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Biomedical subjects

R Osman

Publications and source records attributed to R Osman.

At least 19 recordsLinked to original sources
PubMed1992

Thyrotropin-releasing hormone binding to the mouse pituitary receptor does not involve ionic interactions. A model for neutral peptide binding to G protein-coupled receptors.

Thyrotropin-releasing hormone, TRH (< Glu-His-Proamide), and [N tau-Me-His]TRH (MeTRH) are present as neutral and positively charged forms at physiologic pH, and it was possible that they bind to the TRH receptor (TRH-R) as charged (protonated) species. Binding affinities of TRH and MeTRH to endogenous rat TRH-Rs and to transfected wild type mouse TRH-Rs decreased below pH 7.1. Half-maximal decreases in binding occurred at the approximate pK alpha values of these ligands. Asp to Ala mutations in extracellular loop 1, TM-4, and TM-5 did not decrease binding affinity, but an Asp to Ala mutation in TM-2 caused the affinity to decrease 8-fold. The pH dependences of binding of MeTRH, however, were similar in wild type and all mutant receptors and were consistent with the protonated form of MeTRH binding less well. Thus, the binding of TRH to its receptor does not involve ionic interactions and may be a prototype for binding of neutral peptide ligands to G protein-coupled receptors.

Amino Acid Sequence↗
PubMed1992

On the use of the transmembrane domain of bacteriorhodopsin as a template for modeling the three-dimensional structure of guanine nucleotide-binding regulatory protein-coupled receptors.

The molecular architecture of bacteriorhodopsin (BR) is commonly regarded as a structural template for the three-dimensional structure of membrane receptors that are functionally coupled to guanine nucleotide-binding regulatory proteins (GPCR). More recently, specific molecular models of such GPCR were constructed on the basis of the functional and structural relation of rhodopsin to BR as well as the sequence homology between rhodopsin and the GPCR. Such models of GPCR leave unresolved the difficulty caused by the apparent lack of any significant degree of sequence homology between the seven transmembrane helices (TMH) of BR and the portions in the sequence of the various GPCR that are considered to constitute their transmembrane domains. Evolutionary arguments offered in favor of the structural relation between BR and the opsins, and hence the GPCR, prompted our investigation of the possibility that the sequence homology, including any similarity in the distribution of kink-inducing proline residues among the helices, might have been obscured by the assumption that the TMH maintained their sequential order from BR in the evolution of the mammalian proteins. With a definition of the TMH in the neurotransmitter GPCR guided by hydropathicity predictions, and additional criteria used to define the span of each helix, optimal alignment of each pair of sequences was determined with no gaps allowed in the matching. The resulting alignment proposed here reveals considerable homology between the TMH in BR and those in GPCR, if the sequential order of the helices is ignored. These findings suggest the possibility that exon shuffling could have occurred in the proposed evolution of the GPCR gene from BR and point to a modification of the BR template to account for the correct packing of the helices in the tertiary structures of GPCR. These findings could guide the construction of three-dimensional models of the neurotransmitter GPCR on the basis of specific interhelical interactions observed in BR.

Amino Acid Sequence↗
PubMed1991

Molecular mechanisms of radiation induced DNA damage: H-abstraction and beta-cleavage.

Quantum mechanical simulations of hydrogen abstraction by hydroxyl radical from methanol and ethanol yield barriers that agree very well with those measured experimentally. Analysis of the multiconfigurational wavefunction indicates that the strength of the C-H bond is the electronic parameter that has a major contribution to the barrier for H-abstraction. Similar analysis applied to 2-deoxy-D-ribose shows that the strength of a C-H bond together with the steric accessibility of the hydrogen determine that H4 is the most susceptible hydrogen for abstraction by a hydroxyl radical. Quantum mechanical simulations of beta-cleavage show that a concerted mechanism in which a water molecule assists in the bond breaking process is more likely than a SN1 mechanism. However, the polar transition state suggests that the environment of the DNA and the surrounding water will have an important effect on the reaction.

DNA↗
PubMed1991

Molecular mechanisms of radiation induced DNA damage: H-addition to bases, direct ionization and double strand break.

Structures and properties of G.(-H) and of TH. were obtained from quantum mechanical calculations. New AMBER parameters for these radicals were obtained to fit their structures and charge distributions. Molecular mechanics simulations of the conformational changes induced in a 12-mer of DNA, d(CGCGAATTCGCG), by these radicals show that the distances between the base and the C2' of the sugar becomes shorter. Such changes suggest that the base radical can abstract the H2' and transfer the radical from the base to the sugar. Once the radical becomes centered on the sugar a strand break can follow. A simultaneous formation of guanine and thymine radicals on opposite strands may lead to a double strand break.

