UDP glucuronosyltransferases (UGTs) comprise a multigene family of drug-metabolizing enzymes. The subfamily of UGTs that conjugate bilirubin and phenolic compounds with glucuronic acid has been termed UGT1A1. In man, UGTIAI isoforms are encoded by a single gene, UGT1A1. Protein isoforms encoded by UGT1A1 originate by alternative splicing. In the present study, we used the cDNA of UGT1A1*4, a bilirubin-conjugating isoform, to localize the UGT1A1 locus in the human genome. The UGT1A1 gene was assigned by in situ hybridization to chromosome region 2q37.

The functional heterogeneity of uridine diphosphateglucuronosyltransferase (UDPGT) and its deficiency in human liver were investigated. The monoclonal antibody (MAb) WP1, which inhibits bilirubin and phenol-glucuronidating activity, was used to immunopurify UDPGTs from human liver. Purified UDPGTs were injected into mice to obtain new MAbs. Immunoblotting of microsomes with MAb HEB7 revealed at least three polypeptides in liver (56, 54, and 53 kD) and one in kidney (54 kD). In liver microsomes from four patients (A, B, C, and D) with Crigler-Najjar syndrome type I (CN type I), UDPGT activity towards bilirubin was undetectable (A, B, C, and D) and activity towards phenolic compounds and 5-hydroxytryptamine either reduced (A and B) or normal (C and D). UDPGT activity toward steroids was normal. Immunoblot studies revealed that the monoclonal antibody WP1 recognized two polypeptides (56 and 54 kD) in liver microsomes from patient A and none in patient B. With HEB7 no immunoreactive polypeptides were seen in these two patients. Patient C showed a normal banding pattern and in patient D only the 53-kD band showed decreased intensity. These findings suggest considerable heterogeneity with regard to the expression of UDPGT isoenzymes among CN type I patients. (J. Clin.

Chloroquine (CQ)-resistant (CQR) Plasmodium falciparum malaria parasites show a strong decrease in CQ accumulation in comparison with chloroquine-sensitive parasites. Controversy exists over the role of the plasmodial pfmdr1 gene in the CQR phenotype. pfmdr1 is a member of the superfamily of ATP-binding cassette transporters. Other members of this family are the mammalian multidrug resistance genes and the CFTR gene. We have expressed the pfmdr1-encoded protein, Pgh1, in CHO cells and Xenopus oocytes. CHO cells expressing the Pgh1 protein demonstrated an increased, verapamil-insensitive susceptibility to CQ. Conversely, no increase in drug susceptibility to primaquine, quinine, adriamycin, or colchicine was observed in Pgh1-expressing cells. CQ uptake experiments revealed an increased, ATP-dependent accumulation of CQ in Pgh1-expressing cells over the level in nonexpressing control cells. The increased CQ accumulation in Pgh1-expressing cells coincided with an enhanced in vivo inhibition of lysosomal alpha-galactosidase by CQ. CHO cells expressing Pgh1 carrying two of the CQR-associated Pgh1 amino acid changes (S1034C and N1042D) did not display an increased CQ sensitivity. Immunofluorescence experiments revealed an intracellular localization of both mutant and wild-type forms of Pgh1. We conclude from our results that wild-type Pgh1 protein can mediate an increased intracellular accumulation of CQ and that this function is impaired in CQR-associated mutant forms of the protein. We speculate that the Pgh1 protein plays an important role in CQ import in CQ-sensitive malaria parasites.

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