Vitamin D binding protein sequesters monomeric actin in the circulation of the rat.

Plasma vitamin D binding protein (DBP) may scavenge actin released during cell lysis. We examined the plasma disappearance and tissue appearance of 125I-DBP, 125I-G-actin, and the DBP-G-actin complex after their intravenous administration to rats. The plasma disappearance of DBP and DBP-actin were indistinguishable, with rapid initial (t1/2 = 2.6 h) and slower second (t1/2 = 7 h) slopes. After 125I-G-actin (nanomole) injection, plasma disappearance paralleled that of DBP and DBP-actin. All injected actin was associated with DBP, without evidence of free actin, actin-gelsolin complexes or actin oligomers. Tissue appearances of 125I-apo-DBP (apo) or holo-DBP were similar, with highest accumulations in perfused liver, kidney, and skeletal muscle. Although more complex phenomena (plasma entry of F-actin and intracellular actin binding proteins) would occur in vivo after cell lysis, our results suggest a role for DBP in the sequestration and disposition of actin monomers in the circulation.