Purine substrates for human thiopurine methyltransferase

Area of Science:

• Biochemistry
• Pharmacogenetics
• Enzymology

Background:

• Thiopurine methyltransferase (TPMT) is crucial for metabolizing thiopurine drugs like 6-mercaptopurine (6-MP) and 6-thioguanine (6-TG).
• Genetic variations in TPMT activity significantly impact patient response and toxicity to these medications.
• Understanding TPMT substrate specificity is vital for optimizing thiopurine drug therapy.

Purpose of the Study:

• To investigate the substrate specificity of human kidney TPMT.
• To identify novel purine derivatives that interact with TPMT.
• To explore potential inhibitors of TPMT activity.

Main Methods:

• Purified human kidney TPMT was used to assay methylation activity.
• Eighteen purine derivatives, including ribonucleosides and ribonucleotides, were tested as potential substrates.
• Enzyme kinetics (Km and Vmax) were determined for identified substrates.

Main Results:

• Sixteen of the 18 tested purine derivatives were found to be substrates for TPMT.
• Thiopurines exhibited lower Km values compared to their corresponding ribonucleosides or ribonucleotides.
• 6-Selenopurine derivatives showed the lowest Km values, indicating high affinity.
• Oxidized purines at the 8-position were methylated, while 2-OH compounds acted as potent inhibitors.

Conclusions:

• The study expands the known substrate profile of TPMT, including novel thiopurine derivatives.
• Specific structural features of purine compounds influence their interaction with TPMT.
• Identification of potent inhibitors like 2-OH compounds offers potential for modulating TPMT activity in clinical settings.