| Evidence type | Finding | Quantitative detail | Species/tissue | Source (with DOI/URL and year) |
|---|---|---|---|---|
| Gene/protein identity and expression | Rat **Ugt2a1/UGT2A1** corresponds to the olfactory UDP-glucuronosyltransferase historically termed **UGT-olf/UGTolf**; highly expressed in olfactory tissues and detected in olfactory sensory cilia (pqac-00000004, pqac-00000007) | Absolute expression reported as **~9.8 attomoles/µg total RNA** in rat olfactory epithelium and **~1.6 attomoles/µg total RNA** in rat olfactory bulb; one other isoform reported as **>1,000-fold lower** than UGT2A1 (pqac-00000004, pqac-00000007) | **Rattus norvegicus**; olfactory epithelium, olfactory bulb, olfactory sensory cilia | Heydel et al., *Drug Metab Rev* (2010), DOI: 10.3109/03602530903208363, https://doi.org/10.3109/03602530903208363 (pqac-00000004, pqac-00000007) |
| Enzymatic reaction | UGT2A1 is a **phase II UDP-glucuronosyltransferase** that conjugates **UDP-glucuronic acid** to odorants/xenobiotics, generating more hydrophilic glucuronides for elimination; this reaction is implicated in olfactory perireceptor metabolism and signal termination (pqac-00000001, pqac-00000005) | Reaction class stated; rapid metabolite formation in olfactory tissue reported on the order of **hundreds of milliseconds** in the 2021 study background, but no rat-specific Km/Vmax values were provided in the retrieved evidence (pqac-00000005) | Rat olfactory epithelium / vertebrate olfactory tissues | Neiers et al., *PLOS ONE* (2021), DOI: 10.1371/journal.pone.0249029, https://doi.org/10.1371/journal.pone.0249029; Heydel et al., *Drug Metab Rev* (2010), DOI: 10.3109/03602530903208363, https://doi.org/10.3109/03602530903208363 (pqac-00000001, pqac-00000005) |
| Tissue/cellular localization | UGT2A1 immunolocalizes throughout the olfactory epithelium, especially the **apical region**; present in **sustentacular cells**, **Bowman glands**, and **Bowman gland ducts** (pqac-00000000, pqac-00000006) | Qualitative localization by immunohistochemistry; no abundance values at the cell-type level reported in retrieved evidence (pqac-00000000, pqac-00000006) | Rat olfactory epithelium | Neiers et al., *PLOS ONE* (2021), DOI: 10.1371/journal.pone.0249029, https://doi.org/10.1371/journal.pone.0249029 (pqac-00000000, pqac-00000006) |
| Subcellular localization | UGT2A1 is localized not only in sustentacular-cell endoplasmic reticulum but also at the **plasma membrane of olfactory cilia** of olfactory sensory neurons, placing the enzyme close to odorant receptors (pqac-00000000, pqac-00000008, pqac-00000009, pqac-00000010) | Localization supported by immunogold EM micrographs (Fig. 2E/F in cited paper); no numerical membrane-density values reported in retrieved evidence (pqac-00000008, pqac-00000009, pqac-00000010) | Rat olfactory cilia / olfactory sensory neurons and sustentacular cells | Neiers et al., *PLOS ONE* (2021), DOI: 10.1371/journal.pone.0249029, https://doi.org/10.1371/journal.pone.0249029 (pqac-00000000, pqac-00000008, pqac-00000009, pqac-00000010) |
| Substrate specificity: positive exemplar | **Eugenol** is described as a strong/highly glucuronidated odorant substrate of olfactory UGTs/UGT2A1, used as the positive functional probe in rat olfactory epithelium (pqac-00000005, pqac-00000006) | EOG experiments used **10^-2 M eugenol**; β-glucuronidase treatment increased response amplitude by **+13 ± 0.14%**, with **p = 0.037** versus vehicle condition summarized in the evidence (pqac-00000000) | Rat olfactory epithelium | Neiers et al., *PLOS ONE* (2021), DOI: 10.1371/journal.pone.0249029, https://doi.org/10.1371/journal.pone.0249029 (pqac-00000000, pqac-00000005, pqac-00000006) |
