Photolabelling of cholera toxin by NAD+

When cholera toxin is incubated under u.v. light with NAD+ labelled in either the adenine or the nicotinamide moiety, radioactivity becomes covalently bound to the protein. The reaction is specific for cholera toxin, and is inhibited by excess unlabelled NAD+ or NAD analogues. Only the active A 1 chain of the toxin is labelled. The u.v.-absorption spectrum of the product is very similar to that of NAD+, and shows the same reaction with cyanide. The nature of the product is therefore different from that found when diphtheria toxin is photolabelled [Carroll & Collier (1984) Proc. Natl. Acad. Sci. U.S.A. 81, 3307-3311] in that the yield is lower, but both moieties of the NAD molecule become bound.