TSH Stimulation of Orotic Acid Conversion to Pyrimidine Nucleotides and RNA in Bovine Thyroid

The effects of TSH on the incorporation of orotic acid into pyrimidine nucleotides and RNA have been studied in bovine thyroid slices. The 2-step conversion of orotic acid-carboxyl-14C to UMP in the main pyrimidine pathway results in the stoichiometric loss of the carboxyl carbon as 14CO2 and the 14CO2 formed is indicative of the total amount of pyrimidine nucleotides synthesized from orotic acid. TSH stimulated orotic acid-14C conversion to pyrimidine nucleotides at an orotic acid concentration of 8 μmoles/3.0 ml of incubation medium but had no effect on the conversion of carrier-free orotic acid-14C. The TSH stimulation was observed in the presence and absence of glucose with optimum effects occurring with 4 μmoles of carbohydrate. The TSH effect was apparent after 30 min of incubation and reached a maximum after approximately 2 hr. An increase in pyrimidine nucleotide formation was observed with 0.03 U of TSH /ml and a maximum response was obtained with approximately 0.08 μ/ml. The TSH stimulation was obtained with both crude (1 U/mg) and purified (12 U/mg) TSH. The greatest responses to TSH were obtained in experiments in which tissue was preincubated for 1 hr. Under these conditions bovine, porcine and canine thyroid preparations were stimulated by TSH. The TSH effect on pyrimidine nucleotide formation was accompanied by an increase in orotic acid-14C incorporation into RNA. The TSH stimulation was specific for the thyroid gland, producing no effect in other tissues. The response in thyroid slices appears to be specific for TSH; only FSH, which was contaminated with TSH, produced a similar, but less marked, effect. (Endocrinology84: 534, 1969)