation by NaF activation experiences a transition only at 24 degrees C whereas the rate constant of the beta-receptor-bound agonist decreases monotonously with no "breaks" on the Arrhenium plot. Following the temperature dependence of the fluorescence intensity of dansylphosphatidylethanolamine embedded in both sides of the membrane and exclusively in the outer monolayer, one can assign the thermal transition of 24 degrees C to the inner monolayer and the other two transitions to the outer monolayer (10 and 35 degrees C). We interpret these results as follows. (a) The monomolecular rate constant characterizing the activation of cyclase by the precoupled adenosine receptor experiences both the transition at 24 and 35 degrees C, indicating that the latter may span the bilayer. (b) The bata receptor activates the cyclase units only in fluid areas since it can diffuse exclusively in the fluid areas of the membrane and is unable to interact with cyclase units in "frozen" areas. the linear dependence of the logarithm of the rate constant on 1/T for the bata receptor reflects the change of membrane fluidity as a function of temperature.

Adenosine↗
PubMed1980

GTP-receptor interrelationships in adenylate cyclase systems. Theoretical considerations.

Theoretical considerations concerning the effect of guanyl nucleotides on hormone-dependent adenylate cyclases show that the phenomena observed can be accounted for by postulating only one type of guanyl nucleotide regulatory site. The binding of GTP or of its non-hydrolizable analogues to the guanyl nucleotide regulatory site induces cyclase activation and may induce a decrease in agonist affinity towards the receptor. From basic principles it is shown that the potency ratio of guanyl nucleotides in inducing the decreased agonist affinity dose not necessarily reflect their order of affinities to the GTP regulatory site.

Adenylyl Cyclases↗
PubMed1980

Lateral mobility of beta-receptors involved in adenylate cyclase activation.

Cationized ferritin was found to inhibit the lateral mobility of intramembrane proteins in turkey erythrocyte membranes and the activation of adenylate cyclase by the (--)-epinephrine-bound beta-adrenergic receptor. It was observed that cationized ferritin has only a small direct effect on the beta-receptor and on the adenylate cyclase moiety. It is concluded that the cationized ferritin-induced inhibition of the hormone-dependent cyclase activity results from the inhibition of the lateral mobility of the receptor and therefore a decrease in the bimolecular rate of interaction between the receptor and the enzyme.

Adenylyl Cyclases↗
PubMed1980

Subunit neighbor interactions in enzyme kinetics: half-of-the-sites reactivity in a dimer.

We consider an isologous enzyme dimer in which the subunits, if operating independently, would obey Michaelis-Menten kinetics. However, because of neighbor interactions, the rate constants of the kinetic cycle in either subunit depend on the state (E or ES) of the other subunit. The steady-state behavior of this dimer system, with interactions, is investigated. In what is probably the most important special case, ES x ES is destabilized considerably by the neighbor interaction compared to E x ES. This leads to half-of-the-sites reactivity (one subunit is in state ES; the other subunit cycles between E and ES), negative cooperativity, and a considerable enhancement of enzyme activity relative to the activity of independent subunits.

Allosteric Regulation↗
PubMed1980

Evidence for participation of transglutaminase in receptor-mediated endocytosis.

We report evidence that the enzyme transglutaminase (glutaminyl-peptide gamma-glutamyltransferase; R-glutaminyl-peptide:amine gamma-glutamyltransferase, EC2.3.2.13) participates in receptor-mediated endocytosis. Clustering and internalization of rhodamine-labeled alpha 2-macroglobulin (R alpha 2 M) in normal rat kidney (NRK) cells is inhibited by a wide spectrum of compounds that inhibit transglutaminases, including that from NRK cells. The pattern of clustering inhibition resembles the pattern of transglutaminase inhibition as follows: (i) The most potent transglutaminase inhibitors are dansylcadaverine and the transglutaminase-directed affinity label N-benzyloxy-carbonyl-5-diazo-4-oxonorvaline p-nitrophenyl ester; these were also the most potent inhibitors of clustering and internalization of R alpha 2M. (ii) The inhibition of clustering of R alpha 2M occurs in the same concentration range as that required for transglutaminase inhibition. (iii) Linear primary amines are more effective blockers than the iso-chain primary amines. (iv) The transglutaminase affinity label N-benzyloxycarbonyl-5-diazo-4-oxonorvaline p-nitrophenyl ester irreversibly inhibits a significant fraction of the NRK transglutaminase and the clustering and internalization of R alpha 2M. A closely related compound, N-trifluoroacetyl-6-diazo-5-oxonorleucine ethyl ester, does not significantly inhibit transglutaminase or clustering and internalization. (v) Clustering and internalization is inhibited 10-fold more effectively by the heptapeptide Ac-Gly2-LLeu-LLys-Gly3 than by the heptapeptides Ac-Gly2-LLeu-DLys-Gly3 or AcGly3-DLys-DLeu-Gly2. This is the pattern of stereospecificity for the inhibition of purified transglutaminases.

