RELATIONSHIP OF MEMBRANE PHOSPHOLIPID-COMPOSITION, LACTOSYLCERAMIDE MOLECULAR-SPECIES, AND THE SPECIFICITY OF CMP-N-ACETYLNEURAMINATE, LACTOSYLCERAMIDE ALPHA-2,3-SIALYLTRANSFERASE TO THE MOLECULAR-SPECIES COMPOSITION OF GM3 GANGLIOSIDE

The ceramide molecular species specificity of rat brain neuron CMP-N-a cetylneuraminate:lactosylceramide alpha 2,3-sialyltransferase (LacCer alpha 2,3-ST) was determined using 19 molecular species of lactosylcer amide incorporated into liposomes prepared with purified rat brain pho spholipids. The neuron enzyme displayed a distinct molecular species s pecificity (which was different than the specificity of liver LacCer a lpha 2,3-ST) based on both the long-chain base and the fatty acid comp osition of the lactosylceramide. Specifically, compared to the liver e nzyme, relatively high activities were obtained with d18:1-16:0, d18:1 -22:1, and d18:0-18:0 lactosylceramide molecular species. When the lip id composition of the neuron microsomal membranes was altered to resem ble that of rat liver Golgi membrane lipids, the activities towards d1 8:1-16:0, d18:1-22:1, and d18:0-18:0 lactosylceramide molecular specie s were significantly (P < 0.01) reduced and the molecular species spec ificity of the neuron enzyme resembled that of liver LacCer alpha 2,3- ST. In the reciprocal experiment in which the lipid composition of the rat liver Golgi membranes was altered to resemble neuron microsomal m embrane lipids, the molecular species specificity of liver LacCer alph a 2,3-ST was virtually identical to the specificity obtained with the native neuron enzyme. Analysis of the molecular species composition of lactosylceramide and GM3 in rat liver Golgi membranes revealed that t he molecular species composition of rat liver Golgi membrane GM3 was p recisely what would be expected based on the molecular species specifi city of LacCer alpha 2,3-ST and the molecular species composition of l actosylceramide in the Golgi membrane. Based on these results, we conc lude that the molecular species specificity of LacCer alpha 2,3-ST det ermined in our in vitro assay accurately reflects the specificity of t he enzyme in vivo and that the specificity of the enzyme is determined by the phospholipid molecular species composition of the Golgi membra ne.