INFLUENCE OF SUBSTRATE STRUCTURE ON THE CATALYTIC EFFICIENCY OF HYDROXYSTEROID SULFOTRANSFERASE STA IN THE SULFATION OF ALCOHOLS

Sulfotransferase a (STa) is an isoform of hydroxysteroid (alcohol) sul fotransferase that catalyzes the formation of sulfuric acid esters fro m both endogenous and xenobiotic alcohols. Among its various functions in toxicology, STa is the major form of hepatic sulfotransferase in t he rat that catalyzes the formation of genotoxic and carcinogenic sulf uric acid esters from hydroxymethyl polycyclic aromatic hydrocarbons. The goal of the present study was to elucidate fundamental quantitativ e relationships between substrate structure and catalytic activity of STa that would be applicable to these and other xenobiotics. We have m odified previous procedures for purification of STa in order to obtain sufficient amounts of homogeneous enzyme for determination of k(cat)/ K-m values, a quantitative measure of catalytic efficiency. We determi ned the catalytic efficiency of STa with benzyl alcohol and eight benz ylic alcohols that were substituted with n-alkyl groups (CnH2n+1, wher e n = 1-8) in the para position, and the optimum value for k(cat)/K-m in these reactions was obtained with n-pentylbenzyl alcohol. Correlati ons between logarithms of k(cat)/K-m values and logarithms of partitio n coefficients revealed that hydrophobicity of the substrate was a maj or factor contributing to the catalytic efficiency of STa. Primary n-a lkanols (CnH2n+1OH, where n = 3-16) exhibited an optimum k(cat)/K-m fo r C-9-C-11 and a linear decrease in upsilon(max) of the reaction for C -3-C-14; 15- and 16-carbon n-alkanols were not substrates for STa. The se results indicated limits to the length of the extended carbon chain in substrates. Such limits may also apply to hydroxysteroids, since c holesterol was inactive as either substrate or inhibitor of STa. Furth ermore, the importance of steric effects on the catalytic efficiency o f STa was also evident with a series of linear, branched, and cyclic s even-carbon aliphatic alcohols. In conclusion, our results provide fun damental quantitative relationships between substrate structure and ca talytic efficiency that yield insight into the specificity of STa for both endogenous and xenobiotic alcohols.