Determination of β1,4-galactosyltransferase enzymatic activity by capillary electrophoresis and laser-induced fluorescence detection

We have developed a nonradioactive method to assay UDP-Gal:β-D- GlcNAcβ1,4-galactosyltransferase (β4GalT-I) enzymatic activity. Capillary electrophoresis combined with laser-induced fluorescence detection (CE-LIF) was employed to provide a baseline separation of FITC-conjugated O-GlcNAc- containing substrate peptides and galactose-capped product peptides, while at the same time allowing a level of detection in the low attomole range (10^{- 18}). The addition of 2 mM hexamethylene diamine to the borate-based capillary electrophoretic buffer modulated the electroosmotic flow, resulting in optimum separation of the glycopeptide product from reactant. β4GalT-I activity was dependent upon the addition of both manganese and UDP-galactose. Using this assay, we show that two β4GalT-I constructs, predicted to localize to different intracellular compartments, are enzymatically active when expressed in vitro using a rabbit reticulocyte transcription-translation system. The high sensitivity of product detection by CE-LIF in combination with in vitro transcription-translation is applicable to the facile determination of the enzymatic activity of other newly cloned glycosyltransferases.