Alteration of human UDP-glucuronosyltransferase UGT2B17 regio-specificity by a single amino acid substitution

Substrate selectivity of human intestinal UDP-glucuronosyltransferases (UGTs): in silico and in vitro insights

The current drug development process aims to produce safe, effective drugs within a reasonable time and at a reasonable cost. Phase II metabolism (glucuronidation) can affect drug action and pharmacokinetics to a considerable extent and so its studies and prediction at initial stages of drug development are very imperative. Extensive glucuronidation is an obstacle to oral bioavailability because the first-pass glucuronidation [or premature clearance by UDP-glucuronosyltransferases (UGTs)] of orally administered agents frequently results in poor oral bioavailability and lack of efficacy. Modeling of new chemical entities/drugs for UGTs and their kinetic data can be useful in understanding the binding patterns to be used in the design of better molecules. This review concentrates on first-pass glucuronidation by intestinal UGTs, including their topology, expression profile, and pharmacogenomics. In addition, recent advances are discussed with respect to substrate selectivity at the binding pocket, structural requirements, and mechanism of enzyme actions.

Highly variable pH effects on the interaction of diclofenac and indomethacin with human UDP-glucuronosyltransferases

In vitro glucuronidation assays of diclofenac and indomethacin at pH 7.4 are biased by the instability of the glucuronides due to acyl migration. The extent of this acyl migration may be reduced significantly by performing the glucuronidation reaction at pH 6.0. Testing the human UDP-glucuronosyltransferases (UGTs) of subfamilies 1A, 2A and 2B at pH 7.4 revealed that UGT1A10, UGT2B7 and UGT2B17 are the most active enzymes in diclofenac glucuronidation, while the highest indomethacin glucuronidation rates (corrected for relative expression levels) were exhibited by UGT2A1, UGT1A10 and UGT2B7. Interestingly, lowering the reaction pH to 6.0 increased the activity of many UGTs, particularly UGT1A10, toward both drugs, even if the rate of 4-methylumbelliferone glucuronidation by UGT1A10 at pH 6.0 was significantly lower than at pH 7.4. On the other hand, UGT2B15 lost activity upon lowering the reaction pH to 6.0. UGT1A6 does not glucuronidate diclofenac and indomethacin. Nevertheless, both drugs inhibit the 1-naphthol glucuronidation activity of UGT1A6 and their inhibition was stimulated by lowering the reaction pH, yielding significantly lower IC(50) values at pH 6.0 than at pH 7.4. In conclusion, glucuronidation reactions pH affects their outcome in variable ways and could increase the toxicity of drugs that carry a carboxylic acid.

How many and which amino acids are responsible for the large activity differences between the highly homologous UDP-glucuronosyltransferases (UGT) 1A9 and UGT1A10?

The amino acid sequences of the human UDP-glucuronosyltransferases (UGTs) 1A9 and 1A10 are 93% identical, yet there are large differences in their activity and substrate selectivity. For example, the regioselectivity in propranolol glucuronidation, the regioselectivity in dobutamine glucuronidation, and the glucuronidation rate of alpha- and beta-estradiol differ greatly between UGT1A9 and UGT1A10. To identify the residue responsible for the activity differences, we divided the N-terminal half of the two UGTs into five comparable segments by inserting four unique restriction sites at identical positions in both genes and constructing chimeras in which segments of UGT1A9 were individually replaced by the corresponding segments from UGT1A10. Activity analyses of the resulting mutants, 910A [1A10((1-83))/1A9((84-285))], 910B [1A9((1-83))/1A10((84-147))/1A9((148-285))], 910C [1A9((1-147))/1A10((148-181))/1A9((182-285))], 910D [1A9((1-181))/1A10((182-235))/1A9((236-285))], and 910E [1A9((1-235))/1A10((236-285))] indicated that more than one residue is responsible for the differences between UGT1A9 and UGT1A10. We next prepared four double chimeras, in which two of the above UGT1A9 segments were replaced simultaneously by the corresponding UGT1A10 segments. However, none of the double chimeras glucuronidated either estradiol, propranolol, or dobutamine at rates that resembled those of UGT1A10. On the other hand, studying the kinetics of 1-naphthol glucuronidation yielded more focused results, indicating that residues within segment B (84-147) contribute directly to the K(m) value for this substrate. Further mutagenesis and activity assays suggested that Phe117 of UGT1A9 participates in 1-naphthol binding. In addition, it appears that residues within segment C of the N-terminal domain, mainly at positions 152 and 169, contribute to the higher glucuronidation rates of UGT1A10.

