title: Novel S-adenosylmethionine-dependent indole-N-methylation of beta-carbolines in brain particulate fractions.
abstract: Guinea pig brain S-adenosylmethionine (SAM)-dependent N-methyltransferase activity toward physiologically relevant beta-carboline (BC) substrates was examined with reverse-phase HPLC and radiochemical detection. Representative BCs, norharman and harmine, were enzymatically methylated on the 2[beta]-nitrogen by [3H]CH3-SAM in undialyzed homogenates to yield 2[beta]-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[beta]- 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 Km and Vmax values. Sequential N-methylation of various BCs that have been observed in vivo may be a unique route to centrally retained N2,N9-dimethylated beta-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.
authors.affiliation: Department of Molecular and Cellular Biochemistry, Loyola University of Chicago Stritch School of Medicine, Maywood, Illinois 60153.
authors.forename: K
authors.initials: K
authors.lastname: Matsubara
authors.forename: E J
authors.initials: EJ
authors.lastname: Neafsey
authors.forename: M A
authors.initials: MA
authors.lastname: Collins
mesh_terms: D000818:Animals; D001921:Brain / Q000166:cytology / Q000378:metabolism*; D001923:Brain Chemistry; D002243:Carbolines / Q000032:analysis / Q000378:metabolism*; D002458:Cell Fractionation; D002851:Chromatography, High Pressure Liquid; D005260:Female; D008401:Gas Chromatography-Mass Spectrometry; D006168:Guinea Pigs; D007211:Indoles / Q000032:analysis / Q000378:metabolism*; D008297:Male; D008745:Methylation; D012436:S-Adenosylmethionine / Q000494:pharmacology*
chemical_list: D002243:Carbolines; D007211:Indoles; D012436:S-Adenosylmethionine