We determined higher TPMT activity in wild-type TPMT*1/*1 individuals with high SAM concentrations (n=96) compared to the low SAM level group (n=19; P<0.001). These findings confirm the results of our in vitro studies, which demonstrated that the restriction of L-methionine (Met) in cell growth media reversibly decreased TPMT activity and protein levels.
Selective inhibition of distinct components of Met metabolism was used to demonstrate that SAM is implicitly responsible for direct post-translational TPMT stabilization.
The greatest effect of SAM-mediated TPMT stabilization was observed in the case of wild-type TPMT*1 and variant *3C allozymes.
In addition to TPMT genotyping, SAM may serve as an important biochemical marker in individualization of thiopurine therapy.