Lead exposure decreases TPMT activity, highlighting the need for thiopurine dose adjustment in high-exposure...

Thiopurine drugs are used for the treatment of a wide variety of diseases ranging from autoimmune diseases to various leukemia types. As efficacy of thiopurine drugs is significantly reduced by Thiopurine S-methyltransferase (TPMT)-mediated S-methylation, accurate assessment of individual TPMT-activity levels is crucial for optimizing thiopurine drug dosage regimens for each patient. Given its multifaceted regulation, identifying occupational exposures impacting TPMT enzymatic activity is crucial. The influence of lead(Pb^2 +) on the enzymatic activity of TPMT was initially assessed in vitro, by spiking human blood having normal TPMT levels with increasing levels of Pb²⁺. The effect of occupational Pb²⁺-exposure on TPMT activity was further investigated by comparing blood TPMT activity levels of industrial workers prone to occupational Pb²⁺-exposure. Putative Pb²⁺-docking sites on TPMT enzyme were assessed through in silico analysis via MIB2 and CB-Dock2 servers. In vitro analysis indicated an inverse correlation between blood Pb²⁺-levels and TPMT-activity, which was further supported by in vivo analyses, providing evidence for Pb²⁺-mediated TPMT-inactivation. In silico Pb²⁺-docking analyses suggest two putative regions for Pb²⁺-mediated competitive TPMT-inhibition, four regions for direct/indirect inhibition, and four regions for allosteric inhibition. Further kinetic studies suggest that Pb²⁺ exerts its inhibitory effect through competition with SAM for the active site. The observed inverse correlation between blood Pb²⁺ and TPMT levels emphasizes the clinical importance of individualizing thiopurine drug dosage, especially for patients exposed to high levels of Pb²⁺. In vitro studies involving site-directed mutagenesis and analysis of patients with Pb²⁺-binding site mutations will validate the precise Pb²⁺-binding sites involved in TPMT-inhibition.