| Substrate specificity: negative comparator | **Amyl acetate** was used as a comparator because it is **not considered a UGT substrate** in this rat olfactory assay framework (pqac-00000006) | EOG experiments used **10^-3 M amyl acetate**; β-glucuronidase produced **no significant change** in the response to amyl acetate, supporting substrate selectivity of the observed effect (pqac-00000000) | Rat olfactory epithelium | Neiers et al., *PLOS ONE* (2021), DOI: 10.1371/journal.pone.0249029, https://doi.org/10.1371/journal.pone.0249029 (pqac-00000000, pqac-00000006) |
| Functional modulation assay | Counteracting glucuronidation in situ with **β-glucuronidase** altered odor responses in a substrate-dependent manner, supporting UGT2A1-mediated odorant clearance in the perireceptor space (pqac-00000000, pqac-00000006) | Vehicle alone reduced EOG amplitudes by **-20.3 ± 0.17%** for amyl acetate and **-15.4 ± 0.12%** for eugenol; β-glucuronidase caused **no significant change** for amyl acetate but increased eugenol responses (**+13 ± 0.14%**, **p = 0.037**) (pqac-00000000) | Rat olfactory mucosa / olfactory epithelium | Neiers et al., *PLOS ONE* (2021), DOI: 10.1371/journal.pone.0249029, https://doi.org/10.1371/journal.pone.0249029 (pqac-00000000, pqac-00000006) |
| Experimental conditions | Functional evidence came from ex vivo EOG recordings after topical enzyme delivery onto endoturbinates, directly testing whether deconjugation of glucuronides modifies odor responses (pqac-00000006) | **Twenty rats** were used; β-glucuronidase from *Helix pomatia* at **10 mg/mL**; topical droplets **~1 µL** via **~5 µm** pipettes; odor puff **200 ms** at **200 mL/min** within **1000 mL/min** airflow (pqac-00000006) | Rat olfactory epithelium / endoturbinates IIb and III | Neiers et al., *PLOS ONE* (2021), DOI: 10.1371/journal.pone.0249029, https://doi.org/10.1371/journal.pone.0249029 (pqac-00000006) |
| Physiological role | UGT2A1 is considered a major **odorant-metabolizing enzyme** in the olfactory perireceptor process, contributing to **odorant elimination/clearance**, limiting receptor activation, and promoting **signal termination**; glucuronidated odorants reportedly fail to stimulate cAMP production in olfactory cilia preparations (pqac-00000001, pqac-00000002, pqac-00000013) | Functional relationship is supported qualitatively and by substrate-dependent EOG modulation; no direct in vivo rat kinetic flux measurements were reported in retrieved evidence (pqac-00000000, pqac-00000002, pqac-00000013) | Rat and broader vertebrate olfactory tissues | Neiers et al., *PLOS ONE* (2021), DOI: 10.1371/journal.pone.0249029, https://doi.org/10.1371/journal.pone.0249029; Heydel et al., *Drug Metab Rev* (2010), DOI: 10.3109/03602530903208363, https://doi.org/10.3109/03602530903208363 (pqac-00000001, pqac-00000002, pqac-00000013) |
| Recent relevance (2023–2024) | Although not rat-specific functional work, recent human-focused literature implicates the **UGT2A1/UGT2A2 locus** in susceptibility to COVID-19-related olfactory dysfunction, consistent with an important role for this odorant-metabolizing pathway in the olfactory epithelium (pqac-00000011, pqac-00000012) | Meta-analysis estimated omicron-related olfactory dysfunction prevalence of **11.6%** in European-ancestry populations vs **2.9–5.4%** in other major groups, with global adult prevalence **~5.2%** and **~222.3 million** affected adults; prevalence pattern mirrored UGT2A1 risk-allele frequency differences (pqac-00000011, pqac-00000012) | Human olfactory epithelium / clinical genetics context | von Bartheld & Wang, *medRxiv* (2023), DOI: 10.1101/2022.12.16.22283582, https://doi.org/10.1101/2022.12.16.22283582 (pqac-00000011, pqac-00000012) |


*Table: This table summarizes the key functional-annotation evidence for rat Ugt2a1/UGT2A1, including reaction type, substrate examples, localization, physiological role in olfaction, and the main quantitative findings available from the cited sources.*