Thymidylate synthase, dihydrofolate reductase and dUTPase specific activities were found to remain at a high and constant
level in crude extracts from adult worms of Trichinella spiralis, as well as from muscle larvae of both Trichinella spiralis
(isolated 1–24 months after infection) and Trichinella pseudospiralis (isolated 5·5–13 months after infection). The results
obtained with Trichinella pseudospiralis muscle larvae isolated with the use of pepsin did not differ from those obtained
when pepsin was not used. No thymidine kinase activity could be detected in muscle larvae of either species and thymidine
phosphorylase could be found only in T. pseudospiralis larvae isolated without the use of pepsin. Muscle larvae of both
species contained orotidylate phosphoribosyl transferase activity, pointing to a possibility of 5-fluorouracil activation.
Uridine phosphorylase, another enzyme involved in 5-fluorouracil anabolism, was also present in T. pseudospiralis muscle
larvae. Results of comparative studies on inhibition of purified T. spiralis and rat thymidylate synthases by substrate
(4-thio-5-fluoro-dUMP, 2-thio-5-fluoro-dCMP and N4-hydroxy-dCMP) and cofactor (ZD 9331) analogues indicated only
dUMP analogues to show feeble selectivity towards the parasite enzyme. A hypothesis is discussed, assuming high
expression of thymidylate synthase in muscle larvae to be connected with their cells being arrested in the cell cycle.