ADP-ribosylation of elongation factor 2 by diphtheria toxin. Isolation and properties of the novel ribosyl-amino acid and its hydrolysis products.

We have isolated the novel amino acid, amino acid X, in elongation factor 2 (EF-2) which is ADP-ribosylated by diphtheria toxin (Robinson, E. A., Hendriksen, O., and Maxwell, E. S. (1974) J. Biol. Chem. 249, 5088-5093). The pure ribosyl-amino acid was obtained in milligram quantities following sequential enzymatic degradation of yeast ADP-ribosyl-EF-2 (Van Ness, B. G., Howard, J. B., and Bodley, J. W. (1978) J. Biol. Chem. 253, 8687-8690) with an overall yield of 30 to 60%. In the course of acid hydrolysis, the ribosyl-amino acid was converted first to an intermediate compound which did not contain ribose. Further acid hydrolysis converted the intermediate to amino acid X and ammonia. Alkaline hydrolysis of the ribosyl-amino acid produced a different intermediate which contained ribose. We have isolated the two hydrolysis intermediates and on the basis of the properties of all four compounds we conclude that the ribosyl-amino acid contains an amido function which is not involved in a glycosidic linkage and a net positive charge of 2 at acid pH. We believe that the amino acid amide occurs in EF-2 and for this compound we propose the trivial name diphthamide. Acid hydrolysis of diphthamide yields amino acid X for which we propose the trivial name diphthine.