SPECIlqCITY OF THE COFACTOR (NAD) REQUIREMENT FOR TOXIN ACTION IN CELL-FREE SYSTEMS BY RONALD

I t was shown by Collier and Pappenheimer (1) that inhibition, by diphtheria toxin, of amino acid incorporation into polypeptides in cell-free systems isolated from mammalian cells requires the presence of nicotinamide adenine dinucleotide (NAD) as an essential cofaetor. In the absence of NAD, there is no inhibition of amino acid incorporation, even in the presence of high toxin concentrations. More recently, Collier (2) and ourselves (3) have demonstrated that toxin causes inhibition of peptide bond formation (in vitro) by inactivation of a soluble enzyme, transferase II . However, the mechanism by which NAD helps to bring about this inactivation has not been elucidated. The early studies (1) had already suggested that the NAD requirement is highly specific since NAD cannot be replaced by nicotinamide adenine dinucleotide phosphate (NADP).