Molecular cloning, expression, and characterization of a canine sulfotransferase that is a human ST1B2 ortholog

Sulfotransferases (SULTs), enzymatic and genetic variation in Carnivora: Limited sulfation capacity in pinnipeds

Wild carnivorans are one of the most important species due to their high positions in the food chain. They are also highly affected by numerous environmental contaminants through bioaccumulation and biomagnification. Xenobiotic metabolism is a significant chemical defense system from xenobiotics because it degrades the activity of a wide range of chemicals, generally into less active forms, resulting in their deactivation. Sulfotransferases (SULTs) are one of the most important xenobiotic metabolic enzymes, which catalyze the sulfonation of a variety of endogenous and exogenous chemicals, such as hormones, neurotransmitters, and a wide range of xenobiotic compounds. Although SULTs are of such high importance, little research has focused on these enzymes in wild carnivorans. In this study, we clarified the genetic properties of SULTs in a wide range of mammals, focusing on carnivorans, using in silico genetic analyses. We found genetic deficiencies of SULT1E1 and SULT1D1 isoforms in all pinnipeds analyzed and nonsense mutations in SULT1Cs in several carnivorans including pinnipeds. We further investigated the enzymatic activity of SULT1E1 in vitro using liver cytosols from pinnipeds. Using a SULT1E1 probe substrate, we found highly limited estradiol sulfonation in pinnipeds, whereas other mammals had relatively high sulfation. These results suggest that pinnipeds have severely or completely absent SULT1E1 activity, which importantly catalyzes the metabolism of estrogens, drugs, and environmental toxins. This further implies a high susceptibility to a wide range of xenobiotics in these carnivorans, which are constantly exposed to environmental chemicals throughout their lifetime.

Structure of mouse cytosolic sulfotransferase SULT2A8 provides insight into sulfonation of 7α-hydroxyl bile acids

Cytosolic sulfotransferases (SULTs) catalyze the transfer of a sulfonate group from the cofactor 3’-phosphoadenosine 5’-phosphosulfate to a hydroxyl (OH) containing substrate and play a critical role in the homeostasis of endogenous compounds, including hormones, neurotransmitters, and bile acids. In human, SULT2A1 sulfonates the 3-OH of bile acids; however, bile acid metabolism in mouse is dependent on a 7α-OH sulfonating SULT2A8 via unknown molecular mechanisms. In this study, the crystal structure of SULT2A8 in complex with adenosine 3’,5’-diphosphate and cholic acid was resolved at a resolution of 2.5 Å. Structural comparison with human SULT2A1 reveals different conformations of substrate binding loops. In addition, SULT2A8 possesses a unique substrate binding mode that positions the target 7α-OH of the bile acid close to the catalytic site. Furthermore, mapping of the critical residues by mutagenesis and enzyme activity assays further highlighted the importance of Lys44 and His48 for enzyme catalysis and Glu237 in loop 3 on substrate binding and stabilization. In addition, limited proteolysis and thermal shift assays suggested that the cofactor and substrates have protective roles in stabilizing SULT2A8 protein. Together, the findings unveil the structural basis of bile acid sulfonation targeting 7α-OH and shed light on the functional diversity of bile acid metabolism across species.

Highly selective bioactivation of 1- and 2-hydroxy-3-methylcholanthrene to mutagens by individual human and other mammalian sulphotransferases expressed in Salmonella typhimurium

