Search PubMed⌕ Search

Biomedical subjects

B Witkop

Publications and source records attributed to B Witkop.

At least 37 records · Page 2Linked to original sources
PubMed1977

The interaction of polyxanthylic acid with polyadenylic acid.

The stoichiometry of interaction between polyxanthylic acid (poly(X)) and polyadenylic acid (poly(A)) was investigated by construction of mixing curves as a function of wavelength. Wavelengths were found which exhibited a break at 48-50 mol % poly(A) or two breaks, one at 48-50 mol % poly(A) and the other at 33-30 mol % poly(A). The melting profile (in 0.1 M salt) of the 50 mol % poly(A) mixture was monophasic (Tm=83 degrees C) at all wavelengths but that of the 33 mol % poly(A) mixture was biphasic showing a transition at Tm=40 degrees C and another at Tm=83 degrees C. Construction of a mixing curve between poly(A) and poly(X) at a temperature of 52 degrees C gave rise to plots which showed only one break (at 49--50 mol % poly(A)) at all wavelengths. Thus, while (at 20 degrees C in 0.1 M salt, pH 7) both poly(A)-poly(X) and poly(A)-2 poly(X) form at their respective stoichiometric end-points, the triplex poly(A)-2 poly(X), upon melting undergoes a disproportionation reaction resulting in the loss of one poly(X) strand and the formation of the poly(A)-poly(X) duplex.

Binding Sites↗
PubMed1977

A valid model for the mechanism of oxidation of tryptophan to formylkynurenine-25 years later.

The dye-sensitized photooxygenation of DL-tryptophan in aqueous solution leads to the tricyclic compound 2-carboxy-3a-hydroperoxy-1,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole which, on reduction with dimethyl sulfide, furnishes two diastereoisomeric alcohols separable by fractional crystallization into a higher melting (mp 254 degrees -256 degrees ) and a lower melting (mp 228 degrees ) diastereoisomer. Each of these alcohols was correlated with one of the analogous pair of isomeric 1,2-dicarbomethoxy analogs by alkaline hydrolysis and by x-ray analysis. In this way, the 3a-hydroxy-1,2-dimethoxycarbonyl- 1,2,3,3a,8,8a-hexahydropyrrolo[2,3-b]indole, mp 163 degrees -164 degrees , was shown to have the trans configuration with regard to the relative positions of the hydroxyl and carbomethoxy groups and that, on alkaline hydrolysis, it produced the isomer with mp 228 degrees , which therefore also has the trans configuration. The mechanism of the smooth thermal rearrangement of the (presumably ring-chain tautomeric) tryptophan hydroperoxy intermediates to formylkynurenine is discussed with its implications for the biological oxidation by tryptophan 2,3-dioxygenase.

Journal Article↗
PubMed1977

Histrionicotoxin enhances agonist-induced desensitization of acetylcholine receptor.

Dihydroisohistrionicotoxin inhibits acetylcholine receptor-dependent 22Na+ uptake of cultured chick muscle cells with a KI of 0.2 micrometer. The inhibition is noncompetitive with respect to agonists. The toxin enhances desensitization of the receptor by agonists which is accompanied by a 10-fold increase in receptor affinity for agonists. Dihydroisohistrionicotoxin increases the affinity of the desensitized form of the receptor for agonists but not antagonists. The results suggest that dihydroisohistrionicotoxin inhibits the acetylcholine receptor by causing an increase in the affinity of the desensitized form of the receptor for agonists and thereby stabilizing the desensitized state.

Amphibian Venoms↗
PubMed1977

Perhydrohistrionicotoxin: a potential ligand for the ion conductance modulator of the acetylcholine receptor.

Histrionicotoxin from the Colombian frog Dendrobates histrionicus and its perhydro derivative reversibly block the acetylcholine-sensitive ion conductance system in frog neuromuscular preparations. The perhydro derivative and [3H]perhydrohistrionicotoxin, like histrionicotoxin, caused a significant decrease in the peak amplitude of the end-plate current and shortened its rise time and half-decay time. In membrane preparations from Torpedo electroplax, [3H]perhydrohistrionicotoxin bound reversibly to a limited number of high-affinity sites [dissociation constant, (KD) = 0.4 micronM]. The ratio of perhydrohistrionicotoxin to acetylcholine binding sites in these membrane preparations approached 2. Histrionicotoxins, local anesthetics, and certain cholinergic agonists inhibited binding of perhydrohistrionicotoxin. Binding of perhydrohistrionicotoxin to membranes was decreased by heat or treatment with proteases. Treatment of membranes with Triton X-100 solubilized acetylcholine binding proteins and apparently also perhydrohistrionicotoxin-binding proteins. However, the detergent Triton X-100 also bound [3H]perhydrohistrionicotoxin. This nonspecific binding was not saturable and complicated studies on the antagonism by drugs of binding of [3H]perhydrohistrionicotoxin. In solubilized preparations the binding protein for acetylcholine could be removed by affinity chromatography or immunoprecipitation without affecting binding of perhydrohistrionicotoxin. Sephadex chromatography also separated acetylcholine- from perhydrohistrionicotoxin-binding proteins. Perhydrohistrionicotoxin did not bind significantly to purified acetylcholine-receptor protein but presumably bound to an ion conductance modulator protein that was associated with the acetylcholine-receptor in intact membrane and readily separable from the receptor protein after solubilization.

