Purification and partial characterization of an alpha-2,8-sialyltransferase from human erythroleukemia K562 cells

An alpha-2,8-sialyltransferase (ST8), the enzyme involved in the biosynthesis of polysialic acid chains, has been purified and partly characterized from undifferentiated human erythroleukemia K562 cells. Purification, based on a key step of affinity chromatography utilizing immobilized colominic acid, was greater than 1000-fold. The enzyme molecular weight determined by SDS-PAGE was estimated to be about 40 kDa, in good agreement with literature data. For the determination of the main kinetic parameters (Vmax and K(M)), fetuin turned out to be the unique substrate acceptor. In fact, other compounds such as asialofetuin, transferrin, alpha1-acid glycoprotein, and G(M3), routinely used to explore the different ST8 isoforms' activities, did not serve as substrate acceptors. In all cases, contrary to the routinely adopted protocol where a radioactive substrate donor is employed, for our purpose a non-radioactive, fluorescent substrate donor such as cytidine-5'-monophospho-9-(3-fluoresceinylthioureido)-9-deoxy-N-acetyl-neuraminic acid (CMP-9-fluoresceinyl-NeuAc) was used. Thus, under our experimental conditions, by using fetuin, data reported in a typical Lineweaver-Burk plot gave a Vmax value of about 4 nkatal/mg of protein and a K(M) value around 0.61 mM. Just as with the estimated molecular weight, these kinetic data were also in good agreement with those already reported for the ST8 purified from human neuroblastoma CHP-134 cells. In particular, in both cases, Vmax values were almost similar (4 nkatal/mg of protein for our ST8 purified from K562 cells and 4.35 nkatal/mg of protein for ST8 purified from CHP-134 cells); conversely, the K(M) value we found was about 3.25-fold lower than that found by Stoykova and Glick (0.61 mM vs. 2 mM). Then, although our purification was lower than that obtained by Stoykova and Glick (1080-fold vs. 2910-fold), the enzyme we purified showed a greater apparent affinity.