Two polymorphic forms of human histamine methyltransferase: structural, thermal, and kinetic comparisons

Area of Science:

• Biochemistry
• Pharmacology
• Structural Biology

Background:

• Histamine is a key mediator in allergic responses, gastric acid secretion, asthma, and neurotransmission.
• Histamine's local action is terminated by methylation, primarily by human histamine N-methyltransferase (HNMT).
• A common HNMT polymorphism at residue 105 results in high- (Thr) and low- (Ile) activity phenotypes.

Purpose of the Study:

• To elucidate the structural and kinetic differences between the Thr105 and Ile105 variants of human HNMT.
• To provide structural insights into HNMT inhibitors and their mechanisms of action.

Main Methods:

• Determination of two ternary structures of human HNMT (Thr105 and Ile105 variants).
• Complexation with substrates, reaction products, and inhibitors (quinacrine).
• Steady-state kinetic analysis of recombinant HNMT variants.

Main Results:

• The Ile105 variant showed increased K(M) for AdoMet and histamine, and slightly lower specific activity compared to the Thr105 variant.
• Kinetic differences were observed between 25°C and 45°C.
• The Ile105 variant was only more thermolabile than the Thr105 variant at temperatures of 50°C or higher.

Conclusions:

• HNMT possesses a two-domain structure with the residue 105 on the surface, explaining kinetic data.
• The polymorphism does not affect protein stability at physiological temperatures but influences K(M) values.
• Structural data offer insights into HNMT inhibitors and their inhibition mechanisms.