Diphtheria Toxin In

Recent studies have demonstrated that diphtheria toxin is an enzyme of unusual type. Small amounts of the toxin, added to nicotinamide adenine dinucleotide (NAD)-containing mammalian cell extracts, block peptide-bond formation by catalyzing inactivation of the translocating enzyme, aminoacyltransferase 2 (T2) (1-3). This highly specific reaction involves the splitting of NAD with liberation of free nicotinamide and transfer of the adenosine diphosphate ribosyl moiety (ADPR) to be bound in covalent linkage to T2 (4, 5). Although the reaction is reversible, the equilibrium lies far over on the side of T2 inactivation. Only "free" T2 is inactivated by toxin; ribosome-bound T2 is not attacked (2, 6). The reaction is a highly specific one and no tissue protein other than soluble T2 has been found capable of accepting ADPR from NAD in the presence of toxin. ~ That the same reaction occurs when living ceils in culture are treated with toxin was shown by Gill et al. (5) who found that inactive T2, extracted from intoxicated HeLa cells that were no longer capable of protein synthesis, could be specifically reactivated to full activity by addition of toxin in the presence of excess nicotinamide. Bonventre and Imhoff (8) have reported that when diphtheria toxin is injected into guinea pigs, reduction in the rate of leucine incorporation into protein can be demonstrated in heart and pancreas only. In other organs from intoxicated guinea pigs in which morphological damage is known to occur, they were unable to detect a decreased leucine uptake. More recently, Bowman and Bonventre (9) have examined the rate of 14C-leucine incorporation in extracts of tissue excised from intoxicated guinea pigs and once again concluded that inhibition of protein synthesis is mainly confined to heart muscle. In our laboratory we have failed to observe such organ specificity. In the present