Identification and quantitation of O‐phosphoserine in human plasma fibronectin

Abstract
O‐Phosphoserine was positively identified as the phosphorylated moiety in human plasma fibronectin by 31P‐NMR of intact peptides. These data correlated completely with chemical analyses which demonstrated the presence of O‐phosphoserine at a concentration of 2 residues/molecule. Neither O‐phosphothreonine nor O‐phosphotyrosine was detected in partial acid and partial alkaline hydrolysates, respectively.