The Relationship of Discrete Amino Acid Regions to the Biochemical Properties of Human Phenol Sulfotransferases (SULTs)
Sulfation is important in the metabolism of many different compounds including phenols, monoamine neurotransmitters, steroid and thyroid hormones, and numerous xenobiotics. The addition of the sulfuryl moiety to substrates produces effects such as increased water solubility and renal excretion, decreased activity, activation of certain agents, and formation of unstable esters that react with DNA to form adducts. The most studied form of the sulfotransferase (STILT) enzymes in humans is SULT1 Al. Allozymes (expressed allelic variants) of SULT1A1 have been detected in human blood platelets at different proportions in separate ethnic populations. The SULT1A1 level of activity and thermal stability measured in the human population are variable. Allozymes are associated with the variability measured in human SULT1 Al activities and thermal stability. The reason for the catalytic differences is not completely understood, but low STJLT1 Al activity may increase susceptability to toxins due to an individual’s inability to metabolize specific reagents. The super-family of human SULT enzymes is classified according to amino acid sequence differences. SULT1 Al and STJLT1 A3 are 93% similar in their amino acid sequences, but they differ in biochemical properties including substrate specificity and thermal stability. To gain knowledge of amino acids that affect the different biochemical properties we produced chimeric and mutant SULT1A1 and SULT1A3 enzymes. Specific amino acid residues over the length of the protein affect the biochemical properties of the SULTl A1 and SULT1 A3 enzymes. The N-terrninal portion of the SULT1A1 enzyme is important for measurable activity of the enzyme because when mutated to the residues present in the N-terminus of SULTl A3, no activity is detectable. The presence of the N-terminal of SULTl A1 with the center region of SUI,T1 A3 creates a more hydrophilic protein with greater thermal stability and resistance to NaCl inhibition for these chimeric proteins. Mutations within the C-terminal portion of SULTl A1 and SULTl A3 produce changes in sensitivities to inhibitors when using dopamine as the substrate. The Phe247 of SULTl A1 when mutated alters the ability of SULTl A1 to catalyze the sulfate conjugation of dopamine. When a phenylalanine is inserted into the 247 position of SULTl A3, the thermal stability is almost equal to SULTl Al. Therefore, there are specific regions of the SULT enzyme amino acid sequences that have an effect on the biochemical properties of these enzymes. Knowledge of the interactions between the amino acid sequences of SULT enzymes and their biochemical properties will aid our understanding of the differences between SULT enzymes and allozymes. This is important because it provides a functional component for certain protein residues. The information may help predict whether an allozyme in the human population may have altered abilities in metabolizing certain substrates.
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