ENZYMES OF GLYCEROLIPID SYNTHESIS 461 of enzymes and possible isoenzymes involved, their subcellular locations, and their functions within cells are still being elucidated. Major stumbling blocks in current research include the tight association of these enzymes with endoplasmic reticulum and mitochondrial membranes and the result ing difficulty in solubilization, the lability of the solubilized enzymes, the difficulties in the fractionation of solubilized enzymes, and the dependence of some partially purified enzymes on phospholipids for activity. Workers have also had to contend with serious difficulties in delivering amphipathic and hydrophobic substrates to both membrane-bound and to solubilized enzyme preparations. At present, few glycerolipid synthetic enzymes have been purified to homogeneity.

Although most current research activities on the enzymes of glycerolipid synthesis continue at the descriptive level, important questions relating to the regulation of enzyme activity are beginning to be addressed. Strong evidence indicates that the biosynthesis of triacylglycerol, phosphatidylcho line, and phosphatidylethanolamine occurs asymmetrically on the cytoplas mic surface of the endoplasmic reticulum. Such asymmetrical synthesis now focuses attention on transmembrane movement and the sorting of glycero lipids within the endoplasmic reticulum and other cell membranes. The overriding question in glycerolipid metabolism concerns the regula tion of the types and quantities of glycerolipids synthesized within eu karyotic cells. An appropriate mixture of products must be synthesized to meet the several, and often simultaneous, demands for membrane biogene sis, lipoprotein biogenesis, bile synthesis, etc. Our understanding of regula tion has been limited by the absence of homogeneous enzymes, highly specific enzyme inhibitors, mutants containing defective enzymes, and suit able cell model systems. At present, few substantial conclusions about the regulation of enzymes of glycerolipid biosynthesis can be reached, in spite of much work suggesting changes in several enzyme activities. A useful preadipocyte model now exists, however, in which adipocyte conversion is accompanied by 30 to loo-fold increases in several triacylglycerol synthetic enzyme activities. In isolated hepatocytes, progress has also been made on the regulation at the diacylglycerol branchpoint of triacylglycerol and phos pholipid biosynthesis.