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Biomedical subjects

S M Hecht

Publications and source records attributed to S M Hecht.

At least 109 records · Page 6Linked to original sources
PubMed1981

Copper(I) . bleomycin. A structurally unique oxidation-reduction active complex.

Cu(I) and Cu(II) form stable 1:1 complexes with bleomycin (BLM). The affinity of both metals for the drug is greater than that of Fe(II). Cu(I) . BLM A2 binds to calf thymus DNA with about the same affinity as Fe(II) . BLM, as judged by DNA-induced fluorescence quenching of the bithiazole moiety of BLM. Based on 1H NMR and potentiometric titration data, the Cu(I) complexes of BLM are shown to have geometries very different than those of other BLM . metal(II) complexes studied thus far. As Cu(I) . BLM is oxidation-reduction active, its geometry is of importance in defining the structural requirements for BLM activity.

Animals↗
PubMed1980

Metal binding to modified bleomycins. Zinc and ferrous complexes with an acetylated bleomycin.

We have studied the DNA- and metal-binding properties of a bleomycin A2 derivative in which the alpha-amino group of the beta-aminoalanine moiety has been N-acetylated. The modified antibiotic has been shown to be without activity in mediating the in vitro release of [3H]thymine from Pm-2 DNA. Fluorescence experiments indicate that the degree of quenching by DNA of the bithiazole fluorescence is unaffected by N-acetylation of bleomycin. Furthermore, 1H NMR experiments demonstrate that N-acetylation does not alter the stoichiometry of metal binding. The Fe(II)--Ac-bleomycin A2 complex, however, has been found to be stable in the presence of both O2 and CO, and thus inactivation appears to be accounted for by the loss of the ability to bind and/or reduce O2. Comparison of the 1H NMR spectra of the Fe(II)--bleomycin and Fe(II)--Ac-bleomycin A2 complexes indicates that either a drastic reorganization of the ligands with respect to the central iron atom has occurred or that an altered spin state is stabilized. These experiments establish that the ability of bleomycin to cause DNA damage is sensitive to even minor structural alterations within the antibiotic.

Animals↗
PubMed1980

Amino acid substitutions in protein biosynthesis. Poly(A)-directed polyphenylalanine synthesis.

The fidelity of protein biosynthesis in vitro was studied quantitatively in a well defined system that employed poly(A) as a message for the elaboration of phenylalanine-containing polypeptides from Escherichia coli phenylalanyl-tRNALys. Admixture of phenylalanyl-tRNALys and lysyl-tRNALys in three different ratios resulted in the efficient formation of peptides that contained the amino acids in the same ratios in which they had been utilized in the individual incubation mixtures. The incorporation was also shown to be codon-specific in a quantitative sense; the poly(A)-directed incorporation of [14C]phenylalanine from phenylalanyl-tRNALys was unaffected by [3H]phenylalanyl tRNAPhe, by arginyl-tRNA (one species of which responds to the codon triplet AGA) or by unfractionated E. coli tRNA. These findings suggest that the transfer of amino acids from (misacylated) tRNAs into polypeptides in vitro is as predicted by the adapter hypothesis and that such systems can operate with sufficient fidelity to permit the preparation of proteins having defined amino acid substitutions.

Amino Acyl-tRNA Synthetases↗
PubMed1980

There is a correlation between the DNA affinity and mutagenicity of several 3-amino-1-methyl-5H-pyrido[4,3-b]indoles.

3-Amino-1-methyl-5H-pyrido[4,3-b]indole, previously reported to be a component of tryptophan pyrolysates, is an intensely mutagenic compound requiring microsomal activation for expression of mutagenicity. We have found that this species and several synthetic analogs interact noncovalently with calf thymus DNA, as judged by both fluorescence quenching and differential dialysis. Remarkably, this noncovalent interaction correlated with the mutagenic potential of the compounds in a bacterial mutagenesis assay. Therefore, it is suggested that the mechanism of mutagenesis involves metabolic activation followed by physicochemical interaction with DNA; for these compounds, the latter step may be limiting for the expression of mutagenicity.

Animals↗
PubMed1979

Transfer RNA control of the activation of isomeric tRNATrp's.

