The inactivation of bovine heart glyceraldehyde-3-phosphate dehydrogenase by phosphatidylinositol (PI) and phosphatidylserine (PS) in the form of liposomes was investigated in the presence and absence of NAD excess. In the absence of NAD, the enzyme activity decreased to about 50% of its initial value at 0.6 mM PI and 0.8 mM PS (lipid-to-protein molar ratio 600 and 800, respectively). In the same lipid concentration range almost full regainment of the activity was observed in the presence of 80 μM NAD. It was shown that the excess of NAD protects the enzyme against conformational change induced by the phospholipids. Centrifugation experiments showed that both PI and PS bind significant amounts of NAD.
