Identification and characterisation of two N-acetylglucosaminyltransferases associated with Trypanosoma Brucei...

Microsomes forom Trypanosoma brucei contain glycosyltransferases able to incorporate N-[14C]acetylglucosamine into two different types of acceptors. A first transferase catalyzes the transfer of N-[14C]acetylglucosamine 1-phosphate from uridine diphosphate N-[14C]acetylglucosamine into dolichol monophosphate. The enzymatic activity requires Mn2+, is time and temperature dependent, has an optimum pH of 7.4 and is completely inhibited by the antibiotic tunicamycin. Exogenous dolichol monophosphate enhances the glycosyltransferase activity. The kinetics of incorporation are characterized by a Km of 2.6 microM for uridine diphosphate N-acetylglucosaminyltransferase are comparable to those reported for the first enzyme of the dolichol cycle described in several eukaryotes. N-Acetyl-glucosaminylpyrophosphoryl-dolichol is essentially the only product of the reaction. A second type of activity which is responsible for the direct transfer of N[14C]acetylglucosamine from uridine diphosphate N[14C]acetylglucosamine into several endogenous polypeptide acceptors, is also associated with T. brucei microsomes. The reaction, which might be due to more than one enzyme, is dependent on Mn2+, but differs from the other transferase in all other characteristics. Time course and optimal temperature are different, and the optimum pH is 6.5. The reaction is independent of the external addition of dolichol monophosphate and tunicamy cin has no inhibitory effect on the enzymatic activity. AKm of 1.6 microM was calculated fr uridine diphosphate N-acetylglucosamine.