Drug Repurposing to Inhibit Histamine N-Methyl Transferase.

Lower activity of the histaminergic system is associated with neurological disorders, including Alzheimer's disease (AD). Thus, the enhancement of histaminergic neurotransmission by inhibition of histamine N-methyl transferase (HNMT), which degrades histamine, appears as an important approach. For this purpose, rigid and flexible molecular docking studies of 185 FDA-approved drugs with the HNMT enzyme were carried out to select two compounds to perform molecular dynamics (MD) simulations to evaluate the binding free energies and stability of the enzyme-drug complexes. Finally, an HNMT inhibition assay was performed to corroborate their effect towards HNMT. Molecular docking studies with HNMT allowed the selection of dihydroergotamine and vilazodone since these molecules showed the lowest Gibbs free energy values. Analysis of the binding mode of vilazodone showed interactions with the binding pocket of HNMT with Glu28, Gln143, and Asn283. In contrast, dihydroergotamine binds to the HNMT active site in a different location, apparently because it is overall the more rigid ligand compared to flexible vilazodone. HNMT inhibitory activity for dihydroergotamine and vilazodone was corroborated (IC50 = 72.89 µM and 45.01 µM, respectively) by in vitro assays. Drug repurposing of HNMT was achieved by employing computational studies.

Femtomole isotopic-enzymatic microassay of histamine in microgram amounts of brain tissue.

We present a modification of a previously described radioenzymatic technique (Snyder et al., Journal of Pharmacology and Experimental Therapeutics, 153: 544-549, 1966) for the determination of histamine. This assay is based on the incubation of tissue histamine with a partially purified preparation of histamine-N-methyltransferase (S-adenosyl-L-methionine: histamine N-methyltransferase, E.C. 2.1.1.8; HMT) in the presence of the natural donor of methyl groups, [3H]-methyl-S-adenosyl-L-methionine ([3H]-SAMe). We have found that HMT retains its histamine N-methylating activity at 4 degrees C. Incubation at low temperature results in a considerable increase in the sample to blank ratio. The reduction of the total amount of [3H]-SAMe used and addition of anhydrous sodium sulfate before the last organic extraction step further reduced the blank levels. With these modifications, the sensitivity of the method was increased to the lower femtomole range. The present assay can be used for the determination of as little as 1 to 2 pg of histamine in samples from individual rat brain nuclei containing 2 to 10 micrograms of protein.