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The prosthetic groups of UDPgalactose-4-epimerase from yeast andE. coli can be reduced by incubation with D-galactose or L-arabinose provided that 5′ uridylic acid (5′UMP) is also present. This type of concerted reduction can also be performed using sodium borohydride. The latter type of reduction also depends on the addition of 5′UMP. Enzymatic analysis of the released prosthetic group by means of lactic dehydrogenase and pyruvate revealed that the pyridine nucleotide of the reduced epimerase generated in the concerted reaction is dehydrogenated in the 4-position of the pyridine ring.The catalytic activity of the reduced epimerase is much lower than that of epimerase (10-15% of native epimerase). This reductive inactivation was also found in crude epimerase preparations fromE. coli , again provided 5′UMP is present. FractionatedE. coli epimerase subjected to the concerted reduction show a striking fluorescent enhancement of the order of magnitude of that observed in the purified yeast epimerase. Unlike the latter, however,E. coli epimerase also becomes fluorescent upon addition of the specific substrate UDPgalactose.The physiological aspects of the phenomenon as related to induction of the galactose genes is discussed.

The Reductive Inactivation of UDPgalactose-4-Epimerase from Yeast and E. coli