Phosphorylation of human plasma alpha(2)-Heremans-Schmid glycoprotein (human fetuin) in vivo

A fraction of alpha (2)-Heremans-Schmid (alpha (2)-HS) glycoprotein (human fetuin) isolated from plasma was phosphorylated at serine-120 and serine-31 2 as shown by MS and peptide fragment sequencing after tryptic digestion. S erine-312-containing peptides were phosphorylated to 77% as determined from relative peak heights in the mass spectrum, which together with the phosph orylation of serine-120 implies a molar degree of phosphorylation of at lea st 1. Approximately 20% of the circulating fetuin plasma pool was phosphory lated to approx. I mot of phosphate/mol of protein. The remainder did not c ontain phosphate, resulting in an average phosphorylation degree for the pr otein in plasma of approx. 0.2 mol/mol. The isolated alpha (2)-HS glycoprot ein was a heterodimer in which the entire C-terminal part of the connecting peptide including threonine-321 was present, but traces of C-terminally tr immed connecting peptide fragments were also found. The short B-chain was O -glycosylated to approx. 40%, whereas the N-glycosylation of asparagine-138 and asparagine-158 seemed to be 100%. This finding, for the first time, th at circulating human plasma fetuin is partly phosphorylated, implies that t he effects of phosphorylated alpha (2)-HS glycoprotein on insulin signal tr ansduction seen in different cell systems could be relevant to its physiolo gical function in vivo.