Processive lipid galactosyl/glucosyltransferases from Agrobacterium tumefaciens and Mesorhizobium loti display multiple specificities.
Glucosylceramide synthase (GCS) catalyses the transfer of glucose from UDP-glucose (UDP-Glc) to ceramide to form glucosylceramide, the common precursor of most higher-order glycosphingolipids. Inhibition of GCS activity has been proposed as a possible target of chemotherapeutic agents for a number of diseases, including cancer. Design of new GCS inhibitors with desirable pharmaceutical properties is hampered by lack of knowledge of the secondary structure or catalytic mechanism of the GCS protein. Thus we cloned the rat homologue of GCS to begin studies to identify its catalytic regions. The histidine-modifying agent diethyl pyrocarbonate (DEPC) inhibited recombinant rat GCS expressed in bacteria; this inhibition was rapidly reversible by hydroxylamine and could be diminished by preincubation of GCS with UDP-Glc. These data suggest that DEPC acts on histidine residues within or near the UDP-Glc-binding site of GCS. Mutant proteins were expressed in which the eight histidine residues in GCS were individually replaced by other amino acids. H193A (His193-->Ala) and H193N (His193-->Asn) mutants were unaffected by 0.1 mM DEPC, a concentration that inhibited other histidine mutants and the wild-type enzyme by at least 60%. These results indicate that His193 is the primary target of DEPC and is at, or near, the UDP-Glc-binding site of GCS. His193 mutants were also insensitive to the GCS inhibitor d-threo-1-phenyl-2- decanoylamino-3-morpholinopropan-1-ol, at concentrations which inhibited the wild-type enzyme by >80%. These results have significance for both an understanding of the GCS active site and also for the possible design of new and specific inhibitors of GCS.