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UDP-glucuronosyltransferase (UGT) isozymes detoxify metabolites, drugs, toxins, and environmental chemicals via conjugation to glucuronic acid. Based on the extended UGT1 locus combined with Northern blot analysis and in situ hybridization, we determined the distribution of UGT1A1 and UGT1A7 through UGT1A10 mRNAs and found them for the first time segmentally distributed in the mucosal epithelia layer of the gastrointestinal tract. Biochemically, recombinant isozymes exhibited pH optima of 5.5, 6.4, 7.6, 8.5, and/or a broad pH range, and activities were found to be unaffected or progressively inhibited by increasing substrate concentrations after attaining V max for certain chemicals. Under different optimal conditions, all exhibited wide substrate selections for dietary and environmentally associated chemicals. Evidence also suggests tandem effects of isozymes in the time for completion of reactions when comparing short-and long-term incubations. Moreover, treatment of colon cells with certain dietassociated constituents, curcumin and nordihydroguaiaretic acid, reversibly targets UGTs causing inhibition without affecting protein levels; there is no direct inhibition of control UGT using curcumin as substrate in the in vitro assay. In summary, we demonstrate that UGTs are located in gastrointestinal mucosa, have vast overlapping activities under differential optimal conditions, and exhibit marked sensitivity to certain dietary substrates/constituents, representing a first comprehensive study of critical properties concerning glucuronidating isozymes in alimentary tissues. Additionally, the highly dynamic, complex, and variable properties necessarily impact absorption of ingested chemicals and therapeutic drugs.The gastrointestinal (GI) 1 system has the critical function of absorbing nutrients from the diet and, simultaneously, acting as a barrier to ingested toxins and unwanted chemicals. To this end, the human UGT1 complex locus, which encodes multiple UDP-glucuronosyltransferase (UGT) isozymes distributed in the GI tract, participates in the broad and critical function of detoxifying lipid-soluble phenols derived metabolically or ingested as part of the diet, its contaminants, and as medications. UGT conjugates glucuronic acid to the acceptor substrate, converting the lipophile to an inactive glucuronide that undergoes facilitated excretion. The isozymes metabolize endogenous substrates such as bilirubin, steroids, bile acids, retinoic acid, and thyroid hormones. Equally important, isozymes also metabolize four categories of dietary phytochemicals that could pose risks for humans and animals when absorbed in excess. Highly abundant dietary flavonoids found in plants are shown to inhibit aromatase (1), and less abundant anthraquinones and two types of phytoestrogens (2-4) can also cause cellular damage. Particular anthraquinones were mutagenic when bioactivated (3) and were carcinogens in long-term feeding experiments in mice (5). Injurious effects of diets were demonstrated by extensive lesions in the reprodu...

UDP-glucuronosyltransferase (UGT) isozymes catalyze detoxification of numerous chemical toxins present in our daily diet and environment by conjugation to glucuronic acid. The special properties and enzymatic mechanism(s) that enable endoplasmic reticulum-bound UGT isozymes to convert innumerable structurally diverse lipophiles to excretable glucuronides are unknown. Inhibition of cellular UGT1A7 and UGT1A10 activities and of [ 33 P]orthophosphate incorporation into immunoprecipitable proteins after exposure to curcumin or calphostin-C indicated that the isozymes are phosphorylated. Furthermore, inhibition of UGT phosphorylation and activity by treatment with PKC-specific inhibitor peptide supported PKC involvement. Coimmunoprecipitation, colocalization by means of immunofluorescence, and crosslinking studies of PKC and UGT1A7His revealed that the proteins reside within 11.4 Å of each other. Moreover, mutation of three PKC sites in each UGT isozyme demonstrated that T73A͞G and T202A͞G caused null activity, whereas S432G-UGT1A7 caused a major shift of its pH-8.5 optimum to 6.4 with new substrate selections, including 17␤-estradiol. S432G-UGT1A10 exhibited a minor pH shift without substrate alterations. PKC involvement was confirmed by the demonstration that PKC overexpression enhanced activity of UGT1A7 but not of its S432 mutant and the conversion of 17␤-[ 14 C]estradiol by S432G-UGT1A7 but not by UGT1A7. Consistent with these observations, treatment of UGT1A7-transfected cells with PKC-specific inhibitor peptide or general PKC inhibitors increased 17␤-estradiol catalysis between 5-and 11-fold, with parallel decreases in phosphoserine-432. Here, we report a mechanism involving PKC-mediated phosphorylation of UGT such that phosphoserine͞threonine regulates substrate specificity in response to chemical exposures, which possibly confers survival benefit.PKC ͉ PKC inhibitors ͉ detoxifying enzymes ͉ UDP-glucuronosyltransferase mutants ͉ immunofluorescence M ammalian UDP-glucuronosyltransferase (UGT) isozymes detoxify numerous lipid-soluble chemicals, including metabolites, such as neurotoxic bilirubin, as well as dietary and environmental agents͞toxins and medicines (1). UGT facilitates detoxification of chemicals by attaching glucuronic acid molecules to form water-soluble, readily excretable glucuronides. Failure to attach glucuronic acid (glucuronidation) to toxins allows high levels to accumulate in tissues, causing deleterious effects, including mutations of genes and the development of cancer (2). Mutations in UGT1 genes lead to lethal hyperbilirubinemic Crigler-Najjar disease (3) and to toxic tissue levels of the commonly used analgesic acetaminophen (Tylenol) in rats (4). The fact that a limited number of UGT isozymes facilitate excretion of a vast number of structurally diverse chemicals suggests a mechanism evolved to confer flexibility on an isozyme to metabolize multiple toxins. Because UGTs are bound to membranes of the endoplasmic reticulum (ER) that cause difficulties associated with purification and s...