Previous studies of the homologous aminoacylations of Escherichia coli and yeast tRNATrp's terminating in 2'- and 3'-deoxyadenosine established that E. coli tryptophanyl-tRNA synthetase activates its cognate tRNA preferentially on the 2' position, while the corresponding yeast enzyme utilizes the 3' position on its homologous substrate tRNA. As this seemed to be the only change in positional specificity during evolution, the heterologous activations were investigated in an effort to determine the basis for this change. Remarkably, E. coli tRNATrp terminating in 3'-deoxyadenosine was found to be the preferred substrate for both the E. coli and yeast activating enzymes, while the same tryptophanyl-tRNA synthetase preparations both activated the isomeric yeast tRNATrp's preferentially on the 3' position. Thus, the preferred position of activation was found to be specified by the tRNA rather than the activating enzyme and, additionally, to be due to some process not reflected in initial velocity measurements. The variable utilization of individual modified aminoacyl-tRNA's as substrates in an enzyme-catalyzed deacylation process appears to provide the most likely explanation for the experimental observations.

Amino Acyl-tRNA Synthetases↗
PubMed1979

Both positional isomers of aminoacyl-tRNA's are bound by elongation factor Tu.

Six purified Escherichia coli and yeast tRNA's were converted to positionally defined tRNA's terminating in 2'- and 3'-deoxyadenosine; the modified (amino-acyl) tRNA's were compared for their abilities to bind to elongation factor Tu (EF-Tu) in the presence both of GTP and guanylylimidodiphosphate (GMP-P(NH)P). Formation of aminoacyl-tRNA . EF-Tu . guanine nucleotide ternary complexes was monitored by gel filtration on Sephadex G-100 and Ultrogel ACA 44 columns and also by measurement of the ability of the factor to diminish the rate of chemical hydrolysis of the aminoacyl-tRNA's. The apparent positional specificity of the factor was found to be affected substantially both by the choice of guanine nucleotide and gel filtration resin utilized, but not in any systematic fashion. Likewise, assay of ternary complex formation by diminution of the rate of chemical deacylation failed to reveal any consistent positional preference from one isoacceptor to another. It is worthy of note that each modified aminoacyl-tRNA tested did form a ternary complex with EF-Tu under each of the experimental conditions used for assay, but that in each case the difference in affinity of the factor for isomeric aminoacyl-tRNA's was less than that between either of the modified aminoacyl-tRNA's and the corresponding unmodified species. On the basis of the experiments performed, we conclude that (i) EF-Tu has remarkable conformation flexibility, possibly reflecting its physiological role in recognizing 20 tRNA isoacceptors and (ii) the factor has no obvious preference for a single positional isomer of aminoacyl-tRNA and it is not clear that any preference that might exist could be established convincingly using tRNA's terminating in 2'- and 3'-deoxyadenosine.

Amino Acyl-tRNA Synthetases↗
PubMed1979

Structural studies of of "active complex" of bleomycin: assignment of ligands to the ferrous ion in a ferrous-bleomycin-carbon monoxide complex.

Proton NMR studies at 360 MHz establish the binary Fe(II)-bleomycin complex to be paramagnetic with a spectrum covering 70 ppm. Addition of carbon monoxide generates a stable, diamagnetic Fe(II)-bleomycin-CO complex that is a putative structural analog of the "active" Fe(II)-bleomycin-O2 complex. The following six groups have been determined to be coordinated to the Fe(II) ion from analysis of the highly resolved 1H NMR spectra of this complex: CO, the primary and secondary amine nitrogens of the beta-aminoalanine moiety, the carbamoyl moiety on the 3-position of mannose, the pyrimidine N-1, and the imidazole N-1. The Fe(II)-bleomycin-CO complex binds to DNA, as shown by fluorescence quenching experiments, but Fe(II)-bleomycin-CO does not mediate thymine release. These results necessitate a major revision in the current model for metal coordination to bleomycin.

Bleomycin↗
PubMed1979

Eukaryotic mRNA cap binding protein: purification by affinity chromatography on sepharose-coupled m7GDP.

A 24,000-dalton polypeptide that binds strongly and can be specifically crosslinked to the 5'-terminal cap structure m7GpppN in eukaryotic mRNAs has been detected in protein synthesis initiation factor preparations [Proc. Natl. Acad. Sci. USA (1978) 75, 4843--4847]. This polypeptide has been purified to apparent homogeneity by one chromatographic passage through an affinity resin prepared by coupling the levulinic acid O2',3'-acetal of m7GDP to AH-Sepharose 4B. Translation, in HeLa cell extracts, of capped mRNAs including Sindbis virus, reovirus, and rabbit globin mRNAs was stimulated by the cap-binding protein under conditions that did not increase translation of noncapped RNAs of encephalomyocarditis virus and satellite tobacco necrosis virus.

