Function of secreted glycosyltransferases?

[phillip buckhaults]

Martin Dalziel wrote:
> 
> Dear Glyco-netters,
> 
> Various soluble glycosyltransferases are found in serum, colostrum,
> saliva, etc...
> 
> I am curious to find out what the consensus of opinion is as to what the
> function(s), if indeed there are any, of these forms of the
> glycosyltransferases might be.
> 
> For example, is transferase activity impossible due to the scarcity of
> nucleotide-sugar substrates? Can they still bind to target
> oligosaccharide sequences, without transferase activity, in a lectin
> like manner? or are they simply unwanted secreted cellular products?

many people think proteolytic cleavage of the stem region of a glycosyltransferase is 
a "turnover" mechanism by which cells get rid of glycosyltransferases in order to 
down-regulate their activity inside of a cell.  the secretep portion is active in a 
tube where you provide enough substrate, but may not be active as an enzyme when 
secreted, as most extracellular fluids have concentrations of sugar-nucleotide donors 
that are far below the Km's of the enzymes.  as for lectin-like activity, Barry Shur 
sez that cell-surface located galactosyltransferase functions as a lectin/cell 
adhesion molecule during sperm-egg interactions.  

i think the main purpose of these secreted transferases is to facilitate their 
purification :)  !  

just my $0.02