Increased CMP‐NeuAc:Galβ1,4GlcNAc‐R α2,6 sialyltransferase activity in human colorectal cancer tissues

The sialyltransferase activities of 10 human colorectal specimens were compared with those of the corresponding adjacent normal mucosa. Using asialofetuin as an acceptor we found, in tumor tissues of 9 out of 10 patients, an increased sialyltransferase activity towards the N‐linked chains as determined upon peptide‐N4‐(N‐acetyl‐β‐glucosaminyl)aspara‐gine amidase (PNGase) treatment. On the contrary, the activity towards the O‐linked chains was not significantly changed. When the specificity of the sialyltransferase acting on N‐linked chains was investigated by using N‐acetyl‐lactosamine (Galβ1,4GlcNAc) as an acceptor, we found that the α2,6 sialyltransferase activity expressed by both normal and tumor colorectal tissues was far higher than the α2,3‐activity and that α2,6 was the only sialyltransferase activity increased in tumor tissues. Kinetic analysis revealed that normal and tumor α2,6 sialyltransferases have the same apparent Km for the acceptor substrate (469 and 465 μ,M), but normal enzyme has a higher Km for CMP‐NeuAc (303 μM) than the tumor enzyme (50 μM). The higher affinity of tumor enzyme for the nucleotide‐sugar might partially explain its increased activity in tumor tissues. In addition, tumor tissues contain a lower amount of sialic acid despite the increase in α2,6 sialyltransferase activity.