Affinity Labels↗
PubMed1980

Mechanism of negative cooperativity in glyceraldehyde-3-phosphate dehydrogenase deduced from ligand competition experiments.

It is shown that the modulation in the negative cooperativity of ligand binding by another, competing ligand that binds noncooperatively is accounted for exclusively by the ligand-induced sequential model. It is therefore suggested that whenever such a phenomenon is observed it argues strongly in favor of the sequential model. The advantages and limitations of this approach are evaluated. The binding of the coenzymes NAD+ and nicotinamide-1-N6-ethenoadenine dinucleotide to rabbit muscle apo-glyceraldehyde-3-phosphate dehydrogenase [D-glyceraldehyde-3-phosphate:NAD+ oxidoreductase (phosphorylating; EC 1.2.1.12] exhibits strong negative cooperativity, whereas acetylpyridine adenine dinucleotide, ATP, and ADP-ribose bind noncooperatively to the NAD+ sites. The strong abolished in the presence of acetylpyridine adenine dinucleotide and strongly weakened by ATP, ADP, and AMP, but was not affected by addition of ADP-ribose. These findings demonstrate that the negative cooperativity in coenzyme binding to this enzyme results from sequential conformational changes and exclude the pre-existent asymmetry model as a possible explanation. These results also support the view that the structure of the pyridine moiety of the coenzyme analogs plays a role in orienting the adenine moiety at the adenine subsite, therefore affecting the cooperativity in the binding of the coenzyme analog which is mediated through the adenine subsites.

Adenosine Diphosphate Ribose↗
PubMed1980

The sequential nature of the negative cooperativity in rabbit muscle glyceraldehyde-3-phosphate dehydrogenase.

The binding of nicotinamide--adenine dinucleotide (NAD+), nicotinamide--1,N6-ethenoadenine dinucleotide (epsilon NAD+), acetylpyridine--adenine dinucleotide (AcPyAD+), ATP, and adenosine diphosphoribose (ADP-ribose) to rabbit muscle glyceraldehyde-3-phosphate dehydrogenase (the enzyme) was examined. NAD+ and epsilon NAD+ were found to bind to the apoenzyme in a negatively cooperative manner, whereas AcPyAD+, ATP, and ADP-ribose bind non-cooperatively to the NAD+ sites. The strong negative cooperativity in coenzyme binding was found to be abolished in the presence of AcPyAD+ and strongly weakened by ATP, ADP, and AMP, but was not affected by the addition of ADP-ribose. These findings demonstrate that the mechanism of the negative cooperativity in coenzyme binding to the enzyme involves ligand-induced conformational changes between neighboring sites. These findings cannot be accounted for by the pre-existent asymmetry model. The results support our previous hypothesis that the structure of the pyridine moiety of the coenzyme analogues plays a role in orienting the adenine moiety in the adenine subsite, and thus affects the cooperativity observed in the binding of the coenzyme analogue.

Adenosine Diphosphate Ribose↗
PubMed1979

Ligand competition curves as a diagnostic tool for delineating the nature of site-site interactions: theory.

