PubMed Health⌕ Search

Biomedical subjects

H Rapoport

Publications and source records attributed to H Rapoport.

At least 37 records · Page 2Linked to original sources
PubMed1986

Molecular mechanisms of the potent and stereospecific nicotinic receptor agonist (+)-anatoxin-a.

Anatoxin-a (AnTX) was shown to be a highly potent and stereospecific agonist at nicotinic synapses in frog skeletal muscle and Torpedo electric organs. AnTX binds to the nicotinic-acetylcholine receptor with a higher affinity than for acetylcholine (ACh) but does not bind to sites in the receptor-gated ionic channel. (+)AnTX caused receptor desensitization, i.e., the loss of agonist-stimulated binding of histrionicotoxin to an allosteric site with time, at a rate significantly slower than that of ACh. Single channel patch clamp recordings indicated that the conductance of channels activated by (+)AnTX (28 pS) and ACh (27 pS) were similar. The (+)AnTX-activated channels contained rapid closing events, the burst times caused by the toxin were shorter than those caused by ACh but had similar voltage dependencies, and the number of short closures per burst was constant at all potentials with both agonists. The bursts of rapid openings and rapid closures (tau = 0.4 msec) appear to result from repetitive opening and closing of the (+)AnTX-bound receptor-ion channel. It is concluded that the semirigid molecule and secondary amine (+)AnTX is a more potent agonist than ACh or carbamylcholine because of a higher affinity for the receptor. At various concentrations the toxin activates the appearance of channels with the same conductances as ACh-induced channels but with a shorter channel lifetime.

Acetylcholine↗
PubMed1985

Bilin attachment sites in the alpha, beta, and gamma subunits of R-phycoerythrin. Structural studies on singly and doubly linked phycourobilins.

Three unique bilin peptides, a beta subunit peptide bearing a doubly linked phycourobilin (PUB), and two gamma subunit peptides with singly linked PUB groups, were obtained by enzymatic degradation of Gastroclonium coulteri R-phycoerythrin. These peptides were shown to have the sequences (Klotz, A. V., and Glazer, A. N. (1985) J. Biol. Chem. 260, 4856-4863): (Formula: see text) The sequence of peptide beta-3T was identical to that previously established for a doubly linked phycoerythrobilin (PEB) peptide derived from a B-phycoerythrin (Lundell, D. J., Glazer, A. N., DeLange, R. J., and Brown, D. M. (1984) J. Biol. Chem. 259, 5472-5480). Secondary ion mass spectrometry of beta-3T yielded a protonated molecular ion of 1629 mass units, the same as that given by the doubly linked PEB peptide (Schoenleber, R. W., Lundell, D. J., Glazer, A. N., and Rapoport, H. (1984) J. Biol. Chem. 259, 5481-5484), indicating that the doubly linked PUB and PEB tetrapyrroles were isomeric structures. High resolution 1H NMR analyses of peptides beta-3T, gamma-BV8, and gamma-DP provided unambiguous structural assignments for the singly and doubly linked PUB chromophores and indicated that the peptides in gamma-BV8 and gamma-DP were linked to ring A. The determination of which peptide fragment is linked to ring A and which to ring D in peptide beta-3T was not achieved in this study. 1H NMR analyses of three PEB-peptides from G. coulteri R-phycoerythrin--alpha-1 Cys(PEB)-Tyr-Arg, alpha-2 Leu-Cys(PEB)-Val-Pro-Arg, and beta-1 Met-Ala-Ala-Cys(PEB)-Leu-Arg--showed that they were identical to previously described corresponding chromopeptides from Porphyridium cruentum B-phycoerythrin, with the peptide linked to ring A of PEB in each instance (Schoenleber, R. W., Lundell, D. J., Glazer, A. N., and Rapoport, H. (1984) J. Biol. Chem. 259, 5485-5489). This is the first documented report on the structure of singly or doubly linked phycourobilins.

Bile Pigments↗
PubMed1985

Synthetic and conformational studies on anatoxin-a: a potent acetylcholine agonist.