The benzylic alcohols 1- and 2-hydroxy-3-methylcholanthrene (OH-MC) are major primary metabolites of the carcinogen 3-methylcholanthrene (MC). We investigated them for mutagenicity in TA1538-derived Salmonella typhimurium strains expressing mammalian sulphotransferases (SULTs). 1-OH-MC was efficiently activated by human (h) SULT1B1 (2400 revertants/nmol), weakly activated by hSULT1C3 and hSULT2A1 (2-9 revertants/nmol), but not activated by the other hSULTs studied (1A2, 1A3, 1C2 and 1E1). Mouse, rat and dog SULT1B1 activated 1-OH-MC (8-100 revertants/nmol) with much lower efficiency than their human orthologue. The other isomer, 2-OH-MC, was activated to a potent mutagen by hSULT1A1 (4000-5400 revertants/nmol), weakly activated by hSULT1A2 or hSULT2A1 (1-12 revertants/nmol), but not activated by the other hSULTs. In contrast to their human orthologue, mouse, rat and dog SULT1A1 did not appreciably activate 2-OH-MC (<1 to 6 revertants/nmol), either. Instead, mouse and rat SULT1B1, unlike their human and canine orthologues, demonstrated some activation of 2-OH-MC (15-100 revertants/nmol). Docking analyses indicated that 1- and 2-OH-MC might bind to the active site of hSULT1A1 and hSULT1B1, but only for (S)-2-OH-MC/hSULT1A1 and (R)-1-OH-MC/hSULT1B1 with an orientation suitable for catalysis. Indeed, 1- and 2-OH-MC were potent inhibitors of the hSULT1A1-mediated sulphation of acetaminophen [concentration inhibiting the enzyme activity by 50% (IC50) 15 and 13nM, respectively]. This inhibition was weak with mouse, rat and dog SULT1A1 (IC50 ≥ 4 µM). Inhibition of the SULT1B1 enzymes was moderate, strongest for 1-OH-MC/hSULT1B1. In conclusion, this study provides examples for high selectivity of bioactivation of promutagens by an individual form of human SULT and for pronounced differences in activation capacity between orthologous SULTs from different mammalian species. These characteristics make the detection and evaluation of such mutagens extremely difficult, in particular as the critical form may even differ for positional isomers, such as 1- and 2-OH-MC. Moreover, the species-dependent differences will complicate the verification of in vitro results in animal studies.

Sulfation and glucuronidation of phenols: implications in coenyzme Q metabolism

Phase II conjugation of phenolic compounds constitutes an important mechanism through which exogenous or endogenous toxins are detoxified and excreted. Species differences in the rates of glucuronidation or sulfation can lead to significant variation in the metabolism of this class of compounds. Conjugation of the hydroxyl groups of phenols can occur with glucuronate or sulfate. Quinone metabolism, deactivation, and detoxification are also affected by the same conjugatory systems as phenols; however, reduction of quinones to hydroquinols seems to be a prerequisite. This work reviews current knowledge on phenol conjugation and its implications on hydroquinone metabolism with special consideration for coenzyme Q metabolism.

Characterization of the rates of testosterone metabolism to various products and of glutathione transferase and sulfotransferase activities in rat intestine and comparison to the corresponding hepatic and renal drug-metabolizing enzymes

Metabolism of testosterone to various products (catalyzed by several different CYP isozymes) and the activities of phenol sulfotransferase (pST) and glutathione transferase (GST) in S9 fractions prepared from the mucosa of the duodenum, jejunum, ileum, caecum and upper and lower colon of male Sprague-Dawley rats were determined and compared to the corresponding hepatic and renal activities. Incubation of the S9 fraction prepared from the jejunum with testosterone and NADPH resulted in the formation of 2alpha-, 6alpha-, 6beta- and 16alpha-hydroxytestosterone and androstenedione at rates that were 1.6, 24, 1.3, 0.6 and 1.3%, respectively, of the corresponding hepatic values. The production of 2alpha-hydroxytestosterone was catalyzed only by the preparations from the duodenum and jejunum; whereas 6alpha-, 6beta- and 16alpha-hydroxytestosterone and androstenedione were produced in all regions of the intestine. In the case of the rat kidney, the rates of formation of the different testosterone metabolites were between 0.6 and 35% of the corresponding liver activity. The activity of glutathione transferase was approximately 12-26% of the corresponding hepatic activity throughout the intestine. The highest activity of phenol sulfotransferase was observed in the lower colon (almost 6% of the liver activity) and the lowest activity in the duodenum (1%). The renal activities of GST and pST were 70 and 1%, respectively, of the corresponding liver values. In summary, the metabolism of testosterone and the activities of GST and pST in rat intestine are generally low to very low in comparison to the corresponding activities in rat liver. In most cases, these activities are present throughout the entire intestine and not restricted to a particular portion(s) of this organ.

Amino acid residue 247 in canine sulphotransferase SULT1D1: a new determinant of substrate selectivity

