Vitronectin is phosphorylated by a cAMP-dependent protein kinase released by activation of human platelets with thrombin.

Activation of freshly isolated human platelets with a physiological stimulant (thrombin) causes them to release a cAMP-dependent protein kinase which specifically phosphorylates one plasma protein (Mr 75000). This protein is immunochemically and biochemically identified as vitronectin (also know as S protein), which was previously implicated in blood clotting, complement function and cell adhesion.