Effect of cycloheximide and actinomycin D on thymidylate synthase and thymidine kinase in regenerating rat liver after partial hepatectomy.

Thymidylate synthase and thymidine kinase, which catalyze the formation of thymidylate via the de novo and salvage pathways, respectively, are rate-determining enzymes in DNA synthesis. The increases in the activities of hepatic thymidylate synthase and thymidine kinase were significantly suppressed at 24 h after 70% partial hepatectomy in rats that had been administered cycloheximide. Concomitantly, other regenerative parameters such as the liver weight and contents of protein, RNA, and DNA were also significantly reduced in 24-h regenerating liver of cycloheximide-treated rats. When actinomycin D was injected, the activity of thymidine kinase, the liver weight, and contents of protein, RNA, and DNA were completely depressed in 24-h regenerating liver. However, the activity of thymidylate synthase in actinomycin D-administered rats rose to the level similar to the control (70% partially hepatectomized). The immunoblotting assay showed that thymidylate synthase is newly synthesized during liver regeneration after partial hepatectomy without being affected with actinomycin D.