Animals↗
PubMed1978

2'-Versus 3'-OH specificity in tRNA aminoacylation. Further support for the "secondary cognition" proposal.

Purified Escherichia coli tRNAAla and tRNALys were each converted to modified species terminating in 2'- and 3'-deoxyadenosine. The modified species were tested as substrates for activation by their cognate aminoacyl-tRNA synthetases and for misacylation with phenylalanine by yeast phenylalanyl-tRNA synthetase. E. coli alanyl- and lysyl-tRNA synthetases normally aminoacylate their cognate tRNA's exclusively on the 3'-OH group, while yeast phenylalanyl-tRNA synthetase utilizes only the 2' position on its own tRNA. Therefore, the finding that the phenylalanyl-tRNA synthetase activated only those modified tRNAAla and tRNALys species terminating in 3'-deoxyadenosine indicated that the position of aminoacylation in this case was specified entirely by the enzyme, an observation relevant to the more general problem of the reason(s) for using a particular site for aminoacylation and maintaining positional specificity during evolution. Initial velocity studies were carried out using E. coli tRNAAla and both alanyl- and phenylalanyl-tRNA synthetases. As noted in other cases, activation of the modified and unmodified tRNA's had essentially the same associated Km values, but in each case the Vmax determined for the modified tRNA was smaller.

Amino Acyl-tRNA Synthetases↗
PubMed1978

"Chemical aminoacylation" of tRNA's.

Incubation of abbreviated tRNA's (tRNA-C-COH's) with (chemically) preaminoacylated P1, P2-di(adenosine 5'-)diphosphates in the presence of purified RNA ligase effected transfer of an aminoacyladenylate moiety to the 3'-terminus of the abbreviated tRNA's in good yield. Aminoacylated (or misacylated) tRNA's may thus be prepared from fractionated or unfractionated tRNA-C-COH's; each of the five aminoacylated dinucleoside diphosphates tested was utilized as a substrate by RNA ligase. That the resulting "chemically aminoacylated" tRNA's were identical with those prepared by enzymatic aminoacylation was judged by comparison of 1) chromatographic properties on benzolated diethylaminoethyl-cellulose, 2) rates of chemical deacylation, and 3) affinities for elongation factor Tu, as well as 4) the ability of misacylated tRNA's so derived to be deacylated chemically and then reactivated enzymatically with their cognate amino acids.

Adenine Nucleotides↗
PubMed1978

The effect of "cap" analogs on reovirus mRNA binding to wheat germ ribosomes. Evidence for enhancement of ribosomal binding via a preferred cap conformation.

A variety of compounds related to the 5'-terminal "cap" (m7GpppN) of eukaryotic mRNA's were chemically synthesized and tested as inhibitors of reovirus mRNA binding to wheat germ ribosomes. Under our conditions of mRNA binding to ribosomes, 7-methyl-, 7-ethyl-, and 7-benzyl-GDP, but not GDP, decreased stable initiation complex formation by 70 to 80% at a concentration of 0.1 mM indicating that 7-substitution, but not a specific substituent, was required for the effect. Elimination of the positive charge on the imidazole of the 7-substituted compounds by treatment with alkali destroyed their inhibitory activity. Similarly, reduction to 8-hydro-m7GDP reversibly decreased the activity of m7GDP. The results were consistent with the hypothesis that the positive charge resulting from 7-alkylation provides an active cap conformer for binding via interaction of phosphate oxygens with the positively charged imidazole moiety. In accord with this suggestion, 7-carboxymethyl GDP and 7,8-dimethyl GDP were found to be less inhibitory than m7GDP. A 2-amino group was also important since m7IDP was less effective than m7GDP and 0.1 mM m7XDP did not inhibit ribosome binding. Other poor inhibitors were 6-Cl-m7GDP and 1,7-dimethyl GDP but N2,7-dimethyl GDP, 2'-deoxy-m7GDP, and m7GppI had essentially the same activity as m7GDP.

Guanosine Triphosphate↗