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D W Mullins

Publications and source records attributed to D W Mullins.

33 records · Page 2Linked to original sources
PubMed1985

Chirally selective, intramolecular interaction observed in an aminoacyl adenylate anhydride.

All earthly creatures use only L-amino acids in template directed protein synthesis. The reason for this exclusive use of the L-isomer is not yet apparent, although recent experiments by Usher and his colleagues have shown some stereoselectivity in the aminoacylation of di- and polynucleotides. We have separately reported on intramolecular interactions between hydrophobic amino acid side chains and the adenine ring in aminoacyl adenylates. There was a preferential association of Phe greater than Leu = Ile greater than Val with the adenine in these studies, but we made no attempts to address the question of D, L selectivity. Recently, in 1H NMR studies of N-acetylphenylalanyl adenylate anhydride, we noticed evidence that both D- and L-isomers of the amino acid were present and, furthermore, that one isomer seemed to be associating with the adenine ring more strongly than the other. Using HPLC, we have separated the two diastereoisomers and have enzymatically determined that the isomer which associates more strongly is the biologically important one, the L-isomer. We present those studies here and discuss the evolutionary significance of this finding.

Adenosine Monophosphate↗
PubMed1984

Mechanisms of streptozotocin- and alloxan-induced damage in rat B cells.

In studies to evaluate possible inhibitors of the B-cell toxin, streptozotocin, the superoxide scavenger, superoxide dismutase, did not prevent or reduce the toxic effects of streptozotocin as determined by loss of insulin secretion from rat pancreatic B cells in monolayer culture. However, 1,1-dimethyl urea, a scavenger of the hydroxyl radical, did afford significant protection. Both scavengers diminished the cytotoxic effects of alloxan. The inhibitors of poly (ADP-ribose) synthetase, 3-aminobenzamide and nicotinamide, also were effective in attenuating alloxan- and streptozotocin-induced B-cell toxicity. Tests of the hydroxyl-scavenging ability of the three streptozotocin antagonists revealed that 3-aminobenzamide, nicotinamide and 1,1-dimethyl urea were effective scavengers of this free radical. Conversely, 1,1-dimethyl urea, although not as potent as 3-aminobenzamide or nicotinamide, was found to inhibit poly (ADP-ribose) synthetase. These data indicate that these chemicals most likely attenuate alloxan-induced toxicity by scavenging the hydroxyl radical and diminish streptozotocin-induced toxicity by inactivation of the poly (ADP-ribose) system.

Alloxan↗
PubMed1984

The case for the anticode.

The present paper will focus on the developments in our lab related to the origin of the code since the Israel meeting (1,2). Principally these items are: (a) a new set of correlations (3) which include ranked hydrophobicities of amino acids and dinucleotides; (b) binding constants (4) of Phe for the four mononucleotides; and (c) binding constants (5) of Phe, Leu, Ile, Val, and Gly for polyadenylic acid (poly A). The data continue to support a model for the origin of the code based on relationships between amino acids and their anticodons.

Amino Acids↗
PubMed1984

Aminoacyl-nucleotide reactions: studies related to the origin of the genetic code and protein synthesis.

In the present paper, we report on the effect of pH and carbonate on the hydrolysis rate constants of N-blocked and free aminoacyl adenylate anhydrides. Whereas the hydrolysis of free aminoacyl adenylates seems principally catalyzed by OH-, the hydrolysis of the N-blocked species is also catalyzed by H+, giving this compound a U-shaped hydrolysis vs. pH curve. Furthermore, at pH's less than 8, carbonate has an extreme catalytic effect on the hydrolysis of free aminoacyl-AMP anhydride, but essentially no effect on the hydrolysis of N-blocked aminoacyl-AMP anhydride. Furthermore, the N-blocked aminoacyl-AMP anhydride is a very efficient generator of peptides using free glycine as acceptor. The possible significance of the observations to prebiological peptide synthesis is discussed.

Adenosine Monophosphate↗
PubMed1984

Hydrolytic properties of phenylalanyl- and N-acetylphenylalanyl adenylate anhydrides.

