Selenoxides inhibit delta-aminolevulinic acid dehydratase

The effect of two selenides and their selenoxides on delta-aminolevulinic acid dehydratase (delta-ALA-D) from liver of adult rats was investigated. In vivo, selenides can be oxidized to selenoxides by flavin-containing monooxygenases (FMO) and selenoxides can regenerate selenides by thiol oxidation. Phenyl methyl selenide (PhSeCH3) and 1-hexynyl methyl selenide (C4H9Ctriple bondCSeCH3) were converted to selenoxides by reaction with H2O2. PhSeCH3 and C4H9Ctriple bondCSeCH3 had no effect on delta-ALA-D up to 400 microM. Conversely, their selenoxides inhibited delta-ALA-D, and the IC(50) for enzyme inhibition was about 100 and 70 microM, respectively. Partially purified delta-ALA-D (P(55)) from swine liver was also inhibited by these selenoxides. The inhibitory action of selenoxides was antagonized by dithiotreitol (DTT). Moreover, delta-ALA-D from a plant source was inhibited by the selenoxides, suggesting a possible involvement of SH groups in a distinct site of the homologous region implicated in Zn2+ binding in mammalian delta-ALA-D. After exposure to PhSeCH3 (500 micromol/kg/day) for 45 or 30 days, the activity of delta-ALA-D from liver of mice decreased to about 50% of the control group. The in vivo inhibitory action of this compound was not antagonized by DTT. PhSeCH3 and C4H9Ctriple bondCSeCH3 had no effect on the rate of DTT oxidation, but their selenoxides oxidized DTT. The results of the present study suggest that hepatic delta-ALA-D of rodents is a potential molecular target for selenides as a consequence of their metabolism to selenoxides by FMO.