Noncompetitive Inhibition of Indolethylamine‑<i>N</i>‑methyltransferase by <i>N</i>,<i>N</i>‑Dimethyltryptamine and <i>N</i>,<i>N</i>‑Dimethylaminopropyltryptamine
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Abstract
Indolethylamine-<i>N</i>-methyltransferase (INMT) is
a Class 1 transmethylation enzyme known for its production of <i>N</i>,<i>N</i>-dimethyltryptamine (DMT), a hallucinogen
with affinity for various serotonergic, adrenergic, histaminergic,
dopaminergic, and sigma-1 receptors. DMT is produced via the action
of INMT on the endogenous substrates tryptamine and <i>S</i>-adenosyl-l-methionine (SAM). The biological, biochemical,
and selective small molecule regulation of INMT enzyme activity remain
largely unknown. Kinetic mechanisms for inhibition of rabbit lung
INMT (rabINMT) by the product, DMT, and by a new novel tryptamine
derivative were determined. After Michaelis–Menten and Lineweaver–Burk
analyses had been applied to study inhibition, DMT was found to be
a mixed competitive and noncompetitive inhibitor when measured against
tryptamine. 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
or PDAT), was shown to inhibit rabINMT by a pure noncompetitive mechanism
when measured against tryptamine with a <i>K</i><sub>i</sub> of 84 μM. No inhibition by PDAT was observed at 2 mM when
it was tested against structurally similar Class 1 methyltransferases,
such as human phenylethanolamine-<i>N</i>-methyltransferase
(hPNMT) and human nicotinamide-<i>N</i>-methyltransferase
(hNNMT), indicating selectivity for INMT. The demonstration of noncompetitive
mechanisms for INMT inhibition implies the presence of an inhibitory
allosteric site. <i>In silico</i> analyses using the computer
modeling software Autodock and the rabINMT sequence threaded onto
the human INMT (hINMT) structure (Protein Data Bank entry 2A14) identified an N-terminal
helix–loop–helix non-active site binding region of the
enzyme. The energies for binding of DMT and PDAT to this region of
rabINMT, as determined by Autodock, were −6.34 and −7.58
kcal/mol, respectively. Assessment of the allosteric control of INMT
may illuminate new biochemical pathway(s) underlying the biology of
INMT