We have used a novel spectrophotometric method to study the hydrolysis of N-acetylphenylalanyl adenylate anhydride (AcPhe-AMP) and phenylalanyl-adenylate anhydride (Phe-AMP) at low concentrations (10(-5) M), 25 degrees C, constant buffer concentration (0.05 M), and as a function of pH. While Phe-AMP is susceptible principally to attack by OH-, with two different rates depending on whether the alpha-amino group of the amino acid is protonated or not, the AcPhe-AMP is susceptible to acid decomposition as well. At pH's 4-8, the Phe-AMP hydrolyzes faster than the AcPhe-AMP, but at pH less than 4 or pH greater than 8, the blocked form hydrolyzes faster. Both forms are also attacked by H2O, and at the same rate. Moreover, the hydrolysis of Phe-AMP is shown to be greatly catalyzed by carbonate, although the AcPhe-AMP is not subject to such catalysis. The rate laws for the various mechanisms and the activation energies for the hydrolyses at pH 7.1 are given.

Adenosine Monophosphate↗
PubMed1984

Binding constants of phenylalanine for the four mononucleotides.

Earlier work has shown that several properties of amino acids correlate directly with properties of their anticodonic nucleotides. Furthermore, in precipitation studies with thermal proteinoids and homopolyribonucleotides, an anticodonic preference was displayed between Lys-rich, Pro-rich and Gly-rich thermal proteinoids and their anticodonic polyribonucleotides. However, Phe-rich thermal proteinoid displayed a preference for its codonic nucleotide, poly U. This inconsistency seemed to be explained by a folding in of the hydrophobic residues of Phe causing the proteinoid to appear more hydrophilic. The present work used nuclear magnetic resonance techniques to resolve a limited question: To which of the four nucleotides does Phe bind most strongly? The results show quite clearly that Phe binds most strongly to its anticodonic nucleotide, AMP.

Adenosine Monophosphate↗
PubMed1983

Complexes of polyadenylic acid and the methyl esters of amino acids.

This report includes studies of the binding of the methyl esters of a series of amino acids to polyadenylic acid. The principal data were obtained using proton NMR; however, some additional data were obtained through the study of insoluble complexes and through ultraviolet spectroscopy. The binding constants are in the order Phe greater than Ile greater than or equal to Leu greater than Val greater than Gly, and show a direct correlation with the hydrophobicities of the amino acids. In most cases they are essentially double the binding constants found by Reuben and Polk (1980) for monomeric AMP. All of these amino acids, except Gly, have A as the middle letter of their anticodons, and Phe is the only one with XAA as its only anticodon. It has the anticodon richest in A and has the highest binding constant for A. These results, coupled with other data, continue to support a model of the origin of the code which is based on weak, but selective affinities between amino acids and their anticodons.

Amino Acids↗
PubMed1980

Studies of the chemical basis of the origin of protein synthesis: initiation and direction of peptide growth.

The data presented in this paper show that the ease of non-enzymatic activation of carboxylic acids by ATP at pH 5 varies directly with the pKa of the carboxyl group, and is consistent with the idea that it is the protonated form of the carboxyl group which participates in the activation reaction. Consequently, since most N-blocked amino acids have higher pKa's than do their unblocked forms, they are activated more readily, and we have demonstrated that this principle applies to peptides as well, which are activated more rapidly than single amino acids. We propose that this fact may be partly responsible for the origin of two important features still observed in contemporary protein synthesis: (1) initiation in prokaryotes is accomplished with an N-blocked amino acid, and (2) elongation in all living systems occurs at the carboxyl end of the growing peptide.

Adenosine Triphosphate↗
PubMed1977

Poly(adenosine diphosphate-ribose) polymerase: the distribution of a chromosome-associated enzyme within the chromatin substructure.

