Purification and immunochemical characterization of a dihydrofolate reductase-thymidylate synthase enzyme complex from wild-carrot cells.

An enzyme complex with dihydrofolate reductase (DHFR, E.C.1.5.1.3.) activity was purified to apparent homogenity from wild-carrot cells. The complex has a mol. wt of 286 kd and contains five polypeptide chains of 95, 70, 50, 45 and 26 kd. The DHFR enzyme activity and methotrexate-binding site are on the 45-kd subunit. Folate analogs (methotrexate, aminopterin and formylaminopterin) as well as SH-group inhibitors [p-hydroxymercuribenzoate, 5,5' -dithiobis(2-nitrobenzoic acid), or N-ethylmaleimide] inhibit DHFR. Thymidylate synthase (TS, E.C.2.1.1.45) activity co-purified with the enzyme complex through each of seven steps and co-eluted from gel filtration columns with the DHFR activity at the mol. wt of the enzyme complex. Further identification of TS within the complex was achieved using a Leishmania DHFR-TS antisera which specifically inhibited the carrot TS, although it immunoprecipitated both TS and DHFR. Polyclonal antisera, raised against and specific for the complex as judged by Ouchterlony double diffusion tests and Western blot analysis, inhibited and immunoprecipitated both DHFR and TS. The Leishmania antisera also identified the 70-kd polypeptide within the purified complex as TS in a Western blot experiment. The functions of the other three polypeptides have not yet been established.