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

Abstract

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 μmol/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 ~30% higher than in women (0.58-1.20 mU/L) and reached a maximum in donors of ~40 years of age. During the menstrual cycle, serum XT activity showed a significant coincidence with the β-estradiol concentration, and in the first trimester of pregnancy we observed a strong increase in serum XT activity.