Structure of hydralazine hydrochloride

Differences between murine arylamine N-acetyltransferase type 1 and human arylamine N-acetyltransferase type 2 defined by substrate specificity and inhibitor binding

Background

The mouse has three arylamine N-acetyltransferase genes, (MOUSE)Nat1, (MOUSE)Nat2 and (MOUSE)Nat3. These are believed to correspond to (HUMAN)NAT1, (HUMAN)NAT2 and NATP in humans. (MOUSE)Nat3 encodes an enzyme with poor activity and human NATP is a pseudogene. (MOUSE)Nat2 is orthologous to (HUMAN)NAT1 and their corresponding proteins are functionally similar, but the relationship between (MOUSE)Nat1 and (HUMAN)NAT2 is less clear-cut.

Methods

To determine whether the (MOUSE)NAT1 and (HUMAN)NAT2 enzymes are functionally equivalent, we expressed and purified (MOUSE)NAT1*1 and analysed its substrate specificity using a panel of arylamines and hydrazines. To understand how specific residues contribute to substrate selectivity, three site-directed mutants of (MOUSE)NAT2*1 were prepared: these were (MOUSE)NAT2_F125S, (MOUSE)NAT2_R127G and (MOUSE)NAT2_R127L. All three exhibited diminished activity towards “(MOUSE)NAT2-specific” arylamines but were more active against hydrazines than (MOUSE)NAT1*1. The inhibitory and colorimetric properties of a selective naphthoquinone inhibitor of (HUMAN)NAT1 and (MOUSE)NAT2 were investigated.

Results

Comparing (MOUSE)NAT1*1 with other mammalian NAT enzymes demonstrated that the substrate profiles of (MOUSE)NAT1 and (HUMAN)NAT2 are less similar than previously believed. Three key residues (F125, R127 and Y129) in (HUMAN)NAT1*4 and (MOUSE)NAT2*1 were required for enzyme inhibition and the associated colour change on naphthoquinone binding. In silico modelling of selective ligands into the appropriate NAT active sites further implicated these residues in substrate and inhibitor specificity in mouse and human NAT isoenzymes.

Conclusions

Three non-catalytic residues within (HUMAN)NAT1*4 (F125, R127 and Y129) contribute both to substrate recognition and inhibitor binding by participating in distinctive intermolecular interactions and maintaining the steric conformation of the catalytic pocket. These active site residues contribute to the definition of substrate and inhibitor selectivity, an understanding of which is essential for facilitating the design of second generation (HUMAN)NAT1-selective inhibitors for diagnostic, prognostic and therapeutic purposes. In particular, since the expression of (HUMAN)NAT1 is related to the development and progression of oestrogen-receptor-positive breast cancer, these structure-based tools will facilitate the ongoing design of candidate compounds for use in (HUMAN)NAT1-positive breast tumours.

Electronic supplementary material

The online version of this article (doi:10.1186/2050-6511-15-68) contains supplementary material, which is available to authorized users.

Probing the architecture of the Mycobacterium marinum arylamine N-acetyltransferase active site

Treatment of latent tuberculosis infection remains an important goal of global TB eradication. To this end, targets that are essential for intracellular survival of Mycobacterium tuberculosis are particularly attractive. Arylamine N-acetyltransferase (NAT) represents such a target as it is, along with the enzymes encoded by the associated gene cluster, essential for mycobacterial survival inside macrophages and involved in cholesterol degradation. Cholesterol is likely to be the fuel for M. tuberculosis inside macrophages. Deleting the nat gene and inhibiting the NAT enzyme prevents survival of the microorganism in macrophages and induces cell wall alterations, rendering the mycobacterium sensitive to antibiotics to which it is normally resistant. To date, NAT from M. marinum (MMNAT) is considered the best available model for NAT from M. tuberculosis (TBNAT). The enzyme catalyses the acetylation and propionylation of arylamines and hydrazines. Hydralazine is a good acetyl and propionyl acceptor for both MMNAT and TBNAT. The MMNAT structure has been solved to 2.1 Å resolution following crystallisation in the presence of hydralazine and is compared to available NAT structures. From the mode of ligand binding, features of the binding pocket can be identified, which point to a novel mechanism for the acetylation reaction that results in a 3-methyltriazolo[3,4-a]phthalazine ring compound as product.