C2 toxin (C2T) elaborated by Clostridium botulinum types C and D is composed of two separate
protein components, designated components I and II, which individually have little activity, but, when mixed and treated with
trypsin, exert the potent activity. The present study provides the evidence that component I of the toxin catalyzes the hydrolysis of
NAD into
nicotinamide and
ADP-ribose, whereas component II does not, indicating that component I of C2T has
NAD-glycohydrolase activity, which ability is shared with
cholera and
diphtheria toxins. However, C2T affected neither
glycerol production of fat cells nor
protein synthesis in cell-free system. Component I of C2T in the presence of [alpha-32P]
NAD radiolabeled a
protein of Mr 46,000 in the supernatant fractions of mouse tissue homogenates; the
protein was abundant in brain, lung and intestine, whereas there was little or none of the
protein in muscle. These results indicate that component I can catalyze the covalent attachment of the
ADP-ribose moiety of
NAD to intracellular
protein, which differs from those modified with
cholera and
diphtheria toxins. The present data, together with previous findings, suggest that the
biological activity of C2T is elicited by ADP-ribosylation activity of component I, which is internalized into the cells after binding to the receptor site introduced with the binding of component II to the cell surface membrane.