The biosynthesis of DNA by insects: II. The origin of thymidylic acid during adult development of the cecropia silkworm

Abstract

Alternate pathways of TMP production were examined in developing pupae of the silkmoth Hyalophora cecropia. It was found that injected [ 14C]thymidine was more effective in labeling DNA thymine when the label was in the pyrimidine ring [2- 14C] rather than in the 5-methyl group, indicating that a demethylation step may be involved in the metabolism of thymidine. A labeled intermediate, 5-hydroxymethyl deoxyuridine, was found when cell-free preparations of whole pupae were supplied with either ring- or methyl-labeled thymidine. Another labeled product, deoxyuridine, was formed from ring-labeled thymidine. Labeled-TMP, TDP and TTP were also detected under both labeling conditions. Partially purified preparations of thymidine kinase also showed thymidine 7-hydroxylase activity which was less sensitive than the kinase to inhibition by 2-mercaptoethanol, dCDP, and high substrate concentrations. Cell-free preparations of whole pupae convert dCMP into both dCTP and precursors of TTP, thus demonstrating that the reduction of cytidine phosphate compounds could contribute to the production of both pyrimidine nucleotide precursors of DNA.