Application of 5-azido-UDP-glucose and 5-azido-UDP-glucuronic acid photoaffinity probes for the determination of the active site orientation of microsomal UDP-glucosyltransferases and UDP-glucuronosyltransferases.

A new approach to determining the active site orientation of microsomal glycosyltransferases is presented which utilizes the photoaffinity analogs [32P]5-Azido-UDP-glucose ([32P]5N3UDP-Glc) and [32P]5-Azido-UDP-glucuronic acid ([32P]5N3UDP-GlcA). It was previously shown that both photoprobes could be used to photolabel UDP-glucose:dolichol phosphate glucosyltransferase (Glc-P-Dol synthase), as well as the family of UDP-glucuronosyltransferases in rat liver microsomes. The effects of detergents, proteases, and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) on the photolabeling of these enzymes were examined in intact rat liver microsomes. Photolabeling of Glc-P-Dol synthase by either photoprobe was the same in intact or disrupted vesicles, was susceptible to trypsin digestion, and was inhibited by the nonpenetrating inhibitor DIDS. Photolabeling of the UDP-glucuronosyltransferases by [32P]5N3UDP-GlcA was stimulated 1.3-fold in disrupted vesicles as compared to intact vesicles, whereas photolabeling of these enzymes by [32P]5N3UDP-Glc showed a 14-fold increase when vesicles were disrupted. Photolabeled UDP-glucuronosyltransferases were only susceptible to trypsin digestion in disrupted vesicles, and this was further verified by Western blot analyses. The results indicate a cytoplasmic orientation for access of UDP-sugars to Glc-P-Dol synthase and a lumenal orientation of most UDP-glucuronosyltransferases.