The interaction between rat and human liver cytochrome P-450 with tentoxin, a natural phytotoxic cyclotetrapeptide having chlorotic properties, was studied by difference ultraviolet visible spectroscopy. Tentoxin interacted with rat liver microsomes and the difference spectrum was characteristic of binding to a protein site close to the heme. The intensity of this spectrum was clearly dependent on the amounts of P-450 3A in the microsomes and was optimal in dexamethasone-treated rat microsomes. Tentoxin exhibited a high affinity for P-450 3A (Ks approximately 10 microM). Similar results were observed with human P-450 isozymes expressed in yeast. Only P-450 3A4 and 3A5 were able to give spectral interactions with tentoxin. Liver microsomes from rats pretreated with dexamethasone, a specific inducer of P-450 3A, were found to be particularly active for the oxidation of tentoxin, which occurs mainly on its Ala(Me) function leading to demethylation. Yeast-expressed P-450 3A also exhibited high activity to metabolize tentoxin. The metabolites were identified by their ultraviolet and mass spectra in fast atom bombardment and collision-activated dissociation modes. In addition to the major N-demethylated metabolite, other hydroxylated metabolites were formed. Preliminary analysis showed that as tentoxin, some metabolites were still efficient chloroplast ATPase inhibitors, while at least one of them exhibited even at low concentration stimulatory effects.