The enol tautomer of indole-3-pyruvic acid as a biological switch in stress responses.

Indole-3-pyruvic acid (IPA) undergoes in solutions to the keto-enol tautomerism, which appears responsible of its pharmacological effects, as only the enol tautomer is an easy target for oxygen free-radicals and can be transformed directly to kynurenic acid (KYNA). Contrary to expectations, the IPA enol tautomer is rather stable in mammalian tissues, due to the presence of specific tautomerases, favouring the formation of KYNA in the presence of free-radicals. Because of the synergistic effects between glucocorticoids, free-radicals and excitatory aminoacids in chronic stress, the enol tautomer of IPA and KYNA are proposed as physiological metabolites produced in order to shut-off the chronic stress cycle.