Of the selenenylsulfides, only RX1 and G1 were reduced by mitochondrial TrxR2, matching the trend of greater reactivity for the piperazinamides, and only at high TrxR2 (50 -100 nM; Fig. , Fig. ). Some TrxR2 response was expected, as it has the same Sec-Cys motif as TrxR1, but this mirrors ample literature precedents of TrxR2 being slower to process small molecule substrates than TrxR1. Since cytosolic TrxR1 has many-fold higher total expression in cells, we therefore expected the faster-reactive TrxR1 to be the dominant cellular reductant for our selenenylsulfide probes.