Anatoxin-a is a powerful nicotinic acetylcholine receptor agonist. Its recently reported synthesis has been further optimized to provide anatoxin-a of greater than 99% optical purity in 10% overall yield. The geometry of solid anatoxin-a has been determined by X-ray crystallography of its hydrochloride. The solution conformation has been determined by 500-MHz 1H NMR spectroscopy, utilizing 2D NMR methods and homonuclear decouplings. For further comparisons, force field calculations have been employed to evaluate the differences in energy between the various conformations available for anatoxin-a. The molecule is seen to adopt the same ring conformation both in solution and in the crystal. Comparison of this conformation with the models proposed for acetylcholine receptor activation shows good agreement and allows for further inferences concerning the stereodiscrimination by the receptor.

Bacterial Toxins↗
PubMed1985

Analgesics of the 6,14-ethenomorphinan type. 6-deoxy-7 alpha-orvinols and 6-deoxy-8 alpha-orvinols.

6-Deoxythebaine (3) has been prepared as a precursor to C-6 alkyl substituted orvinols 15 and 17. The C-6 methyl group was introduced by addition of methyllithium to codeinone. Transformation of 6-methylcodeine to its 6-methyl ether and 1,4-elimination of methanol with potassium tert-butoxide in Me2SO then gave 6-deoxythebaine (3) in 49% overall yield. Diels-Alder addition of methyl vinyl ketone to this diene yielded four ketones: three regio- and stereoisomeric 6,14-endo-ethenomorphinans and one exo adduct. The major ketone isomer provided the set of C-19 diastereomeric orvinols 15 in which the pendant carbon has the 7 alpha configuration. Regioisomeric ketone 8, in which the acetyl group is at C-8, was formed in 3% yield and was similarly converted to the corresponding orvinols 17. Orvinol (R)-15 (R at C-19) is an analgesic of very high potency, 2200 times that of morphine; regioisomeric orvinols 17, in which the pendant tertiary alcohols are on C-8, are much less potent. The higher activity of the C-6 methyl and methoxyl analogues (R)-15 and (R)-22 relative to hydrogen-substituted (R)-19 indicates that C-6 alkyl substitution enhances analgesic potency.

Analgesics↗
PubMed1985

Morphine and other opiates from beef brain and adrenal.

We describe nonpeptide opioids found in extracts of beef hypothalamus and adrenal, which are recognized by antisera raised against morphine. Four have been purified to homogeneity. One is morphine. The structures of the other three have not been determined yet. None of them are derived from morphine or normorphine after extraction from the tissues. It is not known whether the opiates described here are of endogenous or exogenous origin.

Adrenal Glands↗
PubMed1984

Bilin attachment sites in the alpha and beta subunits of B-phycoerythrin. Structural studies on a doubly peptide-linked phycoerythrobilin.

A bilin-containing fragment of the beta subunit of Porphyridium cruentum B-phycoerythrin produced by cleavage with thermolysin was shown by sequence analysis (Lundell, D.J., Glazer, A.N., DeLange, R.J., and Brown, D.M. (1984) J. Biol. Chem. 259, 5472-5480) to have the following structure. (Formula: see text) Secondary ion mass spectrometry of this bilin-peptide yielded a protonated molecular ion of 1629 mass units corresponding to that predicted from the composition of the fragment, and indicated that the heptapeptide is linked to ring A and the tripeptide to ring D. NMR spectra provided definitive evidence for a thioether linkage at the C-3' carbon of ring A and a second thioether linkage at teh C-18' carbon of ring D of the bilin. This is the first documented report of a bilin linked through two thioether linkages to a polypeptide.

Amino Acid Sequence↗
PubMed1984

Bilin attachment sites in the alpha and beta subunits of B-phycoerythrin. Structural studies on the singly linked phycoerythrobilins.

