Mechanism of action ofVibrio cholerae enterotoxin

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Abstract
The characteristics of the cholera toxin-stimulated adenylate cyclase of toad (Bufus marinus) and rat erythrocyte plasma membranes have been examined, with special emphasis on the response to purine nucleotides, fluoride, magnesium and catecholamine hormones. Toad erythrocytes briefly exposed to low concentrations of cholera toxin (40,000 to 60,000 molecules per cell) and incubated 2 to 4 hr at 30°C exhibit dramatic alterations in the kinetic and regulatory properties of adenylate cyclase. The approximateK m for ATP, Mg++ increases from about 1.8 to 3.4mm in the toxinstimulated enzyme. The stimulation by cholera toxin increases with increasing ATP, Mg++ concentrations, from 20% at low levels (0.2mm) to 500% at high concentrations (greater than 3mm). Addition of GTP, Mg++ (0.2mm) restores normal kinetic properties to the toxin-modified enzyme, such that stimulation is most simply explained by an elevation ofV max. GTP enhances the toxin-treated enzyme activity two-to fourfold at low ATP concentrations, but this effect disappears at high levels of the substrate. At 0.6mm ATP and 5mm MgCl2 the apparentK a for GTP, Mg++ is 5 to 10μm. The control (unstimulated) enzyme demonstrates a very small response to the guanyl nucleotide. 5′-ITP also stimulates the toxin-treated enzyme but cGMP, guanine, and the pyrimidine nucleotides have no effect. Cholera toxin also alters the activation of adenylate cyclase by free Mg++, decreasing the apparentK a from about 25 to 5mm. (−)-Epinephrine sensitizes the toad erythrocyte adenylate cyclase to GTP and also decreases the apparentK a for free metal. Sodium fluoride, which cause a 70- to 100-fold activation of enzyme activity, has little effect on sensitivity to GTP, and does not change the apparentK a for Mg++; moreover, it prevents modulation of these parameters by cholera toxin. Conversely, cholera toxin severely inhibits NaF activation, and in the presence of fluoride ion the usual three- to fivefold stimulation by toxin becomes a 30 to 60% inhibition of activity. The toxin-stimulated enzyme can be further activated by catecholamines; in the presence of GTP the (−)-epinephrine stimulation is enhanced by two- to threefold. The increased catecholamine stimulation of toad erythrocyte adenylate cyclase induced by cholera toxin is explained primarily by an increase in the maximal extent of activation by the hormones. Rat erythrocyte adenylate cyclase is also modified by cholera toxin. In the mammalian system the apparent affinity for the hormone appears to be increased. Cholera toxin thus induces profound and nearly permanent changes in adenylate cyclase by a unique process which mimics the stimulation by hormones in important ways, and which also accentuates the normal hormonal response. The relevance of these findings to the mechanism of action of cholera toxin is considered.