Pyrimidine and purine salvage deoxyribonucleoside metabolism in hepatic and renal homogenates from rats pretreated with propylthiouracil or L-thyroxine

OBJECTIVES: In vitro activities of thymidine kinase (TK, EC 2.7.1.21), adenosine kinase (AK, EC 2.7.1.20) and deoxycytidine kinase (dCK, EC 2.7.1.74) enzymes involved in the salvage pathway of DNA precursor synthesis, in homogenates of the rat liver and kidney, were examined. Type I iodothyronine-5'-deiodinase (5'D-I) is the main enzyme responsible for peripheral metabolism of thyroid hormones. This occurs especially in the liver, kidney and muscle. The activity of 5'D-I is inhibited by propylthiouracil (PTU), an antithyroid drug. METHODS: The liver and kidney were collected from rats pretreated in vivo with either a 0.1% solution of PTU in drinking water for 2 weeks or injected with levothyroxine (L-T₄, 50 μg/kg BW, daily) for 2 weeks. The enzyme activities were measured by ascending chromatography and expressed as the amounts of radioactive reaction products of the phosphorylation of dThd (for TK), of dAdo (for AK and dCK) and of dGuo (for dCK). RESULTS: In liver homogenates, PTU-pretreatment decreased the activities of the three enzymes when compared to control values and those of L-T₄-treated animals; also L-T₄ injections decreased the AK and dCK activities in the liver homogenates. PTU-pretreatment increased TK activity and the rate of dGuo phosphorylation in kidney homogenates, when compared to controls and to the L-T₄-pretreated animals. Conversely, both PTU- and L-T₄-pretreatment reduced the rate of dAdo phosphorylation in kidney homogenates. CONCLUSION: Changes in the activities of examined enzymes which participate in pyrimidine or purine metabolism of the salvage pathway of DNA synthesis in the liver after PTU-pretreatment (as shown herein) are similar to the changes of the 5'D-I activity after PTU-treatment (as reported by others). Thus, the observations suggest a role of the salvage pathway of DNA synthesis in the peripheral metabolism of thyroid hormones.