Substrate specificity of human UDP-glucuronyltransferase in cultured lymphocytes

Abstract

1. This study establishes the presence of UDP-glucuronyltransferase activity for non-steroidal as well as steroidal substrates, in cultured human B-lymphocytes. Glucuronidation of α;-naphthol and testosterone was demonstrated in homogenates of two cell lines, SN1006 and RPMI-1788, and that of phenolphthalein, 4-methylumbelliferone, p-nitrophenol and estradiol in the cell line with the higher glucuronyltransferase activity, SN1006.

2. Kinetic studies of testosterone glucuronidation in homogenates of both cell lines revealed a similarity in the behaviour of glucuronyltransferase of these cells. Thus, comparable apparent K_m values for UDPGA (0.63 mM) and for testosterone (14 μm) were observed, although apparent maximal velocities, V_max, differed several-fold (3.0 versus 05.5 pmol/10⁶ cells per min, in SN1006 and RPMI-1788 cells, respectively).

3. Kinetic studies of glucuronidation of testosterone, estradiol, phenolphthalein, α;-naphthol, 4-methylumbelliferone, and p-nitrophenol yielded comparable apparent K_m values for UDPGA (0.56-0.67mM), suggesting that the same, or similar, glucuronyltransferase(s) catalyse(s) glucuronidation of this wide range of substrates in lymphocytes. This was reinforced by the observation of competitive inhibition of testosterone glucuronidation by α;-naphthol (K_i 0.25 mM), 4-methylumbelliferone (K_i 0.8mM) and p-nitrophenol (K_i0.8mM). Thus, lymphocyte glucuronyltransferase activity with a broad substrate specificity, for steroidal and non-steroidal aglycones, is indicated.