The SULT (sulphotransferase) family plays a critical role in the detoxification and activation of endogenous and exogenous compounds as well as in the regulation of steroid hormone actions and neurotransmitter functions. The structure-activity relationships of the human SULTs have been investigated with focus on the amino acid 146 in hSULT1A3 and its impact on dopamine/PNP (p-nitrophenol) specificity. In the present study, we have generated canine SULT1D1 (cSULT1D1) variants with mutations at amino acid residues in the substrate-binding pocket [A146E (Ala-146-->Glu), A146D, A146Q, I86D or D247L]. These mutation sites were chosen with regard to their possible contribution to the marked dopamine/PNP preference of cSULT1D1. After characterization, we found that the overall sulphation efficiencies for the cSULT1D1 A146 and the I86 mutants were strongly decreased for both substrates compared with wild-type cSULT1D1 but the substrate preference was unchanged. In contrast, the D247L mutant was found to be more than 21-fold better at sulphating PNP (120-fold decrease in K(m) value) but 54-fold less efficient in sulphating dopamine (8-fold increase in K(m) value) and the preference was switched from dopamine to PNP, indicating the importance of this amino acid in the dopamine/PNP preference in cSULT1D1. Our results show that Asp-247 has a pronounced effect on the substrate specificity of cSULT1D1 and thus we have identified a previously unrecognized contributor to active-site selectivity.

A proposed nomenclature system for the cytosolic sulfotransferase (SULT) superfamily

A nomenclature system for the cytosolic sulfotransferase (SULT) superfamily has been developed. The nomenclature guidelines were applied to 65 SULT cDNAs and 18 SULT genes that were characterized from eukaryotic organisms. SULT cDNA and gene sequences were identified by querying the GenBank databases and from published reports of their identification and characterization. These sequences were evaluated and named on the basis of encoded amino acid sequence identity and, in a few cases, a necessity to maintain historical naming convention. Family members share at least 45% amino acid sequence identity whereas subfamily members are at least 60% identical. cDNAs which encode amino acid sequences of at least 97% identity to each other were assigned identical isoform names. We also attempted to categorize orthologous enzymes between various species, where these have been identified, and the nomenclature includes a species descriptor. We present recommendations for the naming of allelic variants of SULT genes and their derived allozymes arising from single nucleotide polymorphisms and other genetic variation. The superfamily currently comprises 47 mammalian SULT isoforms, one insect isoform and eight plant enzymes, and collectively these sequences represent nine separate SULT families and 14 subfamilies. It is hoped that this nomenclature system will be widely adopted and that, as novel SULTs are identified and characterized, investigators will name their discoveries according to these guidelines.

Conjugation metabolism of acetaminophen and bilirubin in extrahepatic tissues of rats

An anhepatic rat model was used to explore the extrahepatic conjugating metabolism of acetaminophen and serum bilirubin. The recovery of glucuronide- and sulfate-acetaminophen was 47.5% in normal control and 13.4% in model rats in the urine collected for 6 h after administration of acetaminophen 20 mg kg(-1). Following the increase of acetaminophen dose to 150 mg kg(-1), the recovery of urinary glucuronide-acetaminophen increased by 53.9% in normal control; but it decreased by 36.4% in model rats. In contrast to normal control, the pretreatment with phenobarbital did not affect acetaminophen and its metabolite levels in plasma and urine in model rats. After the establishment of anhepatic model the serum direct bilirubin rose dramatically. Urinary bilirubin test was positive in model rats, but not in normal control. No changes were observed in serum total bilirubin and ratio of direct/total bilirubin after the pretreatment with ranitidine or phenobarbital 50 mg kg (-1), i.p. for 5 days in model rats. The results indicate that the glucuronide- and sulfate-acetaminophen formed in the extrahepatic tissues of model rats is 28.2% of normal control, serum free bilirubin can be transformed into conjugated bilirubin in extrahepatic tissues, and the regulation mechanism of phase II conjugating enzymes is different between the hepatic and extrahepatic tissues.

Canine sulfotransferase SULT1A1: molecular cloning, expression, and characterization

Sulfotransferases (SULTs) are involved in detoxification and activation of various endogenous and exogenous compounds including important drugs and hormones. SULT1A, the phenol-SULT subfamily, is the most prominent subfamily in xenobiotic metabolism and has been found in several species, e.g., human, rat, and mouse. We have cloned a phenol-sulfating phenol SULT from dog (cSULT1A1) and expressed it in Escherichia coli for characterization. cSULT1A1 showed 85.8, 82.7, 76.3, and 73.6% identities to human P-PST, human M-PST, rat PST-1, and mouse STp1, respectively. It consists of 295 amino acids, which is in agreement with the human ortholog and sulfate substrates typical for the SULT1A family, i.e., p-nitrophenol (PNP), alpha-naphthol, and dopamine. The K(m) for PNP was found to be within the nanomolar range. It also sulfates minoxidil and beta-estradiol but not dehydroepiandrosterone. Western blot analysis indicated that this newly cloned enzyme was found to be ubiquitously expressed in canine tissues with highest expression in male and female liver.