Inhibition of soluble transferase II activity in cell-free systems by diphtheria toxin and NAD can be prevented or reversed in the presence of a sufficient concentration of nicotinamide. Quantitative studies on inhibition of peptide bond formation in cell-free extracts by toxin and NAD have indicated that two successive reversible reactions are involved. First, toxin and NAD interact mole for mole to form a relatively dissociable complex. This toxin-NAD complex then reacts with transferase II to form an enzymatically inactive product that is but slightly dissociated. In the presence of sufficient nicotinamide, however, the latter complex can be broken down to yield active transferase II once more.

Based on the above model, an equation has been derived that accurately predicts the per cent inhibition of amino acid incorporation in cell-free systems at any given toxin and NAD level. The observed inhibition appears to be independent of the sensitivity to toxin of the cell species from which the extracts were derived, and depends only on the toxin and NAD concentrations.

Although the model satisfactorily explains inhibition of peptide bond formation by toxin in cell-free systems, further assumptions are needed to explain how still lower concentrations of toxin are able to arrest protein synthesis completely in the living cell.

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