The effect of a temperature-induced phase change within membrane lipids on the regulatory properties of microsomal uridine diphosphate glucuronyltransferase.

UDP-glucose, UDP-mannose, and UDP-xylose inhibit microsomal UDP-glucuronyltransferase in assays at 6 degrees. None of these compounds affects activity in assays at 37 degrees. Studies with UDP-glucose revealed an abrupt temperature-dependent change in the effect of this compound on the activity of UDP-glucuronyltransferase. Inhibition by UDP-glucose was observed only at temperatures below 16 degrees. UDP-glucose had no effect on activity in assays at temperatures of 16 degrees and above. UDP-N-acetylglucosamine activates UDP-glucuronyltransferase at temperatures at or above 16 degrees, but has no effect on activity below 16 degrees. The lipid portion of the microsomal membrane is known to undergo a phase change at 16 degrees. These data indicate, therefore, that significant alterations in the regulatory properties of UDP-glucuronyltransferase are associated with temperature-induced phase separations within the lipid portion of the microsomal membrane.