Phosphatidylethanolamine synthesis by castor bean endosperm. Intracellular distribution and characteristics of CTP:ethanolaminephosphate cytidylyltransferase.

The intracellular distribution and catalytic properties of CTP: ethanolaminephosphate cytidylyltransferase from endosperm of castor bean (Ricinus communis L. var. Hale) have been studied. This enzyme was confined to membranes, with about 80% of the activity occurring in mitochondria and the rest in endoplasmic reticulum (ER) following sucrose density gradient centrifugation. The mitochondrial location of this enzyme was supported by further purifying mitochondria on Percoll density gradients. The mitochondrial cytidylyltransferase was detected largely in outer membrane fractions, and lost its activity after trypsin treatment, indicating that the active sites are exposed to the cytoplasm. Both mitochondrial and ER cytidylyltransferase required cations for activity; Mg2+ was preferred over Mn2+ and Ca2+. The pH optima both were 6.5. The apparent Km values for ethanolamine phosphate were 143 and 83 microM and those for CTP were 125 and 1010 microM, respectively, for the mitochondrial and ER activities. The mitochondrial cytidylyltransferase reached a maximal velocity of 3.0 nmol/min/mg protein, whereas ER cytidylyltransferase was 0.424 nmol/min/mg protein. These findings reveal that the majority of the cytidylyltransferase activity in castor bean endosperm is not closely associated with ethanolaminephosphotransferase (predominantly in ER) which catalyzes the subsequent reaction in the synthesis of phosphatidyl-ethanolamine by a nucleotide pathway. The possible roles of these enzymes in phosphatidylethanolamine synthesis in plants are discussed.