A few molecular models have been developed in recent years to explain the mechanism of cooperative ligand binding. The concerted model of Monod, Wyman and Changeux and the sequential model of Koshland, Némethy and Filmer were formulated to account for positively cooperative binding. The pre-existent asymmetry model and the sequential model can account for negatively cooperative ligand binding. In most cases, however, it is virtually impossible to deduce the molecular mechanism of ligand binding solely from the shape of the binding isotherm. In the present study we suggest a new strategy for delineating the molecular mechanism responsible for cooperative ligand binding from binding isotherms. In this approach one examines the effect of one ligand on the cooperativity observed in the binding of another ligand, where the two ligands compete for the same set of binding sites. It is demonstrated that the cooperativity of ligand binding can be modulated when a competitive ligand is present in the protein-ligand binding mixture. A general mathematical formulation of this modulation is presented in thermodynamic terms, using model-independent parameters. The relation between the Hill coefficient at 50% ligand saturation with respect to ligand X in the absence, h(x), and in the presence of a competing ligand Z, h(x,z), is expressed in terms of the thermodynamic parameters characterizing the binding of the two ligands. Then the relationship between h(x) and h(x,z), in terms of the molecular parameters of the different allosteric models, is explored. This analysis reveals that the different allosteric models predict different relationships between h(x,z) and h(x). These differences are especially focused when Z binds non-cooperatively. Thus, it becomes possible, on the basis of ligand binding experiments alone, to decide which of the allosteric models best fits a set of experimental data.

Binding Sites↗
PubMed1979

Adenosine receptor permanently coupled to turkey erythrocyte adenylate cyclase.

The mode of coupling of the adenosine receptor to adenylate cyclase in turkey erythrocyte membranes was probed by two independent approaches. The progressive inactivation of the adenosine receptor by an adenosine receptor affinity label resulted in the proportional reduction in the adenosine plus GppNHp dependent specific activity. In contrast, the intrinsic rate constant (k3), characterizing the process of adenylate cyclase activation by the adenosine-adenosine receptor complex, is independent of the extent of receptor inactivation. This behavior favors the precoupled mechanism, A + R.E: formula: (see text), where the receptor R and the enzyme E are permanently coupled to each other and the adenosine A binds to the receptor and induces the first-order process of cyclase activation to its active form ARE'. The finding that adenosine receptor is permanently coupled to the cyclase catalytic unit is corroborated by the observation that the progressive increase in membrane fluidity has no effect on the rate constant (k3) of adenylate cyclase activation by the adenosine-adenosine receptor complex and that the dose-response curve for adenosine is noncooperative.

Adenosine↗
PubMed1979

Evidence for ligand-induced conformational changes in rabbit-muscle glyceraldehyde-3-phosphate dehydrogenase.

The tetrameric glyceraldehyde-3-phosphate dehydrogenase from rabbit muscle binds NAD+ and some of its analogues in a negatively cooperative manner, whereas other NAD+ analogues bind non-cooperatively to this enzyme. Subsequent to alkylation of a fraction of the active sites of the enzyme with the fluorescent SH reagent N-iodoacetyl-N'-(5-sulfo-1-naphthyl)-ethylenediamine, it was found that the alkylated sites bind NAD+ and NAD+ analogues with a markedly reduced affinity as compared with non-alkylated sites. It was therefore feasible to measure the fluorescence and the circular polarization of the luminescence of the enzyme-bound alkyl groups as a function of binding of NAD+ and of NAD+ analogues to the non-alkylated sites. The changes observed indicate that ligand binding to the non-alkylated sites induces changes in the fluorescence properties of the alkyl groups bound to neighbouring subunits, most likely through the protein moiety. The nature of these changes appears to depend on the structure of the coenzyme analogue. The binding of the non-cooperative binders acetyl-pyridine--adenine dinucleotide, ATP and ADP-ribose induce different conformational changes in the neighbouring vacant subunit, as monitored by the spectroscopic properties of the bound alkyl group. These results in conjunction with other data support the view that the negative cooperativity in NAD+ binding to glyceraldehyde-3-phosphate dehydrogenase results from ligand-induced conformational changes. Furthermore, these results further support the view that subtle structural changes in the coenzyme molecule determine the nature of the conformational changes induced within the enzyme tetramer.

Animals↗