Novel S‐Adenosylmethionine‐Dependent Indole‐N‐Methylation of β‐Carbolines in Brain Particulate Fractions

Abstract

Guinea pig brain S-adenosylmethionine (SAM)-dependent N-methyltransferase activity toward physiologically relevant β-carboline (BC) substrates was examined with reverse-phase HPLC and radiochemical detection. Representative BCs, norharman and harmine, were enzymatically methylated on the 2[β]-nitrogen by [³H]CH₃-SAM in undialyzed homogenates to yield 2[β]-methylated BCs and subsequently on the 9[indole]-nitrogen to generate 2,9-dimethylated BC products. This may be the first account of mammalian indole N-methyl transfer. There was no HPLC evidence for 9-methyl BC or (from carbon methylation) 2,6-dimethyl BC products. Capillary gas chromatography-mass spectrometry analysis confirmed the structures of the 2,9-dimethyl and 2-methyl products of norharman. The 2[β]- and 9[indole]-N-methylation activities were mainly in the nuclear fractions and were negligible in undialyzed cytosol. This differs from the cytosolic SAM-dependent N-methylations reported with other azaheterocyclics, including 1,2,3,4-tetrahydro-BCs. The involvement of a single enzyme was suggested because the two N-methyl transfers with BC substrate had similar subcellular activity patterns, regional brain distributions, and K_m and V_max values. Sequential N-methylation of various BCs that have been observed in vivo may be a unique route to centrally retained N²,N⁹-dimethylated β-carbolinium ions. Because they resemble the synthetic parkinsonian toxicant, N-methyl-4-phenylpyridinium, with respect to structure and neurotoxic activity, such “bioactivated” carbolinium ions could be endogenous causative factors in Parkinson's disease.