Determination of xylosyltransferase activity in serum with recombinant human bikunin as acceptor

Xylosyltransferase (XT) is the chain-initiating enzyme of the biosynthesis of chondroitin sulfate. So far, XT activity has been detected by incorporation of [14C]xylose in chemically deglycosylated cartilage proteoglycan or silk fibroin. However, these acceptors allow no reliable determination in blood. We found that recombinant bikunin is an excellent acceptor for XT. The Michaelis-Menten constants for the xylosylation of silk, deglycosylated cartilage proteoglycans, and bikunin were 545, 155, and 0.9 micromol/L, respectively. With recombinant bikunin as acceptor, we developed a sensitive assay that allows a precise determination of XT activity in serum. We measured the serum XT activities of 500 blood donors and observed a considerable sex and age dependence. XT activities in men (0.77-1.50 mU/L) were approximately 30% higher than in women (0.58-1.20 mU/L) and reached a maximum in donors of approximately 40 years of age. During the menstrual cycle, serum XT activity showed a significant coincidence with the beta-estradiol concentration, and in the first trimester of pregnancy we observed a strong increase in serum XT activity.