Five unique phycoerythrobilin (PEB) peptides were prepared from Porphyridium cruentum B-phycoerythrin by a combination of tryptic and thermolytic digestion without alteration in the spectroscopic properties of the bilin (Lundell, D.J., Glazer, A.N., DeLange, R.J., and Brown, D.M. (1984) J. Biol. Chem. 259, 5472-5480). alpha-1 Cys(PEB)-Tyr-Arg alpha-2 Leu-Cys(PEB)-Val-Pro-Arg beta-1 Met-Ala-Ala-Cys(PEB)-Leu-Arg beta-2T Phe-Ala-Ala-Gly-Asp-Cys(PEB)-Thr-Ser (Formula: see text) where alpha and beta refer to the subunits from which the peptides were derived High resolution 1H NMR analysis of peptides alpha-2, beta-1, and beta-2T combined with earlier studies of peptide alpha-1 (Schoenleber, R.W., Leung, S.-L., Lundell, D.J., Glazer, A.N., and Rapoport, H. (1983) J. Am. Chem. Soc. 105, 4072-4076) has provided proof that all of the singly linked PEB peptides contain a thioether bond to the 3' position of ring A, and strong evidence in support of a trans-dihydro ring A in each of these chromopeptides. The circular dichroism spectra of the four singly linked PEB peptides show that the configuration at C-16 is R in each instance. The present study coupled with previously reported results on peptide beta-3T (Schoenleber, R.W., Lundell, D.J., Glazer, A.N., Rapoport, H. (1984) J. Biol. Chem. 259, 5481-5484 provides the first comprehensive analysis of the structure of all the polypeptide-linked prosthetic groups on the alpha and beta subunits of B-phycoerythrin.

Amino Acid Sequence↗
PubMed1984

Analgesics of the orvinol type. 19-Deoxy and 6,20-epoxy derivatives.

A series of new ethenotetrahydrooripavine derivatives has been prepared in which the C-19 hydroxyl group has been replaced by hydrogen. The syntheses proceed via the thioanisole adducts of the thevinones. These adducts are converted to 1,2-epoxides, which are rearranged to aldehydes and then reduced to primary alcohols. Further conversion of these alcohols result in the deoxy derivatives and the 6,20-epoxy analogues, which contain a fused tetrahydrofuran ring. The compounds in this series, some of which offer the possibility of strong hydrogen-bonding interactions and others of which contain sterically fixed lipophilic side chains, have been evaluated as analgesics by the rat tail-flick method. Based on these and previous results, a proposal is made for the interaction of the orvinols with the opiate receptor involving both lipophilic (L) and hydrophilic (H) subsites. This proposal can be extended to suggest an optimum conformation for the enkephalins.

Analgesia↗
PubMed1984

8-Methoxypsoralen-nucleic acid photoreaction. Effect of methyl substitution on pyrone vs. furan photoaddition.

We have synthesized a series of 8-[3H]methoxypsoralens in which methyl and hydrogen are systematically varied at the 4- and 5'-positions. Analysis of the products resulting from the photoaddition of these four psoralens with the nucleic acid poly(dA-dT) reveals that the product distribution depends on the presence or absence of a 4-methyl substituent. Compounds with the 4-methyl group show an overwhelming preference (approximately 98%) for addition to the furan double bond, while compounds without the 4-methyl show a substantial amount (approximately 18%) of addition to the pyrone double bond.

Chemical Phenomena↗
PubMed1982

Enzymic conversion of reticuline to salutaridine by cell-free systems from Papaver somniferum.

A cell-free extract from the opium poppy, Papaver somniferum, was prepared that utilized hydrogen peroxide to convert (+/-)-[3-3H]reticuline to [3H]salutaridine in 80-85% yield based on consumed [3-3H]reticuline. The phenolic-coupling enzyme activity was not detected in the crude homogenate of whole plants. Reticuline conversion to salutaridine was accomplished only after methods were developed to separate both alkaloid and hydrogen peroxide degrading activities from the desired enzyme activity by centrifugal fractionation of carefully prepared stem and root extracts. Purity of the enzymatically produced [3H]salutaridine was established by chromatographic methods and synthetic conversion to [3H]-thebaine.

