Phosphorylation of human plasma α2-Heremans‒Schmid glycoprotein (human fetuin) in vivo

Abstract

A fraction of alpha2-Heremans-Schmid (alpha2-HS) glycoprotein (human fetuin) isolated from plasma was phosphorylated at serine-120 and serine-312 as shown by MS and peptide fragment sequencing after tryptic digestion. Serine-312-containing peptides were phosphorylated to 77% as determined from relative peak heights in the mass spectrum, which together with the phosphorylation of serine-120 implies a molar degree of phosphorylation of at least 1. Approximately 20% of the circulating fetuin plasma pool was phosphorylated to approx. 1 mol of phosphate/mol of protein. The remainder did not contain phosphate, resulting in an average phosphorylation degree for the protein in plasma of approx. 0.2 mol/mol. The isolated alpha2-HS glycoprotein was a heterodimer in which the entire C-terminal part of the connecting peptide including threonine-321 was present, but traces of C-terminally trimmed 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, that circulating human plasma fetuin is partly phosphorylated, implies that the effects of phosphorylated alpha2-HS glycoprotein on insulin signal transduction seen in different cell systems could be relevant to its physiological function in vivo.

Fetuin-A-Containing Calciprotein Particles Reduce Mineral Stress in the Macrophage

“…Haglund et al demonstrated that 7–20% of the fetuin-A circulating in plasma was phosphorylated, predominantly at Ser312 and to a lesser extent at Ser120 [51]. Recent mass spectrometry based studies of human phosphoproteome have identified four other serine residues that may be partially phosphorylated: Ser134, Ser138, Ser325, Ser334 [52], [53].…”

Extended‐release niacin decreases serum fetuin‐A concentrations in individuals with metabolic syndrome

“…Phosphofetuin‐A (Ser312) was reduced in 11 of 14 participants but did not reach statistical significance ( p = 0.08), which may be attributed to the small sample size. Human fetuin‐A is phosphorylated on Ser120 and Ser312 with most of the phosphorylation on Ser312 35. The phosphorylation status has been shown to be critical for the inhibitory action of fetuin‐A in rats and implicated in humans 5, 11, 12.…”

Alterations of Serum Ser312‐Phosphorylated Fetuin‐A from Exercise‐Induced Moderate Body Weight Loss in Individuals with Obesity

“…Fet‐A exists in circulation in both phosphorylated (~20%) and dephosphorylated (~80%) forms . Of the two phosphorylation sites on Fet‐A, serine312 (Ser312) was shown to be the dominant phosphorylation site, displaying ~77% phosphorylation compared with serine120 . We and others have shown that phosphorylation status is critical for its inhibitory effects on insulin‐stimulated insulin receptor autophosphorylation, insulin receptor tyrosine kinase activity, glucose uptake, and glycogen synthesis .…”