Biosynthesis of N-Methylisoquinolinium Ion, a Potent Inhibitor of Catecholamine Metabolism, from 1,2,3,4-Tetrahydroisoquinoline Through N-Methyl-1,2,3,4-Tetrahydroisoquinoline in Human Brain
The discovery of N-methyl-4-phenyl-l,2,3,6-tetrahydropyridine (MPTP) as a neurotoxin that elicits symptoms very similar to those of parkinsonism in humans’ indicates that similar compounds may accumulate in human brain and cause neurodegeneration of the central nervous system. In human brain, 1,2,3,4-tetrahydroisoquingline (TIQ) was found and its amount was increased in a parkinsonian brain2. TIQ produced parkinsonism in primates by systemic administration, with a reduction in dopamine and biopterin contents and in the activity of tyrosine hydroxylase [TH; tyrosine, tetrahydropteridine: oxygen oxidoreductase (3-hydroxylating), EC 22.214.171.124] in the nigro-striatal region.3 Of the derivatives of TIQ, the N-methylisoquinolinium ion (NMIQ+) has a chemical structure similar to that of an oxidative product of MPTP, N-methyl-4-phenylpyridinium jon (MPP+). NMIQ+ is a potent inhibitor of TH in rat striatal slices4, and of monoamine oxidase [MAO; monoamine: oxygen oxidoreductase (deaminating), EC 126.96.36.199].5 More recently, in a rat clonal pheochromocytoma cell line, PC12h, as a model of dopaminergic neurons, NMIQ+ inhibited in vitro and in vivo activity of TH, MAO, and aromatic L-amino acid decarboxylase [aromatic L-amino acid carboxy-lyase, EC 188.8.131.52].6 These results suggest that NMIQ+ may be an endogenous inhibitor similar to MPP+. To confirm the biosynthesis of NMIQ+ from TIQ in human brain, N-methylation of TIQ was studied, using an enzyme sample prepared from human brain. This paper reports the synthesis of NMTIQ by an N-methyltransferase in human brain and its oxidation into NMIQ+ by MAO. The enzymatic studies on the two steps of reactions were discussed in relation to the effect of TIQ derivatives on catecholamine metabolism in human brain.
KeywordsHuman Brain Tyrosine Hydroxylase Monoamine Oxidase Methyl Transferase Pyridinium Salt
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