Stereoselective sulfation of R,S-4'-hydroxypropranolol by canine hepatic cytosol and partially purified phenolsulfotransferases.

The stereoselective formation of 4'-hydroxypropranolol sulfate (HOPS) from racemic 4'-hydroxypropranolol (HOP) has been investigated using canine hepatic cytosol and partially purified phenolsulfotransferases (PSTs). Pseudo first order rate constants, Vmax/Kmapp, were significantly greater (P less than .05) for the formation of R-HOPS by canine hepatic cytosol, S/R-HOPS = 0.72. Moreover, double reciprocal replots for the sulfation of both HOP enantiomers were biphasic, suggesting the presence of multiple PST enzymes. Pentachlorophenol (PCP) inhibited the sulfation of both HOP enantiomers with an IC50 of 244 nM, suggesting that phenol (P) forms of the PST enzymes may be primarily responsible for the cytosolic sulfation. Three distinct PST fractions were partially purified from the cytosol and were found to possess unique stereoselectivities toward HOP. Fraction 1 was most stereoselective, S/R-HOPS = 0.51, but was not inhibited by PCP, suggesting that it is a monoamine (M) form PST. Fraction 3 was the most active fraction and mimicked the cytosol in both stereoselectivity, S/R-HOPS = 0.73 vs. 0.72, and susceptibility to PCP inhibition, IC50 = 93 nM. The S/R-HOPS ratio produced by fraction 2 was 0.63. These data demonstrate the presence of multiple PST enzymes with differing stereoselectivities for the enantiomers of HOP in canine hepatic cytosol. Further studies with homogeneous PST isozymes, ideally from human tissue, and other chiral drugs are needed to define the enzymatic mechanism of this reaction and its role in chiral drug metabolism.