SUBSTRATE-SPECIFICITY OF CTP - PHOSPHOCHOLINE CYTIDYLYLTRANSFERASE

The specificity of CTP: phosphocholine cytidylyltransferase from rat liver for phosphorylated bases has been investigated. The apparent K(m) for phosphocholine was 0.17 mM. As the number of methyl substituents on the phospho-base decreased, the apparent K(m), increased: 4.0 mM for phosphodimethylethanolamine, 6.9 for phosphomonomethylethanolamine and 68.4 for phosphoethanolamine. The V(max) for the reaction was similar for phosphocholine (12.6-mu-mol/min per mg protein), phosphomonomethylethanolamine (13.5-mu-mol/min per mg protein) and phosphoethanolamine (9.2-mu-mol/min per mg protein). When phosphodimethylethanolamine was the substrate, the V(max) was 3-fold higher (40.3-mu-mol/min per mg protein). Phosphoethanolamine, phosphomonomethylethanolamine and phosphodimethylethanolamine were competitive inhibitors of the cytidylyltransferase when phosphocholine was used as substrate with K(i) values of 18.5 mM, 9.3 mM and 1.5 mM, respectively. The results show that the cytidylyltransferase is highly specific for phosphocholine.