Influence of N-terminal domain histidine and proline residues on the substrate selectivities of human UDP-glucuronosyltransferase 1A1, 1A6, 1A9, 2B7, and 2B10

An N-terminal domain histidine [corresponding to position 39 of UDP-glucuronosyltransferase (UGT) 1A1] is conserved in all UGT1A and UGT2B subfamily proteins except UGT1A4 (Pro-40) and UGT2B10 (Leu-34). Unlike most UGT1A and UGT2B xenobiotic-metabolizing enzymes, UGT1A4 and UGT2B10 lack the ability to glucuronidate 4-methylumbelliferone (4MU) and 1-naphthol (1NP), both planar phenols, and naproxen (a carboxylic acid). However, only UGT1A4 glucuronidates the tertiary amines lamotrigine (LTG) and trifluoperazine (TFP). In this study, we sought to elucidate the influence of specific N-terminal histidine and proline residues on UGT enzyme substrate selectivity. The conserved N-terminal domain histidine of UGT1A1, UGT1A6, UGT1A9, and UGT2B7 was mutated to proline and leucine 34 of UGT2B10 was substituted with histidine, and the capacity of the wild-type and mutant proteins to glucuronidate 4MU, 1NP, LTG, TFP, and naproxen was characterized. Whereas UGT1A1(H39P), UGT1A6(H38P), and UGT1A9(H37P) lacked the ability to metabolize 4MU, 1NP, and naproxen, all glucuronidated LTG. K(m) values for UGT1A1(H39P) and UGT1A9(H37P) were 774 and 3812 microM, respectively, compared with 1579 microM for UGT1A4. UGT1A1(H39P) also glucuronidated TFP with a V(max)/K(m) value comparable to that of UGT1A4. In contrast to the wild-type enzyme, UGT2B10(L34H) glucuronidated 4MU and 1NP with respective K(m) values of 260 and 118 microM. UGT2B7(H35P) lacked activity toward all substrates. The data confirm a pivotal role for an N-terminal domain proline in the glucuronidation of the tertiary amines LTG and TFP by UGT1A subfamily proteins, whereas glucuronidation reactions involving proton abstraction generally, although not invariably, require a histidine at the equivalent position in both UGT1A and UGT2B enzymes.

Activators of the farnesoid X receptor negatively regulate androgen glucuronidation in human prostate cancer LNCAP cells

Androgens are major regulators of prostate cell growth and physiology. In the human prostate, androgens are inactivated in the form of hydrophilic glucuronide conjugates. These metabolites are formed by the two human UGT2B15 [UGT (UDP-glucuronosyltransferase) 2B15] and UGT2B17 enzymes. The FXR (farnesoid X receptor) is a bile acid sensor controlling hepatic and/or intestinal cholesterol, lipid and glucose metabolism. In the present study, we report the expression of FXR in normal and cancer prostate epithelial cells, and we demonstrate that its activation by chenodeoxycholic acid or GW4064 negatively interferes with the levels of UGT2B15 and UGT2B17 mRNA and protein in prostate cancer LNCaP cells. FXR activation also causes a drastic reduction of androgen glucuronidation in these cells. These results point out activators of FXR as negative regulators of androgen-conjugating UGT expression in the prostate. Finally, the androgen metabolite androsterone, which is also an activator of FXR, dose-dependently reduces the glucuronidation of androgens catalysed by UGT2B15 and UGT2B17 in an FXR-dependent manner in LNCaP cells. In conclusion, the present study identifies for the first time the activators of FXR as important regulators of androgen metabolism in human prostate cancer cells.

