Affinity labelling of the NADP+-binding site of glucose 6-phosphate dehydrogenase from Candida utilis.

1. Periodate-oxidized NADP+ inhibits the catalytic activity of glucose 6-phosphate dehydrogenase from Candida utilis, competing with NADP+. 2. Incubation of the enzyme with the coenzyme analogue causes partial reversible inactivation of the enzyme as a result of affinity labelling of the coenzyme-binding site. 3. Some kinetic values of the reaction were calculated. 4. The inactivation can be made irreversible by treatment with NaBH4, which reduces a Schiff base formed between an aldehyde group on the coenzyme analogue and a lysine residue on the enzyme. 5. Complete inactivation can be correlated with the binding of only one inhibitor to each enzyme subunit. 6. The lysine residue involved in the binding of the inhibitor is present at the coenzyme-binding site.