Mechanism of delta-aminolevulinate dehydratase inhibition by phenyl selenoacetylene involves its conversion to diphenyl diselenide

The mechanism of δ-aminolevulinate dehydratase (δ-ALA-D) inhibition by phenyl selenoacetylene in vitro was investigated in this study. Phenyl selenoacetylene (40-400 μM) inhibition of δ-aminolevulinate dehydratase from rat liver (low speed supernatant fraction, S1 fraction) was partially prevented by incubation under argon atmosphere and completely prevented by dithiothreitol. After incubation with S1 fraction from rat liver or cysteine (40 mM), phenyl selenoacetylene was partially converted into diphenyl diselenide, which is a stronger inhibitor of δ-aminolevulinate dehydratase than phenyl selenoacetylene. Diphenyl diselenide increased the rate of oxidation of -SH groups, while phenyl selenoacetylene did not affect such oxidation. δ-Aminolevulinate dehydratase purified from bovine liver (Sigma®) was less sensitive to phenyl selenoacetylene and diphenyl diselenide than the enzyme from S1 fraction. We propose that the lower sensitivity of purified enzyme to selenides could be related to the formation of selenols due to the presence of dithiothreitol (a reducing agent) in the incubation medium. In agreement, incubation of purified enzyme (Sigma®) with diphenyl diselenide (2 μM) and sodium borohydride (a reducing agent) under argon atmosphere significantly increased enzyme activity. Results obtained suggest that δ-aminolevulinate dehydratase inhibition by phenyl selenoacetylene is dependent on its conversion into diphenyl diselenide that induces oxidation of essential -SH groups of δ-aminolevulinate dehydratase. We propose that oxygen could be important in the regeneration of diphenyl diselenide leading to a catalytic oxidation of the enzyme.