COMPARED SELECTIVITIES OF THE PHOSPHATIDYLINOSITOL-SYNTHASE FROM MAIZE COLEOPTILES EITHER IN MICROSOMAL-MEMBRANES OR AFTER SOLUBILIZATION

PI-synthase selectivity from etiolated maize coleoptiles was studied e ither associated with the microsomal membranes or after solubilization by CHAPS and prepurification on a DEAE-trisacryl M column. When maize microsomes were incubated with [H-3]inositol without any exogenous CP M-PA, the most heavily labelled molecular species were 16:0/18:2-PI (7 7%), 16:0/18:3- plus 18:2/18:2-PI (15%), 16:0/18:1-PI (4%) and 18:0/18 :2-PI (4%). Addition to the incubation medium of up to 300 mu M 16:0/1 6:0-CMP-PA unexpectedly resulted in the formation of very little label led 16:0/16:0-PI. When the solubilized fraction from microsomes was in cubated with [H-3]inositol in absence of 16:0/16:0-CPM-PA, the same PI molecular species as above were synthesized. However, with increasing concentrations of 16:0/16:0-CMP-PA in the medium, increasing amounts of labelled 16:0/16:0-PI appeared as well. With prepurified PI-synthas e eluted from a DEAE column, endogenous CMP-PA was poorly utilized for PI biosynthesis whereas the exogenous 16:0/16:0-CPM-PA was used activ ely. With time, the endogenous CMP-PA was utilized first and the exoge nous substrate was utilized, albeit, much more slowly. The results dem onstrate that the selectivity displayed by PI-synthase towards various molecular species of CMP-PA depends on the integration of the enzyme in the membrane structure. Solubilization of the enzyme, i.e., inclusi on of the protein in micelles with detergents and lipids, results in a n apparent loss of the selectivity for CMP-PA.