Amphibian Venoms↗
PubMed1976

Polynucleotide displacement reactions: detection by interferon induction.

A large variety of displacement reactions between homopolynucleotides and complexes thereof has been demonstrated by interferon induction data obtained in primary rabbit kidney cell cultures superinduced with metabolic inhibitors. The polymers involved in these helix-coil displacement studies were: poly(adenylic acid), poly(inosinic acid), poly(cytidylic acid), poly(uridylic acid), poly(ribothymidylic acid), polylaurusin, poly(7-deazaadenylic acid), poly(7-deazainosinic acid), poly(5-bromocytidylic acid), and poly(5-bromouridylic acid). As monitored by ultraviolet absorbance-temperature profiles, all displacement reactions were directed toward the formation of the helix with the higher thermal stability. Concomitantly, the resulting helix was invariably more active as interferon inducer than the reactant helix, except for some reactions in which poly(7-deazaadenylic acid) was involved. For the latter reactions both the reactant and resultant helices were inactive as interferon inducer. The interferon induction data revealed that all displacement reactions proceeded to completion within 1 h even at temperatures well below the Tm of the reactant helix. The helix-coil displacement reaction could also be monitored by sucrose velocity gradient analysis, and, as evidenced for poly(A)-2poly(I) + 2poly(C) leads to 2poly(I)-poly(C) + poly(A), readily occurred at the cellular level, presumably at the cell surface.

Animals↗
PubMed1976

Triple-helical polynucleotides. Mixed triplexes of the poly(uridylic acid)-poly(adenylic acid)-poly(uridylic acid) class.

By the techniques of interferon induction in primary rabbit kidney cells "superinduced" with metabolic inhibitors, ultraviolet absorbance-temperature profiles, sensitivity to pancreatic ribonuclease A, and sucrose velocity gradient ultracentrifugation, a number of reactions between double-helical RNA and single-stranded RNA or DNA homopolymers were investigated. The polymers involved in these studies were poly(adenylic acid), poly(uridylic acid), poly(ribothymidylic acid), poly(5-bromouridylic acid), poly(deoxythymidylic acid), poly(deoxyuridylic acid), poly(3-methyluridylic acid), poly(2'-O-methyluridylic acid), and poly(2'-azido-2'-deoxyuridylic acid). Two different reaction courses, both leading to the formation of triple helices, were noted: (1) poly(Ux)-poly(A) + poly(Uy) leads to poly(Ux)-poly(A)-poly(Uy) if the Tm of poly(Ux)-poly(A) was higher than the Tm of poly(Uy)-poly(A); (2) poly(Ux)-poly(A) + poly(Uy) leads to poly(Uy)-poly(A)-poly(Ux) if the Tm of poly(Ux)-poly(A) was lower than the Tm of poly(Uy)-poly(A). In these equations, the homopolymer written to the left of poly(A) implies Watson-Crick hydrogen bonding whereas the polymer to the right of poly(A) is involved in Hoogsteen hydrogen bonding.

Animals↗
PubMed1976

Dependence of interferon induction on nucleic acid conformation.

UV and circular dichroism characteristics of duplex analogs belonging to the (A)n-(U)n and (I)n-(C)n series were determined to assign qualitatively the nature of conformational differences caused by 5-pyrimidine and c7 purine substitutions in such duplexes. Evidence is presented which shows that 5-pyrimidine substitution by bromine or methyl changes the duplex conformation of both series in a similar way, if at all. A c7 substitution in the purine ring affects the duplex conformation of the members in the same series similarly, but the conformational change appears to be different for the two series. To wit, it is proposed that in the duplexes the effect of the change (A)n leads to (c7A)n is an increase of the positive base tilt, whereas the change (I)n leads to (c7I)n causes a decrease where (c7A)n is poly (7-deazaadenylic acid) and (c7I)n is poly(7-deazainosinic acid), respectively. Poly(5-bromocytidylic acid) (br5C)n proved to be useful as a sensor strand for the intepretation of the spectroscopic data. The circular dichroism findings correlate well with observations made earlier on the interferon inducing ability for such duplexes, namely, duplexes based on the (c7A)n are inactive as interferon inducers, whereas duplexes based on (c7I)n are potent inducers. Furthermore, a 5-pyrimidine substitution does not substantially affect the interferon inducing ability, unless the thermal stability of the analog becomes critical, as in the case of (A)n-(br5U)n. Thus, this study provides the first evidence to link the interferon-inducing ability of a nucleic acid to a defined physical parameter of double helix, and reinforces the concept that interferon induction is dependent on the recognition of a particular spatial and steric organization of a double-stranded RNA.

