Glucuronide conjugation of 6- n-propyl-2-thiouracil and other antithyroid drugs by guinea pig liver microsomes in vitro

Abstract

Guinea pig liver microsome UDP glucuronyl transferase and UDPGA were incubated with the radioactive antithyroid drugs 6- n-propyl-2-thiouracil (PTU). 1-methy1-2-mercaptoimidazole (methimazole, MMI) and 2-thiouracil (TU). Radioactive metabolites were produced with PTU and thiouracil and, in each case, were identified as the corresponding β-glucuronide conjugate. No measurable glucuronidation of MMI was observed. Kinetic studies with the microsomal preparation demonstrated a K m rmvalue of 7.2 × 10 −4 M for PTU and 6.7 × 10 −3 M for thiouracil. Glucuronide conjugation of PTU was linear for 1 hr. declining thereafter while conjugation of phenolphthalein was linear for 2 hr. Conjugation of phenolphthalein by microsomes stored in 0.154 M KCl at −20° for 14 days was 41 per cent higher than in fresh microsomes, whereas conjugation of PTU was 67.4 per cent lower. PTU glucuronidation did not occur in the absence of UDPGA and was essentially linear with respect to enzyme concentrations. Under the same conditions, spontaneous N-glucuronidation of PTU by glucuronate was not measurable. The pH optimum for PTU glucuronidation was 8.0 and similar to the broad optimums of 7.3 to 7.9 for UDP glucuronyl transferases from a variety of sources rather than to non-enzymatic N-glucuronidation, which has a reported pH optimum of 3–4. The conjugating enzyme for PTU was located primarily in the guinea pig liver microsomes with this fraction exhibiting 75 per cent of the total activity of whole homogenates. PTU conjugation was inhibited by MMI but not by thiouracil, thiourea or 6-methy1-2-thiouracil. The results obtained demonstrate that β-glucuronide conjugation of the antithyroid drugs PTU and thiouracil, but not MMI. is readily catalyzed by a guinea pig liver microsomal UDP glucuronyl transferase in vitro.