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-beta-glucosaminyl)asparagine
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 (Gal beta 1,4GlcNAc) as an acceptor, we found that the alpha 2,6
sialyltransferase activity expressed by both normal and
tumor colorectal tissues was far higher than the alpha 2,3-activity and that alpha 2,6 was the only
sialyltransferase activity increased in
tumor tissues. Kinetic analysis revealed that normal and
tumor alpha 2,6
sialyltransferases have the same apparent Km for the acceptor substrate (469 and 465 microns), but normal
enzyme has a higher Km for
CMP-NeuAc (303 microns) than the
tumor enzyme (50 microns). 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 alpha 2,6
sialyltransferase activity.