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

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(Pb2 +) 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.