Enzymatic formation of uridine diphosphate N-acetyl-D-mannosamine.

An enzyme that catalyzes the interconversion of UDP-N-acetyl-D-glucosamine and UDP-N-acetyl-D-mannosamine was purified about 700-fold from the supernatant fraction of Bacillus cereus, and the properties of this enzyme were studied. This enzyme was not stimulated by NAD+, NADH, or any metal ions. The optimum pH was between 7.5 and 8.0. At equilibrium of the reaction, the ratio of UDP-N-acetylglucosamine to UDP-N-acetylmannosmaine was about 9:1. The enzyme was inactive toward free N-acetylhexosamines, their phosphate esters, UDP-glucose, and UDP-N-acetylgalactosamine. A stimulatory role of UDP-N-acetylglucosamine was demonstrated. In the reaction with UDP-N-acetylglucosamine, the rate as a function of substrate concentration showed a sigmoidal relationship with a Hill coefficient of 1.8 and an apparent Km value for UDP-N-acetylglucosamine of 1.1 mM. The reverse reaction with UDP-N-acetylmannosamine required the presence of UDP-N-acetylglucosamine. The UDP-N-acetylglucosamine concentration required for half-maximal activation was about 0.5 mM. The apparent Km for UDP-N-acetylmannosamine measured in the presence of 0.5 mM UDP-N-acetylglucosamine was 0.22mM. Other nucleotides or hexosamine derivatives were not stimulatory. The same activity was found in cell extracts from several bacterial species.