Alkylation of A/2 in Deoxyguanosine by Dehydroretronecine, a Carcinogenic Metabolite of the Pyrrolizidine Alkaloid Monocrotaline1

Dehydroretronecine, an antimitotic and carcinogenic agent, reacted with deoxyguanosine at pH 7.4 in vitro to yield in nearly equal quantities two major adducts, which were isolated by thin-layer and high-pressure liquid chromatography. The adducts were stable at pH 8.7 and 30°for at least 24 hr, showed pK.s at acidic and alkaline pHs, reacted with 4-(p-nitrobenzyl)pyridine, had a nonexchangeable guan-8-yl proton, and con tained equimolar quantities of the pyrrole and nucleoside moie ties. From the above data and comparison of proton magnetic resonance spectra of dehydroretronecine, deoxyguanosine, and the adducts, both products were identified as derivatives with a bond between C-7 of dehydrosupinidine and N2 of deoxyguanosine. Mass spectral fragmentation patterns and infrared and ultraviolet absorbance spectra were also consist ent with A/2substitution. Circular dichroism spectra established the identities of each of the adducts as 7-(deoxyguanosin-/V2yl)dehydrosupinidine; they are enantiomeric at C-7. These re sults demonstrate that the reactive electrophile derived from protonated dehydroretronecine readily alkylates the N2 posi tion of deoxyguanosine at C-7 in an S,V1reaction to yield a racemic mixture of products.