Critical Roles of Residues 36 and 40 in the Phenol and Tertiary Amine Aglycone Substrate Selectivities of UDP-Glucuronosyltransferases 1A3 and 1A4

Despite high sequence identity, UGT1A3 and UGT1A4 differ in terms of substrate selectivity. UGT1A3 glucuronidates the planar phenols 1-naphthol (1-NP) and 4-methylumbelliferone (4-MU), whereas UGT1A4 converts the tertiary amines lamotrigine (LTG) and trifluoperazine (TFP) to quaternary ammonium glucuronides. Residues 45 to 154 (which incorporate 21 of the 35 amino acid differences) and 45 to 535 were exchanged between UGT1A3 and UGT1A4 to generate UGT1A3-4((45-535)), UGT1A3-4((45-154))-3, UGT1A4-3((45-535)), and UGT1A4-3((45-154))-4 hybrid proteins. Although differences in kinetic parameters were observed between the parent enzymes and chimeras, UGT1A4-3((45-535)) and UGT1A4-3((45-154))-4 [but not UGT1A3-4((45-535)) and UGT1A3-4((45-154))-3] retained the capacity to glucuronidate LTG and TFP. Likewise, UGT1A3-4((45-535)) and UGT1A3-4((45-154))-3 retained the capacity to glucuronidate 1-NP and 4-MU, but UGT1A4-3((45-535)) and UGT1A4-3((45-154))-4 exhibited low or absent activity. Within the first 44 residues, UGT1A3 and UGT1A4 differ in sequence at positions 36 and 40. "Reciprocal" mutagenesis was performed to generate the UGT1A3(I36T), UGT1A3(H40P), UGT1A4(T36I), and UGT1A4 (P40H) mutants. The T36I and P40H mutations in UGT1A4 reduced in vitro clearances for LTG and TFP glucuronidation by >90%. Conversely, the I36T and H40P mutations in UGT1A3 reduced the in vitro clearances for 1-NP and 4-MU glucuronidation by >90%. Introduction of the single H40P mutation in UGT1A3 conferred LTG and TFP glucuronidation, whereas the single T36I mutation in UGT1A4 conferred 1-NP and 4-MU glucuronidation. Thus, residues 36 and 40 of UGT1A3 and UGT1A4 are pivotal for the respective selectivities of these enzymes toward planar phenols and tertiary amines, although other regions of the proteins influence binding affinity and/or turnover.

Amino terminal domains of human UDP-glucuronosyltransferases (UGT) 2B7 and 2B15 associated with substrate selectivity and autoactivation

Despite the important role of UDP-glucuronosyltransferases (UGT) in the metabolism of drugs, environmental chemicals and endogenous compounds, the structural features of these enzymes responsible for substrate binding and selectivity remain poorly understood. Since UGT2B7 and UGT2B15 exhibit distinct, but overlapping, substrate selectivities, UGT2B7-UGT2B15 chimeras were constructed here to identify substrate binding domains. A UGT2B7-15-7 chimera that incorporated amino acids 61-194 of UGT2B15 glucuronidated the UGT2B15 substrates testosterone and phenolphthalein, but not the UGT2B7 substrates zidovudine and 11alpha-hydroxyprogesterone. Derived apparent K(m) values for testosterone and phenolphthalein glucuronidation by UGT2B7-15((61-194))-7 were similar in magnitude to those determined for UGT2B15. Moreover, glucuronidation of the non-selective substrate 4-methylumbelliferone (4MU) by UGT2B7-15((61-194))-7 and UGT2B15 followed Michaelis-Menten and weak substrate inhibition kinetics, respectively, whereas 4MU glucuronidation by UGT2B7 exhibited sigmoidal kinetics characteristic of autoactivation. Six UGT2B7-15-7 chimeras that incorporated smaller domains of UGT2B15 were subsequently generated. Of these, UGT2B7-15((61-157))-7, UGT2B7-15((91-157))-7 and UGT2B7-15((61-91))-7 glucuronidated 4MU, but activity towards the other substrates investigated here was not detected. Like UGT2B7, the UGT2B7-15((61-157))-7, UGT2B7-15((91-157))-7 and UGT2B7-15((61-91))-7 chimeras exhibited sigmoidal 4MU glucuronidation kinetics. The sigmoidal 4MU kinetic data were well modelled using both the Hill equation and the expression for a two-site model that assumes the simultaneous binding of two substrate molecules at equivalent sites. It may be concluded that residues 61-194 of UGT2B15 are responsible for substrate binding and for conferring the unique substrate selectivity of UGT2B15, while residues 158-194 of UGT2B7 appear to facilitate the binding of multiple 4MU molecules within the active site.

