Regulation of microsomal UDP-glucuronyltransferase by metal ions. Differential effects of Mn2+ on forward and reverse reactions.

Mn2+ activates UDP-glucuronyltransferase (assayed with p-nitrophenol as aglycone) in assays of the forward reaction. The kinetic mechanism of this activation is an increase in activity at V. The rate of the reverse reaction catalyzed by UDP-glucuronyltransferase (UDP-dependent hydrolysis of p-nitrophenylglucuronic acid) is inhibited by Mn2+. The basis for this inhibition is an increase in KUDP, and a small increase in KNphGlcUA. Mn2+ appears to have no effect of the activity of the reverse reaction of saturating concentrations of both substrates. The differential effects of divalent metal ions on rates of the forward and reverse reactions catalyzed by UDP-glucuronyltransferase are due in part to differences in the affinities of UDP-glucuronic acid and UDP for metals. Keq for the formation of UDP-Mn2+ is 8.6 X 10(3); Keq for the formation of UDP-glucuronic acid-Mn2+ is 4.8 X 10(2). High concentrations of UDP thus can compete for metal ions bound to UDP-glucuronyltransferase, but UDP-glucuronic acid is less likely to do so. This competition for metal ions between the enzyme and substrates accounts for the failure of Mn2+ to increase the activity at V of the reverse reaction. Mn2+-induced inhibition of the reverse reaction, due to an increase of KUDP, reflects that the affinity of the affinity of the enzyme for UDP is greater than that for UDP-Mn2+. On the other hand, formation of a complex between UDP-glucuronic and Mn2+ does not alter the affinity of UDP-glucuronyltransferase for this nucleotide derivative