5-Aminolevulinic acid dehydratase. The role of sulphydryl groups in 5-aminolevulinic acid dehydratase from bovine liver.

The thiophilic reagent 5,5'-dithiobis(2-nitrobenzoic acid) Nbs2) reacts with four sulphydryl groups in native 5-aminolevulinic acid dehydratase from bovine liver (groups I, II, III and IV). All four of these groups exhibit various degrees of half-site reactivity. Groups I and II are highly reactive and their rates of reaction with Nbs2 have been investigated using stopped-flow analysis. The reaction of these groups with Nbs2 results in the formation of an intramolecular disulphide bond which may be reduced with dithioerythritol to regenerate the free sulphydryl groups. Groups I and II appear to be at, or near, the catalytic site whereas group III is involved in the maintenance of conformation in the native enzyme.