Purification and characterization of CTP: cholinephosphate cytidylytransferase from rat liver cytosol

Two forms of CTP:cholinephosphate cytidylyltransferase have been identified in the cytosol of rat liver. The L-form has a molecular weight of about 2.0 X 10(5) and low specific activity. The activity of the L-form is markedly stimulated by liposomes made from rat liver lipids. The L-form is the major species present in fresh cytosol. When the cytosol is incubated at 4 degrees for 5 days, the H-form (high molecular weight) of the enzyme is generated from the L-form. The H-form consists of multiple copies of the L-form and requires a lipid-containing fraction for optimal activity. The H-form has a wide range of molecular weights with a median value of 1.3 X 10(6). It was possible to dissociate partially the H-form of the enzyme into the L-form by incubation with 0.05% sodium dodecyl sulfate. The H-form of the enzyme could be generated by incubation of the L-form with liposomes. The L-form of the cytidylyltransferase was purified from rat liver cytosol. The enzyme gave a single band when stained for protein or assayed for activity after polyacrylamide gel electrophoresis. Both forms of the enzyme were active over a wide range of pH with an optimum at pH 7.0. The K'c (pH 6.4) was 0.80. The true Michaelis constants for the forward and reverse reactions of both forms of the enzyme are given. The H-form of the enzyme from the cytosol could be precipitated by centrifugation at 100,000 X g for 1 h.