The self-glucosylation of beef kidney glycogenin was inhibited by the following pyrimidine nucleotides and nucleotide sugars, listed in order of decreasing effectiveness: CDP-glucose, CDP, UDP-xylose, UDP-N-acetylglucosamine, UDP-galactose, UDP, CTP, CDP-choline, UDP-glucuronic acid, beta-S-UDP-glucose, and CMP. In contrast, the purine nucleotide sugars, ADP-glucose and GDP-glucose, were essentially ineffective, as was the pyrimidine nucleoside, cytidine. UDP-Xylose may be utilized by glycogenin as an alternative sugar donor instead of UDP-glucose (Rodén, L., Ananth, S., Campbell, P., Manzella, S., and Meezan, E. (1994) J. Biol. Chem. 269, 11509-11513) and therefore presumably inhibited the glucosyl transfer reaction by being a competitive substrate. Like glucosyl transfer, xylosyl incorporation into glycogenin was also inhibited effectively by CDP. On the other hand, UDP-xylose:proteoglycan core protein xylosyltransferase (EC 2.4.2.26) was not affected by CDP, nor was it inhibited by UDP-glucose. Addition of CDP or UDP-glucose to reaction mixtures containing both enzymes therefore made it possible to assay xylosyltransferase EC 2.4.2.26 reliably without the extensive product characterization that is otherwise necessary. The CDP effect on glycogenin further allowed the development of an improved procedure for the purification of this enzyme, in which specific elution of an affinity matrix (UDP-glucuronic acid-agarose) was carried out with CDP as the eluant.