Role of plasma gelsolin and the vitamin D-binding protein in clearing actin from the circulation

We determined the plasma kinetics of both actin and complexes of actin with the two high affinity actin-binding proteins of plasma, gelsolin, and vitamin D-binding protein (DBP). Actin is cleared rapidly from the plasma by the liver (half-disappearance time, 0.5 h). Using radiolabeled actin-binding proteins, we found that actin accelerated the clearance of both plasma gelsolin and the vitamin D-binding protein. In separate experiments we found that DBP-actin complexes were cleared more quickly than gelsolin-actin complexes, at a rate comparable to the clearance of actin from the blood. A low affinity interaction (dissociation constant, 2.9 X 10(-4) M) between actin and fibronectin was found, suggesting that little actin will bind to fibronectin in plasma. We conclude that while plasma gelsolin and DBP may both clear actin from the circulation, DBP appears to play a more important role. By so doing, DBP may conserve the filament-severing activity of plasma gelsolin.