Substrate specificity of the human UDP-glucuronosyltransferase UGT2B4 and UGT2B7. Identification of a critical aromatic amino acid residue at position 33

The human UDP-glucuronosyltransferase (UGT) isoforms UGT2B4 and UGT2B7 play a major role in the detoxification of bile acids, steroids and phenols. These two isoforms present distinct but overlapping substrate specificity, sharing common substrates such as the bile acid hyodeoxycholic acid (HDCA) and catechol-estrogens. Here, we show that in UGT2B4, substitution of phenylalanine 33 by leucine suppressed the activity towards HDCA, and impaired the glucuronidation of several substrates, including 4-hydroxyestrone and 17-epiestriol. On the other hand, the substrate specificity of the mutant UGT2B4F33Y, in which phenylalanine was replaced by tyrosine, as found at position 33 of UGT2B7, was similar to wild-type UGT2B4. In the case of UGT2B7, replacement of tyrosine 33 by leucine strongly reduced the activity towards all the tested substrates, with the exception of 17-epiestriol. In contrast, mutation of tyrosine 33 by phenylalanine exhibited similar or even somewhat higher activities than wild-type UGT2B7. Hence, the results strongly indicated that the presence of an aromatic residue at position 33 is important for the activity and substrate specificity of both UGT2B4 and UGT2B7.

Characterization of a common deletion polymorphism of the UGT2B17 gene linked to UGT2B15

Members of the human UDP-glucuronosyltransferase 2B family are located in a cluster on chromosome 4q13 and code for enzymes whose gene products are responsible for the normal catabolism of steroid hormones. Two members of this family, UGT2B15 and UGT2B17, share over 95% sequence identity. However, UGT2B17 exhibits broader substrate specificity due to a single amino acid difference. Using gene-specific primers to explore the genomic organization of these two genes, it was determined that UGT2B17 is absent in some human DNA samples. The gene-specific primers demonstrated the presence or absence of a 150 kb genomic interval spanning the entire UGT2B17 gene, revealing that UGT2B17 is present in the human genome as a deletion polymorphism linked to UGT2B15. Furthermore, it is shown that the UGT2B17 deletion polymorphism shows Mendelian segregation and allele frequencies that differ between African Americans and Caucasians.

Human uridine diphosphate-glucuronosyltransferase UGT2B7 conjugates mineralocorticoid and glucocorticoid metabolites

Mineralocorticoid and glucocorticoid hormones are metabolized as glucuronide conjugates. Using labeled [(14)C]uridine diphosphate glucuronic acid and microsomal preparations from human embryonic kidney 293 cells stably expressing the different human and monkey uridine diphosphate glucuronosyltransferase (UGT)2B enzymes, it is demonstrated that the two human allelic variants UGT2B7H((268)) and UGT2B7Y((268)) conjugate aldosterone, its A-ring reduced metabolites (5alpha-dihydroaldosterone and 3alpha,5beta-tetrahydroaldosterone), and both 5alpha- and 5beta-tetrahydrocortisone epimers. The two variants of UGT2B4 also glucuronidate tetrahydroaldosterone, whereas all enzymes tested were inefficient to produce cortisol glucuronide derivatives. Kinetic analyses reveal that UGT2B7 polymorphisms glucuronidate mineralocorticoids with a 5.5- to 20-fold higher affinity than glucocorticoids. For the first time, a significant difference between the two allelic variants of UGT2B7 is described, because UGT2B7H((268)) possesses an 11-fold higher aldosterone glucuronidation efficiency (ratio Vmax((app.))/Km((app.))) than UGT2B7Y((268)). RT-PCR experiments demonstrate the expression of UGT2B7 in human kidney and in renal proximal tubule epithelial cells, suggesting that mineralocorticoids and glucocorticoids are metabolized in their target tissue. Measurement of aldosterone glucuronidation and normalization with the UGT2B protein contents in monkey tissues demonstrate that liver and kidney glucuronidate this hormone with a similar velocity. Immunohistochemical studies performed in monkey kidney cortex reveal a restrictive expression of UGT2B proteins in the epithelial cells of the proximal tubules. Because expression of the mineralocorticoid receptor was detected in the distal tubule epithelial cells, the present data suggest a two-cell mechanism of aldosterone action and metabolism in the kidney.

