MODULATION OF THE ACTIVITIES OF ENZYMES OF MEMBRANE LIPID-METABOLISM BY NON-BILAYER-FORMING LIPIDS

The activities of several enzymes which catalyze the synthesis or degr adation of important lipid structural components of membranes are infl uenced by non-bilayer-forming lipids. We present and critique the hypo thesis that enzymes that catalyze interconversions between bilayer- an d non-bilayer-forming lipids are regulated by the lipid packing pertur bations of the non-bilayer-forming component. The consequence of this regulation is lipid polymorphic homeostasis. The enzymes which are mod ulated by non-bilayer components of membranes include a diglucosyl dia cylglycerol synthase from mycoplasma, secretory and cell-associated ph ospholipase A2s, phospholipase C, phosphocholine cytidylyltransferase, phosphatidate phosphohydrolase and diacylglycerol kinase, The effect of the non-lamellar lipids, both reversed phase-forming and micellar-f orming, are discused in terms of bilayer packing strain: which can eff ect enzyme-membrane associations, and lateral domain formation, which can modulate the effective concentration of lipid activators.