The Novel UDP Glycosyltransferase 3A2: Cloning, Catalytic Properties, and Tissue Distribution

Abstract

The human UDP glycosyltransferase (UGT) 3A family is one of three families involved in the metabolism of small lipophilic compounds. Members of these families catalyze the addition of sugar residues to chemicals, which enhances their excretion from the body. The UGT1 and UGT2 family members primarily use UDP glucuronic acid to glucuronidate numerous compounds, such as steroids, bile acids, and therapeutic drugs. We showed recently that UGT3A1, the first member of the UGT3 family to be characterized, is unusual in using UDP N-acetylglucosamine as sugar donor, rather than UDP glucuronic acid or other UDP sugar nucleotides (J Biol Chem 283:36205-36210, 2008). Here, we report the cloning, expression, and characterization of UGT3A2, the second member of the UGT3 family. Like UGT3A1, UGT3A2 is inactive with UDP glucuronic acid as sugar donor. However, in contrast to UGT3A1, UGT3A2 uses both UDP glucose and UDP xylose but not UDP N-acetylglucosamine to glycosidate a broad range of substrates including 4-methylumbelliferone, 1-hydroxypyrene, bioflavones, and estrogens. It has low activity toward bile acids and androgens. UGT3A2 transcripts are found in the thymus, testis, and kidney but are barely detectable in the liver and gastrointestinal tract. The low expression of UGT3A2 in the latter, which are the main organs of drug metabolism, suggests that UGT3A2 has a more selective role in protecting the organs in which it is expressed against toxic insult rather than a more generalized role in drug metabolism. The broad substrate and novel UDP sugar specificity of UGT3A2 would be advantageous for such a function.

Quantitative profiling of the UGT transcriptome in human drug-metabolizing tissues

“…It is worth noting that expression of UGT3A1 in liver and kidney as well as renal expression of UGT8 were well in the range of multiple UGT1 and UGT2 genes, supporting an important contribution of these enzymes to the conjugation of lipophilic substrates in these tissues. 7,15,16 The limited number of individuals initially sequenced from normal tissues (3 pools of at least 3 RNA samples) is a significant limitation of our study. However, data were consistent with those of an independent RNA-seq dataset of 18 individual normal liver and kidney tissues.…”

Identification of Residues That Confer Sugar Selectivity to UDP-Glycosyltransferase 3A (UGT3A) Enzymes

“…Wild-type and mutant UGT3A proteins were detected with anti-UGT3A1 and anti-UGT3A2 antibodies generated previously (8,9) and a peroxidase-conjugated goat anti-rabbit secondary antibody (NeoMarkers). Immunocomplexes were visualized with the SuperSignal West Pico chemiluminescent kit (Thermo Fisher Scientific) and quantified with a GE Healthcare LAS4000 scanner.…”

“…We recently characterized the substrate specificities of the UGT3A enzymes (8,9). In striking contrast to the UGT1 and UGT2 families, we found that neither UGT3A enzyme can utilize the prototypic UGT co-substrate UDP-GlcUA.…”

“…This sugar selectivity is unique among vertebrate UGTs. UGT3A2 preferentially utilizes UDP-Glc with a range of substrates, including bioflavones and estrogens (9). The significance of these unusual sugar specificities remains unclear, although it is possible that the GlcNAc and Glc conjugates have important biological roles in vivo.…”

“…The remaining members of the UGT2 family are encoded by separate genes of six exons each that are arrayed along chromosome 4q13 (6). The UGT3A family has two members denoted UGT3A1 (8) and UGT3A2 (9) that are arranged as a direct repeat on chromosome 5p13. The human UGT8 family contains only one gene (UGT8A1) located on chromosome 4q26.…”