5-Aminolevulinic acid dehydratase: alkylation of an essential thiol in the bovine-liver enzyme by active-site-directed reagents.

1. 5-Aminolevulinic acid dehydratase from bovine liver has been shown to be inactivated by 5-halolevulinic acids and 3-halolevulinic acids. 2. The substrate, 5-aminolevulinic acid, protects the enzyme from modification by 5-halolevulinic acids. 3. Using tritiated chlorolevulinic acids, it was shown that four of the subunits in the octameric enzyme are preferentially modified. 4. The susceptible enzyme group modified is an --SH group of a reactive cysteine at or near the active site. 5. Oxidized enzyme is not affected by either 5-chlorolevulinic acid or 3-chlorolevulinic acid. 6. Evidence is presented which suggests that 5-chlorolevulinic acid is acting as an active-site-directed reagent.