The Interaction of Aminoacyl Transferase II and Ribosomes

Binding of periodate-oxidized guanine nucleotides to eukaryotic elongation factor 2

Periodate-oxidized guanine nucleotides (GTPox and GDPox) were shown to bind stoichiometrically to rat liver elongation factor 2 (EF-2). This binding was quantitatively inhibited in the presence of GTP. After binding, oxidized nucleotides remained on EF-2 despite extensive dialysis. They exchanged, however, with free quanine nucleotides in the course of prolonged (greater than 1 h) incubations. The prior reduction EF-2.GTPox with NaBH4 abolished, to a large extent, this slow exchange. Thus, a Schiff's base was implicated to be formed between EF-2 and oxidized guanine nucleotides. Mg2+ increased the GTPox concentration necessary for a stoichiometric binding to EF-2. EF-2-oxidized nucleotide conjugates bound in the presence of ribosomes a second molecule of GTP (or GTPox). GTPox bound to EF-2 in the presence of ribosomes appeared to exchange readily with free GTP. Moreover, GTPox proved to be active as substrate in EF-2 and ribosome-dependent GTPase reaction: Km values found for GTPox and GTP were 7.7 and 3.4 microM, respectively. The binding of GTPox to EF-2 inhibited only partially the subsequent ribosome-dependent GTP binding, and GTPase reaction or polyphenylalanine (polyPhe) synthesis. On the other hand, the binding of GuoPP[CH2]Pox to EF-2 inhibited all of these reactions strongly. The nature of the binding site involved in the direct interactions of EF-2 with guanine nucleotides is discussed in the light of these results.

Cystathionase activity and glutathione metabolism in redifferentiating rat hepatocyte primary cultures

Capacity to incorporate methionine sulfur into glutathione as well as cystathionase activity were lost in cultured hepatocytes in a biphasic manner with 75% of the total capacity disappearing with a half-life of about 10.6 hr, the remainder with a half-life of greater than 20 hr. Nicotinamide, 25 mM, produced a single phase loss with a t 1/2 of approximately 21 hr for both transsulfuration and cystathionase activity. Loss of both methionine sulfur incorporation and cystathionase activity occurred in transferrin/sodium selenite-supplemented Williams Medium E (TS-HWME) with a t 1/2 of about 96 hr through 72 hr in culture. Addition of the cystathionase inhibitor, propargylglycine, blocked glutathione synthesis in TS-HWME cells through 48 hr in culture, while propargylglycine blocked glutathione synthesis only at 4 hr in HWME cultured cells. Further, the accumulation of gamma-glutamyl transpeptidase was delayed by 48 hr in TS-HWME versus unsupplemented medium. Variation in the transport of sulfur amino acids was also found to occur with culture age. The Km values for cysteine and methionine transport were found to be approximately 150 and 100 microM, respectively, and were unaffected by culture age or the presence of TS-HWME. However, the Vmax for transport of methionine declined from 0.29 to 0.012 nmol/min/mg protein over 48 hr in culture. In TS medium, the Vmax at 48 hr for methionine transport had only decreased to 0.20 nmol/min/mg protein and increased for cysteine transport to 0.17 nmol/min/mg protein. These data suggest that during the redifferentiation of hepatocytes in culture, transsulfuration is regulated by control of the flow of substrate through cystathionase and that cystathionase is regulated by alteration of enzyme activity or content. Variations in the rate of transport of precursor sulfur amino acids are also an important component of the regulation of the net glutathione status of the redifferentiating hepatocyte.

Diphtheria toxin subunit active in vitro

Exposure of diphtheria toxin to dithiothreitol (and similar thiols) resulted in a subunit which was active in catalyzing the adenosine diphosphateribosylation of mammalian aminoacyl-transferase II in the presence of nicotinamide adenine dinucleotide. At the same time there was a marked increase in total ADP-ribosylation activity. A molecule which was apparently identical to the derived subunit in size and activity was detected in partially purified preparations of toxin.