The effects of detergents and phospholipids on a glycolipid galactosyltransferase.

1. Phospholipids activated the enzyme, lactosylceramide: UDP-galactose alpha-galactosyltransferase in hamster cells (NIL 2E clone 8) when assayed in the presence of neutral detergents. 2. Phosphatidylserine and phosphatidylinositol were the most effective phospholipids in activating the enzyme. Other phospholipids were also effective, but sphingomyelin and lysophosphatidylcholine were inhibitory. 3. Considerable enzyme activity was obtained in the absence of any detergent. Most of this activity was due to glycosylation of endogenous acceptors. 4. There was a complex effect of detergents on the enzyme activity. Very low concentrations were sharply inhibitory, but higher concentrations, above the critical micelle concentration for detergent, caused regeneration of activity. 5. The phospholipids, in the absence of a detergent, are required to maintain the lipid substrate, lactosylceramide, in a suitable dispersion where it can be acted upon by the enzyme. In the presence of detergents, it is proposed that the phospholipids also act by affecting the state of the lipid substrate.