Glucuronidation activity of the UGT2B17 enzyme toward xenobiotics

UDP-glucuronosyltransferase (UGT) 2B17 is one of the most important conjugating enzymes in androgen metabolism and shares more than 95% homology with UGT2B15. Although UGT2B15 has been fully characterized for its ability to conjugate drugs, environmental pollutants, and dietary components, UGT2B17 received less attention for its capacity to glucuronidate xenobiotics. In the present study, more than 55 exogenous compounds belonging to several categories of compounds were analyzed as potential substrates for UGT2B17. Glucuronidation activity was observed with several coumarins, anthraquinones, and flavonoids. The higher glucuronidation activity was measured with alizarin (125 pmol x min(-1) x mg protein(-1)), whereas UGT2B17 conjugated eugenol, scopoletin, and galangin with glucuronidation rates of 102.5, 102, and 58 pmol x min(-1) x mg protein(-1), respectively. The characterization of UGT2B17 as a xenobiotics-conjugating enzyme demonstrates that its role is not limited to androgen metabolism and that its specificity for exogenous substrates is different from other UGT2B isoforms. Taken together, these data suggest a role of UGT2B17 for the hepatic detoxification.

Relative enzymatic activity, protein stability, and tissue distribution of human steroid-metabolizing UGT2B subfamily members

Androgens and estrogens play major roles in cell differentiation, cell growth, and peptide secretion in steroid target tissues. In addition to the binding of these hormones to their receptors, formation and metabolism are important in the action of steroids. Metabolism of the potent steroid hormones includes glucuronidation, a major pathway of steroid elimination in liver and several steroid target tissues. Glucuronidation is catalyzed by UDP-glucuronosyltransferases (UGTs), which transfer the polar moiety from UDP-glucuronic acid to a wide variety of endogenous compounds, including steroid hormones. The UGT superfamily of enzymes is subdivided into two families, UGT1 and UGT2, on the basis of sequence homology. To date, six UGT2B proteins have been isolated, namely UGT2B4, UGT2B7, UGT2B10, UGT2B11, UGT2B15, and UGT2B17, all of which have been demonstrated to be active on steroid molecules, except for UGT2B10 and UGT2B11, for which no substrate was found. The relative activity of these enzymes on steroidal compounds remains unknown due to variable levels of UGT2B expression in different in vitro cell line models and various conditions of the enzymatic assays. Comparison of the glucuronidation rates of these enzymes requires a unique system for UGT2B protein expression, protein normalization, and enzymatic assays. In this study we have stably expressed UGT2B4, UGT2B7, UGT2B15, and UGT2B17 in the HK293 cell line, which is devoid of steroid UGT activity; characterized their kinetic properties relative to UGT protein expression; determined their transcript and protein stabilities; and established extensively their tissular distributions. UGT2B7 was demonstrated to glucuronidate estrogens, catechol estrogens, and androstane-3alpha,17beta-diol more efficiently than any other human UGTB isoform. UGT2B15 and UGT2B17 showed similar glucuronidation activity for androstane-3alpha,17beta-diol (30% lower than that of UGT2B7), whereas UGT2B17 demonstrated the highest activity for androsterone, testosterone, and dihydrotestosterone. UGT2B4 demonstrates reactivity toward 5alpha-reduced androgens and catechol estrogens, but at a significantly lower level than UGT2B7, 2B15, and 2B17. Cycloheximide treatment of stably transfected HK293 cells demonstrated that the UGT2B17 protein is more labile than the other enzymes; the protein levels decrease after 1 h of treatment, whereas other UGT2B proteins were stable for at least 12 h. Treatment of stable cells with actinomycin D reveals that UGT2B transcripts are stable for 12 h, except for the UGT2B4 transcript, which was decreased by 50% after the 12-h incubation period. Tissue distribution of the UGT2B enzymes demonstrated that UGT2B isoforms are expressed in the liver as well as in several extrahepatic steroid target tissues, namely, kidney, breast, lung, and prostate. This study clearly demonstrates the relative activities and the major substrates of human steroid-metabolizing UGT2B enzymes, which are expressed in a wide variety of steroid target tissues.