The distribution of a chromatin-bound, nuclear protein modifying enzyme, poly (adenosine diphosphate-ribose) polymerase, and its product, poly(ADP-ribose), among various fractions of sheared and nuclease-digested HeLa cell chromatin has been examined. Epichlorohydrin-tris(hydroxymethyl)aminomethane-cellulose and glycerol gradient fractionation of solubilized chromatin indicated that poly(ADP-ribose)polymerase activity was associated primarily with the template active regions (euchromatin), whereas the transcriptionally inert chromatin fractions were found to contain relatively low levels of ADP-ribosylating activity. When isolated HeLa cell nuclei were digested in situ with micrococcal nuclease and the resultant chromatin was fractionated into nucleosome monomers (v bodies) and oligomers by sucrose gradient centrifugation, only material sedimenting faster than the 11S monomers was found to contain appreciable poly(ADP-ribose) polymerase activity. If, on the other hand, isolated HeLa cell nuclei were first incubated with labeled NAD, the substrate for poly(ADP-ribose) polymerase, prior to the preparation and fractionation of nuclease-digested chromatin, it was found that those chromatin fractions which possess significant poly(ADP-ribose) polymerase activity (nucleosome oligomers) are relatively deficient in the labeled product of this enzyme, and that a considerable portion of the homopolymeric product is ultimately associated with the 11S v bodies. Additional evidence is presented which indicates that the absence of nucleosome monomer-associated poly(ADP-ribose) polymerase activity is not due to the absence of a suitable acceptor on these structures, and that the activity of this enzyme within the chromatin is most probably dependent upon the physical integrity of the oligomeric structures themselves.

Cell Nucleus↗
PubMed1977

A putative role for nicotinamide adenine dinucleotide-promoted nuclear protein modification in the antitumor activity of N-methyl-N-nitrosourea.

Incubation of HeLa cells with the anticancer agent N-methyl-N-nitrosourea (MNU) results in: (a) depression of intracellular nicotinamide adenine dinucleotide levels; (b) stimulation of the chromatin-associated, chromosomal protein-modifying enzyme polyadenosine diphosphoribose [poly(ADP-ribose)] polymerase, which uses nicotinamide adenine dinucleotide as substrate; and (c) some fragmentation of cellular DNA. DNase treatment of HeLa nuclei in vitro also stimulates poly(ADP-ribose) polymerase activity, but not in nuclei derived from MNU-treated cells unless they have been subsequently incubated to allow for recovery from MNU damage. DNA polymerase activity is stimulated in vitro by poly(ADP) ribosylation of nuclear proteins. By using intact nuclei derived from MNU-treated HeLa cells, the repair via elongation of single-strand DNA breaks is demonstrated in vitro. This repair is dependent on DNA polymerase activity and is enhanced by adenosine diphosphate ribosylation of histones. Inhibition of poly(ADP-ribose) polymerase with nicotinamide results in extensive degradation of MNU-damaged DNA. Taken as a whole, these results suggest that poly(ADP-ribose) polymerase may play a role in the repair of alkylation damage to cellular DNA and that the inhibition of this enzyme in vivo might be exploited to potentiate the antitumor and carcinogenic activities of MNU.

Alkylation↗
PubMedDate not supplied

Restoration of hyperthermia-associated increased protein to DNA ratio of nucleoids.

The sedimentation of L1210 nucleoids has been used to demonstrate a hyperthermia-associated increased protein to DNA ratio and an apparent inhibition of processes involved in the restoration of the protein to DNA ratio. The distance of nucleoid sedimentation increased as a function of exposure temperature and exposure time, and was proportional to an increased protein to DNA ratio in the nucleoids. Studies in which control and hyperthermia-treated cells were mixed prior to nucleoid preparation indicated that the association of protein with the nucleoid occurred during hyperthermia treatment and not during nucleoid preparation. However, double-labelling studies suggested an interaction between control and hyperthermia-treated cells since the presence of control cells during lysis resulted in near normalization of nucleoid sedimentation. Treatment with proteinase K also restored the nucleoid sedimentation. Incubation at 37 degrees C following hyperthermia revealed a rapid restoration of nucleoid sedimentation and a slower restoration of the protein to DNA ratio. Ethidium bromide-induced changes in nucleoid sedimentation were altered by the hyperthermia-associated increased protein content of nucleoids and the alterations were overcome by enzymatic digestion of the protein prior to the ethidium bromide exposure. Thus, hyperthermia caused, and inhibited the repair of, an increased protein content of nucleoids. The restoration of the increased protein possibly occurs by a heat-sensitive proteolytic enzyme. The temporal use of an appropriate chemotherapy agent to inhibit the restoration of hyperthermia-associated changes may be a useful treatment option.

Animals↗