Mammalian synthesis of UMP from orotate: The regulation of and conformers of Complex U

Abstract

Complex U contains two enzyme activities, namely, orotate phosphoribosyltransferase and orotidylate decarboxylase, which convert orotate to UMP. These two activities copurify in a 300-fold purification from Ehrlich ascites cells, as studied in this laboratory, and in purified preparation studied in other laboratories from other tissues. We have discussed: 1) How OMP is channeled by Complex U so that it is not readily released into solution after it is formed from orotate and phosphoribosyl pyrophosphate. This channeling makes it important to test inhibitors of the decarboxylase of the complex at low OMP concentrations. 2) The sedimentation characteristics of Complex U are a function of the solvent. Five conformer sizes, namely, 3.8, 4.45, 4.8, 5.1 and 5.5 S are seen. These conformers are related to the kinetic effects of substrates, products or inhibitors on one or the other of the two enzymatic centers of Complex U. It is suggested that the 4.8 S conformer might be expected to exist in the cytosol, in vivo. A major problem that remains to be resolved is whether the two enzyme activities reside on separable polypeptide chains or whether Complex U is a multifunctional protein so that the two enzymatically active centers reside on a singly polypeptide chain.