Noncompetitive Inhibition of Indolethylamine‑<i>N</i>‑methyltransferase by <i>N</i>,<i>N</i>‑Dimethyltryptamine and <i>N</i>,<i>N</i>‑Dimethylaminopropyltryptamine

Abstract

Indolethylamine-<i>N</i>-methyltransferase (INMT) is\na Class 1 transmethylation enzyme known for its production of <i>N</i>,<i>N</i>-dimethyltryptamine (DMT), a hallucinogen\nwith affinity for various serotonergic, adrenergic, histaminergic,\ndopaminergic, and sigma-1 receptors. DMT is produced via the action\nof INMT on the endogenous substrates tryptamine and <i>S</i>-adenosyl-l-methionine (SAM). The biological, biochemical,\nand selective small molecule regulation of INMT enzyme activity remain\nlargely unknown. Kinetic mechanisms for inhibition of rabbit lung\nINMT (rabINMT) by the product, DMT, and by a new novel tryptamine\nderivative were determined. After Michaelis–Menten and Lineweaver–Burk\nanalyses had been applied to study inhibition, DMT was found to be\na mixed competitive and noncompetitive inhibitor when measured against\ntryptamine. The novel tryptamine derivative, <i>N</i>-[2-(1<i>H</i>-indol-3-yl)­ethyl]-<i>N</i>′,<i>N</i>′-dimethylpropane-1,3-diamine (propyl dimethyl amino tryptamine\nor PDAT), was shown to inhibit rabINMT by a pure noncompetitive mechanism\nwhen measured against tryptamine with a <i>K</i>_i of 84 μM. No inhibition by PDAT was observed at 2 mM when\nit was tested against structurally similar Class 1 methyltransferases,\nsuch as human phenylethanolamine-<i>N</i>-methyltransferase\n(hPNMT) and human nicotinamide-<i>N</i>-methyltransferase\n(hNNMT), indicating selectivity for INMT. The demonstration of noncompetitive\nmechanisms for INMT inhibition implies the presence of an inhibitory\nallosteric site. <i>In silico</i> analyses using the computer\nmodeling software Autodock and the rabINMT sequence threaded onto\nthe human INMT (hINMT) structure (Protein Data Bank entry 2A14) identified an N-terminal\nhelix–loop–helix non-active site binding region of the\nenzyme. The energies for binding of DMT and PDAT to this region of\nrabINMT, as determined by Autodock, were −6.34 and −7.58\nkcal/mol, respectively. Assessment of the allosteric control of INMT\nmay illuminate new biochemical pathway(s) underlying the biology of\nINMT.