UDP-glucose 4-epimerase from Saccharomyces fragilis. Involvement of sulfhydryl group(s) at the active site.

UDPglucose-4-epimerase (EC 5.1.3.2) from Saccharomyces fragilis is inactivated by 0.1 mM 5,5'-dithiobis-(2-nitrobenzoate) in 6 min. Unlike p-chloromercuribenzoate-inactivated or heat-inactivated enzymes, the dithiobisnitrobenzoate-inactivated enzyme retains the dimeric structure and NAD is not dissociated from the protein moiety. Inactivation of the enzyme by dithiobisnitrobenzoate can not therefore be attributed to any subsequent loss of structural integrity or to the detachment of the cofactor from the apoenzyme. The inactivated enzyme can be almost fully reactivated in the presence of mercaptoethanol and characteristic properties of native enzyme are regained. The inactivation by dithiobisnitrobenzoate can be substantially protected by UDPglucose or UDPgalactose indicating a possible critical involvement of one or more sulfhydryl groups at the active site.