Base Sequence↗
PubMed1991

Kinetic characterization of 5-hydroxytryptamine receptor desensitization in isolated guinea-pig trachea and rabbit aorta.

Desensitization of the contractile response mediated by the 5-hydroxytryptamine2 (5-HT2) receptor in the isolated guinea-pig trachea and rabbit aorta is a time-dependent process and therefore it has been characterized by an apparent rate constant obtained from a kinetic analysis. Under similar conditions, desensitization of the response in the trachea is 7-fold faster than in the aorta. Desensitization is homologous and reversible and is not affected by inhibition of neuronal and extraneuronal uptake, monoamine oxidase activity, alpha 1 adrenergic, cholinergic muscarinic or histamine H1 receptors. Desensitization does not depend on removal of epithelium from the trachea or endothelium and adventitia from the aorta or on the release of a stable relaxant factor. It is also not affected by the removal of extracellular Ca++, which is needed for tonic contraction. The dependence of desensitization on agonist concentration, number of receptors and the intrinsic activity of the agonist was determined. The observed values of the rate constants for desensitization and of the peak tension (T peak) in trachea show a saturable dependence on the concentration of 5-HT, indicating that occupancy of the 5-HT2 receptor is needed for desensitization. The less efficacious agonists, N-methyl serotonin, dimethyltryptamine, quipazine, 5-methoxytryptamine, 5-methyltryptamine, 5-methoxy dimethyltryptamine, 4-hydroxytryptamine and bufotenine induce significantly slower desensitization than 5-HT. A 25 to 75% reduction in 5-HT2 receptor number by alkylation had no effect on the observed rate constants for desensitization.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

Functional interactions in smooth muscle: kinetic characterization of the relaxation and desensitization responses to a beta adrenergic agonist in the rabbit aorta.

Vascular smooth muscle tone is continuously modulated in vivo by the functional interaction of a variety of vasoconstrictor and vasodilator stimuli. Endogenous substances such as epinephrine simultaneously activate alpha adrenergic receptors that elicit muscle contraction and beta adrenergic receptors that relax the muscle. This study characterizes the beta adrenergic response in the isolated rabbit aorta precontracted with 1 microM phenylephrine. The beta adrenergic agonist isoproterenol (0.03-10 microM) produces a biphasic response that is composed of a rapid relaxation followed by a slower regaining of tension, which is identified as desensitization. An exploratory kinetic model that describes both the relaxation and the desensitization as first-order processes provides a good fit to the experimental data. The parameters used to describe the isoproterenol response are: 1) the observed rate constant for relaxation and its magnitude (krel and R, respectively), 2) the observed rate constant for desensitization and its magnitude (kdes and D, respectively) and 3) the observed delay in the onset of the desensitization response (td). Both the krel and the fractional relaxation were dependent on concentration of isoproterenol in a saturable manner (EC50 = 0.017 and 0.067 microM, respectively). No concentration dependence was observed for kdes, fractional desensitization and td (the average values +/- S.E.M. of these parameters are (4.7 +/- 0.2). 10(-3) sec-1, 0.83 +/- 0.02 and 191 +/- 6 sec, respectively). This work demonstrates that a kinetic approach is necessary to characterize the desensitization response and is also very useful in characterizing the kinetic and steady-state parameters of the relaxation response.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

[Nesidioblastosis. Apropos of 12 new cases].

Two new cases of diffuse hyperplasia of the pancreas are reported. This infrequent condition is caused by intermittent and variable insulin hypersecretion. The hyperinsulinism is responsible for severe, lasting and intractable hypoglycemia that causes seizures and mental retardation. Onset usually occurs in the neonatal period. The diagnosis of hyperinsulinism rests on four criteria: the presence of increased insulin levels in the face of hypoglycemia, the low urinary excretion of ketone bodies during hypoglycemic episodes, the need for more than 15/mg/kg/min glucose to maintain the serum glucose level above 2 mmol/l, and a positive response to glucagon. The topographic diagnosis is often disappointing. Medical treatment of the hypoglycemia with diazoxide is a transient measure. Subtotal pancreatectomy is indispensable. Postoperative results are variable. Insulin deficiency diabetes mellitus is common and unusual in that insulin induces an exaggerated response. Recovery can be observed. If hypoglycemia recurs, diazoxide is often effective.

