Glycobiology, Vol 9, 115-123, Copyright © 1999 by Oxford University Press
IM Van den Nieuwenhof, WE Schiphorst, I Van Die and DH Van den Eijnden
We have identified a novel N -acetylgalactosaminyltransferase activity in
lactating bovine mammary gland membranes. Acceptor specificity studies and
analysis of products obtained in vitro by 400 MHz1H-NMR spectroscopy
revealed that the enzyme catalyses the transfer of N - acetylgalactosamine
(GalNAc) from UDP-GalNAc to acceptor substrates carrying a terminal,
beta-linked N -acetylglucosamine (GlcNAc) residue and establishes a
beta1-->4-linkage forming a GalNAcbeta1-->4GlcNAc ( N, N
'-diacetyllactosediamine, lacdiNAc) unit. Therefore, the enzyme can be
identified as a UDP-GalNAc:GlcNAcbeta-R beta1-->4-N-
acetylgalactosaminyltransferase (beta4-GalNAcT). This enzyme resembles
invertebrate beta4-GalNAcT as well as mammalian beta4-
galactosyltransferase (beta4-GalT) in acceptor specificity. It can,
however, be clearly distinguished from the pituitary hormone-specific
beta4-GalNAcT by its incapability of acting with an elevated activity on a
glycoprotein substrate carrying a hormone-specific peptide motif.
Furthermore, the GalNAcT activity appeared not to be due to a promiscuous
action of a beta4-GalT as could be demonstrated by comparing the
beta4-GalNAcT and beta4-GalT activities of the mammary gland, bovine
colostrum, and purified beta4-GalT, by competition studies with UDP-GalNAc
and UDP-Gal, and by use of an anti-beta4-GalT polyclonal inhibiting
antibody. Interestingly, under conditions where mammalian beta4-GalT forms
with alpha-lactalbumin (alpha-LA) the lactose synthase complex, the mammary
gland beta4-GalNAcT was similarly induced by alpha-LA to act on Glc with an
increased efficiency yielding the lactose analog GalNAcbeta1-->4Glc.
This enzyme thus forms the second example of a mammalian
glycosyltransferase the specificity of which can be modified by this milk
protein. It is proposed that the mammary gland beta4-GalNAcT functions in
the synthesis of lacdiNAc- based, complex-type glycans frequently occurring
on bovine milk glycoproteins. The action of this enzyme is to be considered
when aiming at the production of properly glycosylated protein
biopharmaceuticals in the milk of transgenic dairy animals.
ORIGINAL ARTICLES
Bovine mammary gland UDP-GalNAc:GlcNAcbeta-R beta1-->4-N- acetylgalactosaminyltransferase is glycoprotein hormone nonspecific and shows interaction with alpha-lactalbumin
Department of Medical Chemistry, Faculty of Medicine, Vrije Universiteit, Van der Boechorststraat 7, 1081 BT Amsterdam, The Netherlands.
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