Mono-N-Methylation of 1,2,3,4-Tetrahydro-β-Carbolines in Brain Cytosol: Absence of Indole Methylation

: In an accompanying report we demonstrated enzyme activity in guinea pig brain cell nuclei that catalyzes S-adenosylmethionine (SAM)-dependent N-methylations of heteroaromatic β-carbolines (BCs) on the 2[β]-nitrogen and subsequently on the 9[indole]-nitrogen, ultimately yielding N2,N9-dimethylated BCs. Presented here are the esults of a parallel study of the N-methylation of 1,2,3,4-tetrahydro-BCs (THBCs), which form endogenously via condensations of tryptophan and its derived indoles with carbonyl compounds or, like their BC oxidation products, are environmental constituents and plant alkaloids. THBCs were enzymatically methylated on the 2[β]-nitrogen by [3H]-SAM in undialyzed homogenates of rat or guinea pig brain, but [3H]methyl transfer to the 9[indole]-nitrogen was not observed. The structure of the 2[β]-methyl THBC product was verified with capillary gas chromatography-mass spectrometry. Furthermore, whereas BC N-methylation was largely particulate and displayed micromolar Km values for BC substrate, THBC 2[β]-N-methylation activity was cytosolic and displayed a relatively high (millimolar) Km for THBC substrate. The N-methylation of THBCs may be due to cytosolic N-methyltransferases that others have studied using different azaheterocyclics. Our overall studies indicate that N2,N9-dimethylated BCs could be unique neurotoxic factors that are bioactivated within brain by sequential N-methylations of BCs. These results suggest the possibility of an additional route to the putative 2,9-dimethylated toxins involving, as a first step, 2[β]-N-methylation of environmental or endogenously derived THBCs in the brain and perhaps other organs.