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A K Abraham

Publications and source records attributed to A K Abraham.

32 records · Page 2Linked to original sources
PubMed1981

Effect of polyamines on the fidelity of macromolecular synthesis.

Addition of polyamines to in vitro systems containing suboptimal concentrations of Mg2+ markedly stimulated protein and RNA synthesis. This stimulation is observed only within a marrow range of polyamine concentration. The extend of stimulation of RNA synthesis was dependent on assay conditions. Addition of spermidine to the wheat germ system not only stimulated poly(U) directed polyphenylalanine synthesis, but also reduced the misincorporation of leucine. MS2-coat protein synthesis, studied in an E.coli cell-free system using either one of the two glutamyl-tRNAs as the only source of glutamine, suggested that in the presence of spermidine, codon recognition by these two isoacceptor tRNA molecules was more stringent. From these results it is concluded that polyamines contribute to the specificity of codon/anticodon interactions and thereby increase the fidelity of protein synthesis.

Codon↗
PubMed1980

Turnover of the poly(A) moiety of mRNA in wheat-germ extract.

The synthesis and degradation of poly(A) were examined in wheat germ lysates under conditions used for protein synthesis. The lysate contained both poly(A)-polymerizing and poly(A)-hydrolytic activities. The synthetic activity was dependent on the presence of either poly(A) or polyadenylated mRNA as primer. Concurrent with the synthesis of poly(A), radioactivity was released from labeled poly(A) and from the poly(A) region of mRNA. Both poly(A) synthesis and hydrolysis were independent of Mn2+ and could proceed in the presence of Mg2+, the major divalent metal ion required for protein synthesis. The synthetic activity was favoured at high (1 mM) ATP concentration, whereas the hydrolytic activity was maximal in the absence of ATP or at low (0.1 mM) ATP concentration. These data indicate that the steady-state length of the poly(A) moeity of mRNAs in the cytoplasm may be regulated by the levels of ATP. Translation products of polyadenylated mRNAs isolated from myeloma cells before and after partial deadenylation were separated by dodecylsulfate/polyacrylamide gel electrophoresis. Shortening of the poly(A) moiety did not alter the relative amounts of the peptides synthesized, indicating that this process does not involve a preferential breakdown of certain species of mRNA.

Adenosine Triphosphate↗
PubMed1980

Variable rate of polypeptide chain elongation in vitro. Effect of spermidine.

Translation of rabbit globin mRNA and tobacco mosaic virus mRNA was studied in a wheat germ system. After a short pulse with labeled methionine, the reaction products were separated by polyacrylamide gel electrophoresis in sodium dodecyl sulfate and visualized by fluorography. Discrete bands corresponding to incomplete products were detected both with globin and tobacco mosaic virus RNA as messenger at a time when no release of peptides from tRNA could be detected. In the absence of spermidine certain intermediate products accumulated during the chase period. With increasing incubation time, the radioactivity of most bands was chased into higher-molecular-weight products. This change in size distribution was more pronounced when magnesium was partially replaced by spermidine. Evidence is presented that some of the incomplete products had in part been released from tRNA. No such release of polypeptides could be detected in experiments using the artificial messenger poly(U), indicating that non-specific peptidyl-tRNA hydrolases were not present in the system. The transient accumulation of discrete bands during the chase period indicates that in the wheat germ system polypeptide chain elongation occurs at discontinuous rates. The fact that spermidine increases the rate of elongation and the yield of full-length translation products may be due to its ability to facilitate the movement of the ribosomes beyond regions where the elongation is retarded.

Animals↗
PubMed1978

Hydrolysis of poly (A) to adenine nucleotides by purified poly (A) polymerase.

Highly purified poly(A) polymerase (polynucleotide adenylyltransferase, EC 2.7.7.19), which synthesizes poly(A) from ATP substrate, can also catalyze hydrolysis of poly(A). The enzyme, designated as poly(A) hydrolase, requires either Mn2+ or Mg2+ for activity. Although AMP is the predominant product of the reaction, ADP and ATP are also formed. The enzyme is a 3'-exonuclease that does not degrade poly(A) associated with poly(A) poly(U) helical structure. AMP, ADP, and ATP inhibit the hydrolytic reaction. These data suggest that (i) the levels of adenine nucleotides regulate synthesis and degradation of poly(A), (ii) poly(A) itself is a storage form of adenine nucleotides, (iii) the hydrolytic reaction is responsible for poly(A) shortening or turnover observed in vivo, and (iv) the synthetic and hydrolytic activities are functions of the same protein molecule.

Adenine Nucleotides↗
PubMed1975

On the mechanism of protein-synthesis inhibition by abrin and ricin. Inhibition of the GTP-hydrolysis site on the 60-S ribosomal subunit.

The mechanism of protein synthesis inhibition by the toxic lectins, abrin and ricin, has been studied in crude and in purified cell-free systems from rabbit reticulocytes and Krebs II ascites cells. In crude systems abrin and ricin strongly inhibited protein synthesis from added aminoacyl-tRNA, demonstrating that the toxins act at some point after the charging of tRNA. Supernatant factors and polysomes washed free of elongation factors were treated separately with the toxins and then neutralizing amounts of anti-toxins were added. Recombination experiments between toxin-treated ribosomes and untreated supernatant factors and vice versa showed that the toxin-treated ribosomes had lost most of their ability to support polyphenylalanine synthesis, whereas treatment of the supernatant factors with the toxins did not inhibit polypeptide synthesis. Recombination experiments between toxin-treated isolated 40-S subunits and untreated 60-S subunits and vice versa showed that only when the 60-S subunits had been treated with the toxins was protein synthesis inhibited in the reconstituted system. The incorporation of [3H]puromycin into nascent peptide chains was unaffected by the toxins, indicating that the peptidyl transferase is not inhibited. Both the EF-1-catalyzed and the EF-2-catalyzed ability of the ribosomes to hydrolyze [gamma-32P]GTP was inhibited by abrin and ricin. An 8-S complex released from the 60-S subunit by EDTA treatment possessed both GTPase and ATPase activity, while the particle remaining after the EDTA treatment had lost most of its GTPase activity. Both enzyme activities of the 8-S complex were inhibited by abrin and ricin. The present data indicate that there is a common site on the 60-S subunits for EF-1- and EF-2- stimulated GTPase activity and they suggest that abrin and ricin inhibit protein synthesis by modifying this site.

Abrin↗
PubMedDate not supplied

Total occlusion of the left main coronary artery in chronic stable angina pectoris.

We present the clinical and angiographic profile of three patients with class I stable angina pectoris. All had strong coronary risk factors, and stress testing was positive in stage one of the Bruce protocol. Coronary angiography revealed total occlusion of the left main coronary artery (LMCA), and aortocoronary bypass surgery was performed. Thus, total LMCA occlusion may be an unexpected angiographic finding in patients with class I angina.

Adult↗