9] Uracil and thymine reductases

Abstract

Publisher Summary Evidence for the presence of the enzyme hydropyrimidine dehydrogenase, which catalyzes uracil and thymine reduction reactions are obtained from a number of sources such as: beef, rat, and mouse liver; bacteria including Clostridium uracilum and Pseudomonas aeruginosa; yeast; the rat tapeworm and other parasitic flatworms; and plants including rape seedlings, bean and cocklebur. Much of this work consists of the use of radioactive labeled substrate, both in vitro and in vivo , and then identifying the radioactive degradative products after separation by chromatography. Extensive purification are carried out thus far from three sources—beef liver, rat liver, and C. uracilum . The detailed methods are presented in the chapter. The methods of assay are based upon the change in absorbency at 340 mμ accompanying the interconversions of TPN and TPNH for the beef and rat preparations, and the decrease in absorbancy at 260 mμ accompanying the conversion of uracil to dihydrouracil for the bacterial preparations. The assay system used is similar for both the beef liver enzyme and rat liver enzyme preparation, except for the use of smaller amounts of substrate, and the routine use of uracil (0.5 micromole) as substrate in rat liver assay.