Glycosyltransferase assay

Universal phosphatase-coupled glycosyltransferase assay

A nonradioactive glycosyltransferase assay is described here. This method takes advantage of specific phosphatases that can be added into glycosyltransferase reactions to quantitatively release inorganic phosphate from the leaving groups of glycosyltransferase reactions. The released phosphate group is then detected using colorimetric malachite-based reagents. Because the amount of phosphate released is directly proportional to the sugar molecule transferred in a glycosyltransferase reaction, this method can be used to obtain accurate kinetic parameters of the glycosyltransferase. The assay can be performed in multiwell plates and quantitated by a plate reader, thus making it amenable to high-throughput screening. It has been successfully applied to all glycosyltransferases available to us, including glucosyltransferases, N-acetylglucosaminyltransferases, N-acetylgalactosyltransferases, galactosyltransferases, fucosyltransferases and sialyltransferases. As examples, we first assayed Clostridium difficile toxin B, a protein O-glucosyltransferase that specifically monoglucosylates and inactivates Rho family small GTPases; we then showed that human KTELC1, a homolog of Rumi from Drosophila, was able to hydrolyze UDP-Glc; and finally, we measured the kinetic parameters of human sialyltransferase ST6GAL1.

Glycosphingolipid expression in pig aorta: identification of possible target antigens for human natural antibodies

Total non-acid glycosphingolipids were isolated from the aortas of more than 80 pigs. The glycolipids were separated by HPLC, analysed by thin-layer chromatography, and tested for reactivity with monoclonal anti-blood group antibodies. The fractions were structurally characterized by NMR spectroscopy and mass spectrometry. Reactivity with both anti-blood group A and H antibodies was seen. The major glycosphingolipid constituents were globotri- and globotetraosylceramides and blood group H pentaglycosylceramides based on type 1 and type 2 core saccharide chains. Globopentaosylceramides, blood group H hexaglycosylceramides based on type 4 chain, and blood group A hexaglycosylceramides based on type 1 core chain were also present. Two structures, that may be important targets for human antibodies initiating hyperacute rejection following pig to human xenotransplantation, were present as minor constituents compared to the blood group components. These were Galalpha1,3neolactotetraosylceramide and a Galalpha1, 3Lexstructure. A Leb/Y hexaglycosylceramide was also present.

A rapid and simple assay method for UDP-glucose:ceramide glucosyltransferase

We have developed a simple rapid method for measuring UDP-glucose:ceramide glucosyltransferase; the method utilizes ceramide immobilized on the surface of silica gel and [14C]UDP-glucose as substrate. The reaction product, [14C]glucosylceramide, formed on the surface of the silica gel was easily separated from free [14C]UDP-glucose, either by centrifugation or by filtration. The reliability of this solid phase method was evaluated by using rat brain membrane fraction as an enzyme source. This enzyme had an optimal pH of 6.4-6.5 and required Mn2+, Mg2+ in the presence of 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS). Apparent Km values of 8.7 microM for UDP-glucose and 292 microM for ceramide were determined using the new method. Under the optimal conditions, the solid phase method yielded 2-5-times more product than did the method using micellar system. Moreover, the reaction was highly quantitative in its enzyme dose-activity relationship.