Protamine sulfate stimulates degradation of factor Xa and the factor Xa-antithrombin complex.

The effect of protamine sulfate on factor Xa (FXa) and the factor Xa-antithrombin complex was studied via SDS-PAGE and Western blot analysis. Human factor Xa [(FXaα) ∼52 kDa, FXaβ ∼47 kDa] and human antithrombin (AT ∼55 kDa) form a primary (1°) complex at 109 and 104 kDa, a secondary (2°) complex at 99 and 95 kDa, and a tertiary (3°) complex at 66 and 62 kDa, as detected with polyclonal anti-FXa and anti-AT antibodies. Addition of protamine sulfate stimulates a transformation from free FXaα to FXaβ and degradation thence to inactive FXaγ (∼30 kDa), as well as transformation of FXaα-AT complexes to FXaβ-AT complexes. Additionally, protamine sulfate promotes digestive degradation from 1° to 3° complexes and FXaγ by the active enzyme. It further promotes reduction in total complex formation as a function of stimulation of hydrolysis of FXa moieties from the complex, thereby generating ATM. Protamine sulfate effects are proportional to the protamine sulfate concentration.