DETERMINATION OF XYLOSYLTRANSFERASE ACTIVITY IN SERUM WITH RECOMBINANT HUMAN BIKUNIN AS ACCEPTOR

Xylosyltransferase (XT) is the chain-initiating enzyme of the biosynth esis of chondroitin sulfate. So far, XT activity has been detected by incorporation of [C-14]xylose in chemically deglycosylated cartilage p roteoglycan or silk fibroin. However, these accepters allow no reliabl e determination in blood. We found that recombinant bikunin is an exce llent acceptor for XT. The Michaelis-Menten constants for the xylosyla tion of silk, deglycosylated cartilage proteoglycans, and bikunin were 545, 155, and 0.9 mu mol/L, respectively. With recombinant bikunin as acceptor, we developed a sensitive assay that allows a precise determ ination of XT activity in serum. We measured the serum XT activities o f 500 blood donors and observed a considerable sex and age dependence. XT activities in men (0.77-1.50 mU/L) were similar to 30% higher-than in women (0.58-1.20 mU/L) and reached a maximum in donors of similar to 40 years of age. During the menstrual cycle, serum XT activity show ed a significant coincidence with the beta-estradiol concentration, an d in the first trimester of pregnancy we observed a strong increase in serum XT activity.