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Novel S‐Adenosylmethionine‐Dependent Indole‐N‐Methylation of β‐Carbolines in Brain Particulate Fractions
Author(s) -
Matsubara Kazuo,
Neafsey Edward J.,
Collins Michael A.
Publication year - 1992
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1992.tb09400.x
Subject(s) - methylation , chemistry , indole test , cytosol , in vivo , biochemistry , stereochemistry , high performance liquid chromatography , enzyme , chromatography , biology , microbiology and biotechnology , gene
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 [ 3 H]CH 3 ‐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 N 2 ,N 9 ‐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.