Female↗
PubMed1988

Role of primary and secondary protein structure in neurotransmitter receptor activation mechanisms.

A proton transfer triggered by a ligand interacting with the receptor had been suggested as the initial step in the activation of a receptor for the neurotransmitter serotonin (5-hydroxy-tryptamine; 5-HT). To evaluate the role of the receptor macromolecule in modulating the primary molecular event in ligand-mediated activation, the process of proton transfer was analysed in the environment of a protein model for the 5-HT receptor. In the absence of a detailed receptor structure, the enzyme actinidin was chosen as the model for the receptor based on criteria obtained from structure-activity considerations on the ligands. The first simulation of a mechanism for receptor activation was performed on this model using methods of theoretical chemistry to study the effect of specific structural elements. The premise is that the role of the elements of secondary structure of soluble proteins (e.g. actinidin) in determining structure-function relations in these macromolecules is maintained when these elements are part of membrane-bound receptor proteins. Results from the calculations of the effects of the six alpha helices of actinidin on the proton transfer process from the imidazolium side chain of His 162 to the thiol side chain of Cys 25 in the protein show that the helices contribute in different ways to modulate the energy of proton transfer. The largest helix, A1, opposes the proton transfer through the effect of the helix dipole. The charged residues (primary structure) in helix A3 favor the proton transfer, and mask the effect of its helix dipole (secondary structure) which opposes the transfer. The direction of the proton transfer simulated for the activation mechanism is opposite to that assumed in the catalytic process of the thiol protease, and the entire protein environment opposes the transfer. This supports the specific role of the ligand in triggering the proton transfer as a response to its binding.

Computer Graphics↗
PubMed1987

A molecular model for activation of a 5-hydroxytryptamine receptor.

The extension of a model proposed previously for molecular recognition at a serotonin (5-hydroxytryptamine (5-HT) ) receptor makes possible the formulation of a molecular mechanism of receptor activation. The activation mechanism proposed here is based on the changes induced in the drug and in a model receptor by the interaction mimicking the formation of a drug-receptor complex. This mechanism was simulated by quantum mechanical calculations of molecular interactions between 5-HT and a model for a receptor represented by an imidazolium-ammonia complex that serves as a proton transfer model (PTM). The movement of the proton in the PTM is promoted by the interaction with 5-HT, suggesting a process by which 5-HT can trigger the activation of the receptor. The elements of the activation mechanism revealed by the results of the simulation are: (a) the electrostatic alignment between the PTM and 5-HT, which guides the recognition of 5-HT by the PTM; (b) the contraction of the distance between the hydrogen bonded components of the PTM, induced by the interaction of 5-HT with the PTM, which leads to a decrease in the barrier to proton transfer in the PTM; (c) an additional decrease of the barrier to proton transfer produced by the negative electrostatic potential of 5-HT, which stabilizes the transition state; and (d) the increased preference for product over reactant in the interaction complex between 5-HT and the PTM, which constitutes a driving force for the proton transfer process. According to this model, compounds that activate the 5-HT receptor should bind in a mode that induces the changes described above in the PTM and thus triggers the proton transfer.

Calorimetry↗
PubMed1987

Molecular determinants for recognition of triazole and tetrazole analogs of histamine at H2-receptors.

Calculations of molecular structures, relative stabilities of the various tautomers, and molecular electrostatic potentials (MEPs) were used to examine the molecular properties that determine the actions at H2-receptors of histamine analogs in which the imidazole ring was replaced by triazole or tetrazole. The analysis indicates that the most stable tautomer of 3-ethylamine-1,2,4-triazole (EATRI), is also the most similar to the assumed active form of histamine. Differences in the MEP of EATRI and histamine are observed mainly near the N(2) nitrogen of EATRI which is the steric equivalent of the C(4) position of histamine. Because EATRI is recognized at the receptor in spite of these differences, we conclude that the H2-receptor has no selectivity with respect to the electrostatic or steric properties near this position, in agreement with previous observations from structure-activity relations. This conclusion contrasts with the apparent selectivity of the receptor for the reactivity properties of the position equivalent to C(2) in histamine. Thus, the analysis of the tautomeric forms and MEP of the 5-ethylamine-1,2,3,4-tetrazole, which is not recognized by the H2-histamine receptor, suggests that the negative potential near the N(3) nitrogen, which corresponds to C(2) in histamine, is responsible for the inactivity of this molecule. The mechanism of receptor activation by EATRI is analyzed in relation to results from a theoretical simulation of a proposed activation mechanism of H2-histamine receptors. We find that the discriminant property for receptor activation by EATRI should be the relative energy of the ring protonated tautomers, and our results indicate that only the cation in which the ring protonation is on N(1) and N(4) and the side chain is anchored at the negative receptor site can be recognized at the histamine H2-receptor, and can participate in the proposed activation process.