Cell-Free System↗
PubMed1982

Psoralen-deoxyribonucleic acid photoreaction. Characterization of the monoaddition products from 8-methoxypsoralen and 4,5'8-trimethylpsoralen.

The isolation and structural characterization are described of the major monoaddition products formed in the photoreaction of two naturally occurring psoralens, 8-methoxypsoralen and 4,5',8-trimethylpsoralen, with high molecular weight, double-stranded DNA. Hydrolysis of the psoralen-modified DNA and subsequent chromatography resulted in the isolation of four modified nucleosides from each psoralen. Structural characterization was accomplished by mass spectrometry and 1H NMR analysis. The major products, accounting for 44-52% of the covalently bound psoralen, are two diastereomeric thymidine adducts formed by cycloaddition between the 5,6 double bond of the pyrimidine and the 4',5' (furan) double bond of the psoralen. A minor product, less than 2% of the covalently bound psoralen, is a furan-side adduct to deoxyuridine, derived from an initially formed deoxycytidine adduct by hydrolytic deamination. A fourth product is a thymidine adduct where cycloaddition has taken place between the 5,6 double bond of the pyrimidine and the 3,4 (pyrone) double bond of the psoralen. This pyrone-side adduct accounts for 19% of the covalently bound 8-methoxypsoralen but for less than 3% of the covalently bound 4,5'8-trimethylpsoralen. All of the isolated adducts have cis-syn stereochemistry. The stereochemistry and product distribution of the adducts are determined in part by the constraints imposed by the DNA helix on the geometry of the noncovalent intercalation complex formed by psoralen and DNA prior to irradiation.

Chromatography, High Pressure Liquid↗
PubMed1981

Permethyl analogue of the pyrrolic antibiotic distamycin A.

The synthesis of an analogue of distamycin A, a pyrrolic oligopeptide possessing antiviral and antibiotic activity, is described in which each of the three pyrrole rings is fully methylated. This structural modification results in pyrrole rings which are extraordinarily electron rich and required the development of a new synthetic approach to these polypyrrolic amides. The key reactions involved development of a general method for the synthesis of 3-aminopyrroles and for formation of an amide bond between a pyrrole-2-carboxylic acid and these 3-aminopyrroles. Since the acid is hindered, a poor electrophile, and acid sensitive, while the amine is unstable and a hindered, weak nucleophile, amide bond formation under the usual conditions was poor. A very efficient method, however, was developed involving the isolation of 1-hydroxybenzotriazole active ester prepared in situ from another active ester. Neither the mono-, di-, nor tripyrrolic permethyl analogues were effective antimalarials, and none showed anticancer activity.

Animals↗
PubMed1981

6-Demethoxythebaine and its conversion to analgesics of the 6,14-ethenomorphinan type.

The 6-demethoxy analogue of thebaine has been easily prepared from codeine via isocodeine and its sulfenate ester. This diene, 7, readily undergoes reaction with vinyl ketones to afford Diels-Alder adducts of the 6,14-ethenomorphinan type. Further reactions afford the epimeric 19(R)- and 19(S)-butyl-6-demethoxy-7 alpha-orvinols (16). Pharmacological testing shows the R diastereomer to be highly analgesic and the s diastereomer to be a much less potent agonist, with similar potencies and relationships as found in the corresponding oripavine series. Thus, any hydrogen bonding between the 6-methoxyl group and the tertiary alcohol can be eliminated as contributory to either the activity of, or difference between the epimeric orvinols.

Analgesics↗
PubMed1978

Hemopyrrole and kryptopyrrole are absent from the urine of schizophrenics and normal persons.

We describe a method for detecting hemopyrrole and kryptopyrrole in urine, with a detection limit of 100 mug/liter (1 part in 10(7)). Urine is thoroughly extracted with methylene chloride and the extract is concentrated and examined by gas chromatography and gas chromatography-mass spectrometry. No hemopyrrole or kryptopyrrole could be detected in 52 samples, from 17 controls, 29 schizophrenics, and six persons with acute intermittent porphyria.

Chromatography, Gas↗