As part of the project in which the enzymes of detoxication are being examined as to their catalytic mechanism, a specific sulfotransferase that is active with phenols has been closely examined. The enzyme, tyrosine-ester sulfotransferase, is listed by the nomenclature Committee of the IUBMB as an enzyme for the transfer of the sulfuryl group of 3- phosphoadenosine-5-phosphosulfate to a wide variety of phenols. Indeed, one of the characteristics of the enzymes that are active with foreign compounds, the enzymes of detoxication, is the broad range of compounds that one accepted as substrates. This laboratory has cloned the enzyme from rat liver MRNA and has expressed it in Escherichia coli in very large quantities. The resultant tyrosine-ester sulfotransferase expressed by the bacterium differs in several cogent properties from that in the natural enzyme. In fact, there are differences in substrate specificity, in Ph optima for specific substrate, and even in the resistance of the enzyme to heat. Despite the number of trivial possibilities that exist for this difference, most of them have been eliminated and the current view is that the recombinant enzyme is folded slightly differently from the natural one. Although such characteristics as substrate specificity, Ph optimum and heat sensitivity have been used to characterize the differences among specific enzymes, it should be clear that genetic data is required before concluding that isoforms with overlapping specificity exist as specific enzyme species on the basis of these properties.