Artificial low-molecular-mass substrates of cholera toxin.

A model system, measuring the rate of cholera-toxin-catalysed release of nicotinamide from NAD+, has been used to identify novel compounds which may serve as substrates for toxin-directed ADP-ribosylation. In a series of guanidine-containing compounds, those in which the guanidine group was connected to a large hydrophobic domain greatly stimulated the rate of toxin-catalysed nicotinamide release. The introduction of a charge centre near the guanidine group destroyed all activity. The compounds thus identified were found to inhibit the action of cholera toxin on rat liver adenylate cyclase, and this was associated with a reduction in the amount of [32P]ADP-ribosylation of a 42-kDa protein in the membranes. Guanidine-containing compounds which did not enhance toxin-catalysed release of nicotinamide from NAD+ had no effect on toxin action on adenylate cyclase, and there was a good correlation between the two activities. The results are discussed in relation to the known properties of the guanine nucleotide regulatory protein associated with adenylate cyclase systems, which is the toxin's natural substrate.