Roles of glucuronidation and UDP-glucuronosyltransferases in xenobiotic bioactivation reactions

Glucuronide conjugates represent one of the major types of naturally occurring phase 2 metabolites of xenobiotics and endobiotics. The process underlying their formation, glucuronidation, is normally considered detoxifying, because glucuronides usually possess less intrinsic biological or chemical activity than their parent aglycones and they are rapid excreted. However, a number of glucuronide conjugates are known that are active and may contribute to pharmacological activities or toxicities associated with their parent compounds. These include two classes of glucuronides with electrophilic chemical reactivity (N-O-glucuronides of hydroxamic acids and acyl glucuronides of carboxylic acids) and several types of glucuronides that impart biological effects through non-covalent interactions (morphine 6-O-glucuronide, retinoid glucuronides, and D-ring glucuronides of estrogens). Glucuronides may thus contribute to clinically significant effects, including environmental arylamine-induced carcinogenesis, drug hypersensitivity and other toxicities associated with carboxylic acid drugs, morphine analgesia, and cholestasis from estrogens. This review summarizes the rat and human UDP-glucuronosyltransferases that may be involved in the formation of bioactive glucuronides, including their substrate- and tissue-specificity and genetic and environmental influences on their activity. This knowledge may be useful for enhancing the therapeutic efficacy and minimizing the risk of adverse effects associated with xenobiotics that undergo bioactivating glucuronidation reactions.

Genetic polymorphism of UDP-glucuronosyltransferase 2B7 (UGT2B7) at amino acid 268: ethnic diversity of alleles and potential clinical significance

UGT2B7 catalyses the glucuronidation of a diverse range of drugs, environmental chemicals and endogenous compounds. Hence, coding region polymorphisms of UGT2B7 are potentially of pharmacological, toxicological and physiological significance. Two variant UGT2B7 cDNAs encoding enzymes with either His or Tyr at residue 268 have been isolated. The variants, referred to as UGT2B7*1 and UGT2B7*2, respectively, arise from a C to T transversion at nucleotide 802 of the UGT2B7 coding region. Analysis of genomic DNA from 91 unrelated Caucasians and 84 unrelated Japanese demonstrated the presence of the variant alleles encoding UGT2B7*1 and UGT2B7*2 in both populations. However, while there was an approximately equal distribution of subjects homozygous for each allele in the Caucasian population, subjects homozygous for the UGT2B7*1 allele were over 10-fold more prevalent than UGT2B7*2 homozygotes in Japanese. The frequencies of the UGT2B7*1 and UGT2B7*2 alleles were 0.511 and 0.489, respectively, in Caucasians, and 0.732 and 0.268, respectively, in Japanese. The 95% confidence intervals for the two alleles did not overlap between Caucasians and Japanese. Rates of microsomal androsterone, menthol and morphine (3-position) glucuronidation were determined for genotyped livers from Caucasian donors. Statistically significant inter-genotypic differences were not apparent for any of the three substrates. Although the UGT2B7 polymorphism characterized here is probably not associated with altered enzyme activity, the results highlight the need to consider ethnic variability in assessing the consequences of UGT polymorphisms.

