PHOSPHATIDYLINOSITOL-SPECIFIC ISOENZYMES OF PHOSPHOLIPASE-D FROM CATHARANTHUS-ROSEUS - PURIFICATION AND CHARACTERIZATION

Microsomal membranes from suspension cultured Catharanthus roseus cells were found to contain phospholipase D activity (EC 3.1.4.4) towards phosphatidylinositol (PI). After extraction with buffer containing Triton X-100, two isoenzymes with apparent molecular weights of 50 000 and 125 000 were partially purified. The enzymes accepted only PI as substrate and transphosphatidylation activity could not be detected. Whereas several phospholipids were slightly inhibitory, phosphatidyl glycerol had stimulatory properties. The enzymes required divalent cations for activity, Ca2+ and Mg2+ were equally effective. In the presence of deoxycholate and Ca2+, K-m values for PI were found to be in the range 20-40 mu M. Although the smaller protein had a slightly higher temperature stability and a slightly lower pI value, both isoenzymes revealed equal enzymic properties. This is the first report on a plant PI-specific phospholipase D.