Azoles↗
PubMed1987

Kinetics of competitive drug action at 5-hydroxytryptamine2 receptors in isolated rabbit aorta.

The kinetics of agonist and antagonist interactions with the 5-hydroxytryptamine2 receptor were studied in the isolated rabbit aorta by following the antagonist-induced decrease in the steady-state response to an agonist. A model describing the competitive drug-receptor interactions was fitted to the data and yielded estimates of the association and dissociation rate constants of the agonist and the antagonist. A high concentration of the agonist ([agonist] much greater than KA) was used to reduce the influence of antagonist diffusion to the receptor upon the onset of antagonism. The effect of a diffusion barrier was evaluated by comparing the kinetics of drug competition in the absence and in the presence of the adventitia. The rate constants of the high-affinity antagonists spiperone, methysergide or ketanserin were similar in the absence and in the presence of the adventitia. In contrast, the rate constants of the low affinity antagonist 5-methoxygramine were reduced almost 5-fold in the presence of the adventitia. This observation may be explained by the large partition coefficients of the high-affinity antagonists as compared to the relatively low partition coefficient of 5-methoxygramine. The ratios of the estimated rate constants (k-x/kx) are in good agreement with the dissociation constants of the drugs determined with steady-state methods. In addition the results suggest that the association rate constant is a primary determinant of drug affinity for the receptor. The kinetic rate constants of the high-affinity antagonists measured in this preparation are similar to those previously reported in high-affinity binding studies. We conclude that the kinetic parameters obtained in our experiments reflect primarily the molecular interactions of these drugs with the receptor.

Animals↗
PubMed1986

Effect of local environment and protein on the mechanism of action of superoxide dismutase.

Quantum mechanical simulations of the mechanism of action of superoxide dismutase (SOD) indicate that the presence of Arg-141 in the active site of the enzyme is responsible for the formation of an intermediate complex between superoxide and the enzyme in which the copper is not reduced. The analysis of the local environmental effects of Arg-141 shows that this residue prevents the reduction of copper by forming a hydrogen bond to superoxide and by generating an electric field in the active site that opposes the transfer of an electron from superoxide to copper. The protein enhances the effect of the opposing field generated by Arg-141. Local changes in the environment of the copper ion, simulated by stretching the Cu-NE2 (His-61) bond also do not induce an electron transfer from superoxide to copper. The protein increases the energies required for this stretch through the electric field it generates near the active site. These results are further support for the new proposed mechanism of action of SOD which is based on the inability of superoxide to reduce the cupric ion in the enzyme.

Binding Sites↗
PubMed1986

Kinetic definition of agonist efficacy at a 5-hydroxytryptamine (5-HT2) receptor in the isolated rabbit aorta.

The contractile response of the isolated rabbit aorta elicited by 5-hydroxytryptamine (5-HT) and five partial agonists acting on the 5-HT2 receptor were separated into a phasic and a tonic response by altering the [Ca++] in the buffer. A kinetic analysis of the two responses yields parameters that provide a mechanistic insight into the different nature of these responses. The kinetic parameters of the phasic contraction indicate that the onset of this response depends on the access of the drug to the receptor and that its decay is independent of the nature and the concentration of the agonist. The observed rate constant of the onset of the tonic response, kobs, is saturable with increasing drug concentration, suggesting that the rate determining step is the activation of an effector by the preformed drug-receptor complex. These kinetic characteristics of the 5-HT2-mediated response are similar to those observed previously by us for the alpha-1 adrenergic receptor-mediated response in the rabbit aorta, suggesting that these receptors activate similar mechanisms related to the mobilization of Ca++. Furthermore, it is shown that the maximal values of kobs for the 5-HT2 agonists follow the rank order of maximal amplitudes of the phasic responses and the maximal steady-state levels of the tonic response. It is suggested that the maximal value of kobs may serve as a kinetic measure of drug efficacy.

Animals↗