Reactivity of the essential thiol group of lactate dehydrogenase and substrate binding

Publisher Website
Google Scholar
Abstract
1. The preparation of a derivative of pig heart lactate dehydrogenase in which the essential thiol group has been converted into an S-sulpho group is described. The derivative has unchanged s20,w and is catalytically inactive. 2. The rate of reaction of the essential thiol group is controlled by a system with a pK>9. 3. The essential thiol group is protected by NADH against reaction with maleimide. 4. Lactate dehydrogenase in which the essential thiol group has been converted into an S-sulpho group or alkylated with maleimide still binds one molecule of NADH/subunit but with a three- to four-fold diminished affinity. 5. The inhibited enzymes also bind one molecule of NAD+–sulphite complex/subunit but with affinity decreased 103–104-fold. 6. The inhibited enzymes fail to bind C2 and C3 molecules to give the ternary complexes enzyme–NAD+–pyruvate, enzyme–NADH–oxamate and enzyme–NADH–oxalate. The 1:1:1 stoicheiometry of the last-mentioned complex with the native enzyme was established by gel filtration. 7. Structures that account for these results are discussed.