N-glycosylation and residue 96 are involved in the functional properties of UDP-glucuronosyltransferase enzymes

The recent cloning of several human and monkey UDP-glucuronosyltransferase (UGT) 2B proteins has allowed the characterization of these steroid metabolic enzymes. However, relatively little is known about the structure-function relationship, and the potential post-translational modifications of these proteins. The mammalian UGT2B proteins contain at least one consensus asparagine-linked glycosylation site NX(S/T). Endoglycosidase H digestion of the human and monkey UGT2B proteins demonstrates that only UGT2B7, UGT2B15, UGT2B17, and UGT2B20 are glycosylated. Although UGT2B15 and UGT2B20 contain three and four potential glycosylation sites, respectively, site-directed mutagenesis revealed that both proteins are glycosylated at the same first site. In both proteins, abolishing glycosylation decreased glucuronidation activity; however, the K(m) values and the substrate specificities were not affected. Despite the similarities between UGT2B15 and UGT2B20, UGT2B20 is largely more labile than UGT2B15. Treating HK293 cells stably expressing UGT2B20 with cycloheximide for 2 h decreased the enzyme activity by more than 50%, whereas the activity of UGT2B15 remained unchanged after 24 h. The UGT2B20 protein is unique in having an isoleucine at position 96 instead of an arginine as found in all the other UGT2B enzymes. Changing the isoleucine in UGT2B20 to an arginine stabilized enzyme activity, while the reciprocal mutation in UGT2B15 R96I produced a more labile enzyme. Secondary structure predictions of UGT2B proteins revealed a putative alpha-helix in this region in all the human and monkey proteins. This alpha-helix is shortest in UGT2B20; however, the helix is lengthened in UGT2B20 I96R. Thus, it is apparent that the length of the putative alpha-helix between residues 84 and 100 is a determining factor in the stability of UGT2B enzyme activity. This study reveals the extent and importance of protein glycosylation on UGT2B enzyme activity and that the effect of residue 96 on UGT2B enzyme stability is correlated to the length of a putative alpha-helix.

Isolation and characterization of the human UGT2B15 gene, localized within a cluster of UGT2B genes and pseudogenes on chromosome 4

Glucuronidation is a major pathway of androgen metabolism and is catalyzed by UDP-glucuronosyltransferase (UGT) enzymes. UGT2B15 and UGT2B17 are 95% identical in primary structure, and are expressed in steroid target tissues where they conjugate C19 steroids. Despite the similarities, their regulation of expression are different; however, the promoter region and genomic structure of only the UGT2B17 gene have been characterizedX to date. To isolate the UGT2B15 gene and other novel steroid-conjugating UGT2B genes, eight P-1-derived artificial chromosomes (PAC) clones varying in length from 30 kb to 165 kb were isolated. The entire UGT2B15 gene was isolated and characterized from the PAC clone 21598 of 165 kb. The UGT2B15 and UGT2B17 genes are highly conserved, are both composed of six exons spanning approximately 25 kb, have identical exon sizes and have identical exon-intron boundaries. The homology between the two genes extend into the 5'-flanking region, and contain several conserved putative cis-acting elements including Pbx-1, C/EBP, AP-1, Oct-1 and NF/kappaB. However, transfection studies revealed differences in basal promoter activity between the two genes, which correspond to regions containing non-conserved potential elements. The high degree of homology in the 5'-flanking region between the two genes is lost upstream of -1662 in UGT2B15, and suggests a site of genetic recombination involved in duplication of UGT2B genes. Fluorescence in situ hybridization mapped the UGT2B15 gene to chromosome 4q13.3-21.1. The other PAC clones isolated contain exons from the UGT2B4, UGT2B11 and UGT2B17 genes. Five novel exons, which are highly homologous to the exon 1 of known UGT2B genes, were also identified; however, these exons contain premature stop codons and represent the first recognized pseudogenes of the UGT2B family. The localization of highly homologous UGT2B genes and pseudogenes as a cluster on chromosome 4q13 reveals the complex nature of this gene locus, and other novel homologous UGT2B genes encoding steroid conjugating enzymes are likely to be found in this region of the genome.