Metabolic cleavage of frangufoline in rodents: In vitro and in vivo study

Frangufoline, a sedative 14-membered frangulanine-type cyclopeptide alkaloid, was found to be rapidly converted, via enzymatic process, in vitro and in vivo in rodents to M1 ((S)-(N,N-dimethylphenylalanyl)-(2S,3S)-3-[(p- formylphenoxy)leucyl]-(S)-leucine), which is a substituted linear tripeptide. The reaction did not require low molecular weight cofactors, and mammalian serum failed to catalyze the reaction. Structure-reactivity study of cyclopeptide alkaloid analogs suggested that the enamide bond is the site being cleaved, and the reaction was inhibited by organophosphorus esters such as BPNP and by eserine at higher concentrations but not by eserine at lower concentrations or by EDTA and PCMB. On the basis of these results, a possible mechanism for metabolic conversion of frangufoline to M1 was proposed, in which oxidation of the vinyl group and enzyme-catalyzed hydrolysis of the adjacent amide bond, possibly by B-esterase-like enzyme, proceed in a concerted manner.