Detergent effects in rabbit liver microsomal UDP-glucuronosyltransferase studied by means of a continuous spectrophotometric assay with p-nitrophenol.

UDP-glucuronosyltransferase (UDPGT) has been studied with a continuous spectrophotometric assay employing UDP-glucuronic acid and p-nitrophenol as substrates. Activity is linearly dependent on the microsomal protein concentration. Male rabbit liver phenobarbital-induced microsomes exhibited a rate of 7.10 microM p-nitrophenol conjugated per minute at 37 degrees C. Addition of small amounts of Tergitol NP-10 caused an approximately 4-fold increase in conjugation activity; maximal activation was observed at 0.01% (v/v) detergent. However, inclusion of additional detergent caused significant inhibition of activity, such that 0.5% Tergitol caused the rate to fall 2.5-fold below the activity observed in the absence of detergent. Membrane solubilization was studied by light scattering. At maximal stimulation of p-nitrophenol UDPGT activity, the membrane was solubilized only approximately 17%. At the point of 50% solubilization, activity was still 91% of maximum. Complete membrane solubilization was achieved at approximately 0.2% Tergitol, and transferase activity had fallen slightly below the rate observed in the absence of detergent. Possible explanations for the unusual detergent-dependence of microsomal p-nitrophenol UDPGT activity are discussed.