Summary
Thrombin has been shown to cleave the vitamin K dependent cofactor protein S with
subsequent loss of its cofactor activity. This study examines the control mechanisms
for thrombin cleavage of protein S.
The anticoagulant activity of activated protein C (APC) is enhanced fourteen fold
by the addition of protein S. Thrombin cleaved protein S is seven fold less efficient
than the native protein, and this loss of activity is due to reduced affinity of cleaved
protein S for APC or the lipid surface compared to the intact protein.
In the absence of Ca++, protein S is very sensitive to minimal concentrations of thrombin. As little as
1.5 nM thrombin results in complete cleavage of 20 nM protein S in 10 min and loss
of cofactor activity. Ca++, in concentrations greater than 0.5 mM, will inhibit this cleavage and in the presence
of physiological Ca++ concentrations, no cleavage of protein S could be demonstrated
in spite of high concentrations of thrombin (up to 1 μM) and prolonged incubations
(up to two hours). The endothelial surface protein thrombomodulin is very efficient
in inhibiting the cleavage of protein S by thrombin suggesting that any thrombin formed
on the endothelial cell surface is unlikely to cleave protein S, thus allowing the
intact protein to act as a cofactor to APC.
We conclude that the inhibitory effects of Ca++ and thrombomodulin on thrombin mediated cleavage of protein S imply that this event,
by itself, is unlikely to represent a physiological control of the activity of protein
S.
Keywords
Protein S - Thrombomodulin - Thrombin - Protein C