Circular Dichroism↗
PubMed1976

Conformation of uncomplexed [Phe4, Val6] antamanide crystallized from nonpolar solvents.

[Phe4, Val6] antamanide, a synthetic, biologically active analog of the cyclic decapeptide antitoxin isolated from Amanita phalloides, has been crystallized from a mixture of n-hexane and methyl acetate, and its conformation has been established by the direct method of x-ray diffraction analysis, i.e., without the benefit of any heavy atom. The uncomplexed molecule contains a 2-fold rotation axis and cis peptide linkages between Pro2-Pro3 and Pro7-Pro8. Otherwise, its conformation differs extensively from that of the Na+[Phe4, Val6] antamanide-C2H5OH complex. The 30-membered ring is elongated and relatively planar as compared to the folded ring in the Na+ complex. The six NH groups are directed toward the anterior of the molecule. There is one pair of intra-molecular NH---O=C bonds of the 5 leads to 1 type containing a cis peptide unit. The other four NH groups participate in hydrogen bonds to three H2O sites in the interior of the molecule. The 10 hydrophobic side groups cover the bottom and surround the perimeter of the molecule with the phenyl groups in the four Phe residues folded against the molecule. The conformation found for [Phe4, Val6] antamanide crystallized from nonpolar solvents is different from any conformations proposed for antamanide in solution based on nuclear magnetic resonance data.

Crystallography↗
PubMed1976

Effect of cytosine, arabinoside, iododeoxyuridine, ethyldeoxyuridine, thiocyanatodeoxyuridine, and ribavirin on tail lesion formation in mice infected with vaccinia virus.

Mice infected intravenously with vaccinia virus develop characteristic lesions over the entire tail surface. This experimental virus infection presents a highly sensitive and reliable model for evaluating the antivaccinia activity of antiviral compounds. Ara-C (1-beta-D-arabinofuranosylcytosine), ribavirin (1-beta-D-ribofuranosyl-1,2,4-triazole-3-carboxamide), IUdR (5-iodo-2'-deoxyuridine) as well as two novel analogs of IUdR, EtUdR (5-ethyl-2'-deoxyuridine), and NCSUdR (5-thiocyanato-2'-deoxyuridine), were found to inhibit the formation of vaccinia tail lesions, when administered intraperitoneally once daily for 7 days starting immediately after virus infection. The order of (decreasing) activity was: ara-C greater than IUdR greater than NCSUdR greater than ribavirin greater than EtUdR. Various drug combinations, involving IUdR + ara-C, NCSUdR + ara-C, NCSUdR + IUdR, NSCUdR + ribavirin, etc., were evaluated but none proved more efficacious than either compound administered alone.

Animals↗
PubMed1975

Polynucleotide duplexes based on poly(7-deazaadenylic acid).

In order to find a poly(A)-poly(U) analog which could not form a triple-stranded complex and which would have a sufficiently high thermal stability to survive under physiological conditions, the interaction of poly(7-deazaadenylic acid) (poly(c-7A)) with modified polyuridylic acids was examined. Mixing curves constructed by the method of continuous variation, isosbestic points and thermal melting profiles proved that poly(c-7A) formed only 1:1 complex with polyribothymidylic acid and poly(5-bromouridylic acid) (Tm values of 50 and 72 degrees C respectively, in 0.15 M NaCl, 0.01 M KH2 PO4, 0.001 M MgCl2, pH7). In addition poly(c-7A) formed a 1:1 complex with poly(I) (Tm equals 22 degrees C in 0.46 M salt, pH 7), and presumed duplexes were observed in the interaction of poly(c-7A) with poly(dT), poly(2'-azido-2'-deoxyuridylic acid) and poly(2'-O-methyluridylic acid) (Tm values of 35, 32 and 41 degrees C respectively, in 0.10 M NaCl, pH7).

Adenine Nucleotides↗