| Aryl Sulfotransferase IV (also designated as ST1A1 or rSULT1A1) is a major phenol sulfotransferase in rat liver. It catalyzes the sulfation of many drugs, carcinogens, and xenobiotics. Among these substrates are chiral benzylic alcohols. Many benzylic alcohols are metabolites of stereoselective oxidative or reductive biotransformation pathways. Stereochemical properties of the AST IV isoform are important, owing to use of the rat in model studies on carcinogenesis and drug metabolism, as well as homology with human phenol sulfotransferases.;Homology modeling of AST IV based on the crystal structure of mouse estrogen sulfotransferase revealed three amino acid residues, Phe 77, Phe 138 and Tyr 236 that might provide steric interactions essential to stereochemical substrate recognition. To test this hypothesis, mutants were constructed wherein each of these residues were substituted with alanine. In the case of chiral 1,2,3,4-tetrahydro-1-naphthol (THN), stereospecificity of Wild Type AST IV for R-(-)-THN as substrate was lost in the F77A and F138A mutants. Although overall catalytic function of the Y236A mutant was reduced, stereospecificity was maintained for R-(-)-THN as substrate.;The role of stereochemistry of secondary alcohols in the catalytic efficiency of AST IV was also investigated with a series of chiral 1-phenyl-1-alkanols. While the active site residues Phe 77 and Phe 138 were equally important in determining stereospecificity of AST IV for tetrahydro-1-naphthols, all three residues (Phe 77, Phe 138, and Tyr 236) were important in determining stereoselectivity and catalytic efficiency of AST IV in catalyzing the sulfation of 1-phenyl-1-alkanols. Binding experiments also indicated that conformational changes in the sulfuryl acceptor substrate-binding site were induced by binding of PAP and PADS to the mutant AST IV enzymes.;Our results on stereochemical interactions of rodent AST IV serve as a starting point for comparing structural differences between the rat and human phenol sulfotransferases that influence the stereoselectivity of these enzymes. These studies should prove useful in understanding in vivo metabolic interactions as well as aiding in the design of specific inhibitors to probe the roles of sulfation in xenobiotic metabolism. |
