Purification and properties of a mannosyltransferase solubilized from mitochondrial outer membranes

The enzyme GDPmannose: dolichyl monophosphate mannosyltransferase has been solubilized and purified from mice liver mitochondrial outer membranes. The purification combines detergent extraction of purified outer membranes using Nonidet P-40, with subsequent ion-exchange chromatography on DEAE-cellulose. At this stage, a 400-fold purification is obtained. The partially purified mannosyltransferase is activated choline-containing lipids such as phosphatidylcholine, lysophatidylcholine and sphingomyelin. The reaction is dependent upon the addition of exogenous dolichyl monophosphate. The sole reaction product has been identified as dolichyl posphate-mannose. The partially purified mannosyltrasnferase exhibits a K_m of 1.33 μM for GDPmannose. Enzyme activity, eluted from DEAE-cellulose, could be further purified after incorporation into sphingomyelin vesicles containing dolichyl monophosphate followed by a sucrose density gradient certrifugation. The mannosyltransferase activity is completely associated with the liposomes at the top of the gradient. Significant stabilization and purification (approx. 1600-fold) of enzyme activity associated with these liposomes is obtained. Furthermore, the reconstitution of this purified enzyme within specific liposomes provides a good model membrane to investigate the molecular requirement of this mitochondrial mannosyltransferase.