DEPENDENCE OF RAT-LIVER CMP-N-ACETYLNEURAMINATE - GM(1) SIALYLTRANSFERASE (SAT-IV) ACTIVITY ON THE CERAMIDE COMPOSITION OF GM(1) GANGLIOSIDE

The dependence of CMP-N-acetylneuraminate:GM(1) sialyltransferase (SAT IV) activity of rat liver Golgi apparatus on GM(1) ganglioside cerami de composition was evaluated. SAT IV activity was assayed on GM(1) mol ecular species carrying homogeneous ceramide moieties containing long chain bases of different length (18 or 20 C atoms) unsaturated or not, linked to 14:0, 16:0, 18:0 or 22:0 fatty acids. The results obtained in the presence of the detergent Triton CF-54, when enzyme and substra te are presumably part of the same supramolecular structure, show that either the long chain base or the fatty acid composition can affect e nzyme activity. This feature was not displayed when GM(1) was embedded in dipalmitoylphosphatidylcholine vesicles in the absence of detergen t. Under the latter conditions, the enzyme was not sensitive to the li pid composition of GM(1) but to the ganglioside/phospholipid ratio in the vesicles. These results indicate for the first time that SAT IV is affected by the lipid composition of the substrate and strengthen the hypothesis that glycosyltransferases may contribute to control the ce llular glycosphingolipid ceramide pattern.