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Gelsolin is a Ca(2+)-regulated actin-modulating protein found in a variety of cellular cytoplasm and also in blood plasma. Affinity separation of human plasma gelsolin was successfully accomplished by eluting the protein with a low concentration of nucleoside polyphosphate from immobilized Cibacron Blue F3GA (1, 2). This finding was followed by the demonstration that the protein had one class of ATP binding site with Kd = 2.8 x 10(-7) M, which saturated at an ATP/gelsolin ratio of 0.6 in the absence of Ca2+ (3). To obtain further information on the nucleotide binding properties of gelsolin, binding studies were done in the presence of EGTA with GTP, ADP, and GDP by equilibrium dialysis. Incubation of plasma gelsolin with GTP resulted in binding of 0.6 mol of GTP per mol of protein with a dissociation constant of 1.8 x 10(-6) M, indicating that ATP binds to gelsolin with higher affinity than GTP. Neither ADP nor GDP at up to 100 microM appreciably bound to gelsolin at a physiological salt concentration. Then, the effects of divalent metal ions on the ATP binding to plasma gelsolin were examined. Gelsolin bound to ATP with Kd = 2.4 x 10(-6) M in a solution containing 2 mM MgCl2, whereas micromolar free Ca2+ concentrations inhibited ATP binding. Furthermore, addition of Ca2+ rapidly reversed the preformed nucleotide binding to gelsolin, suggesting that Ca2+ binding to gelsolin leads to a conformational change which disrupts a nucleotide binding fold in the protein molecule.

Human plasma contains two actin-binding proteins, plasma gelsolin and vitamin D-binding protein. These proteins are considered to play an important role in the disposition of actin derived from injured tissue. To evaluate this actin-scavenger system, gelsolin concentrations were measured in serial plasma samples obtained from patients with acute liver injury using an enzyme-linked immunosorbent assay. Plasma gelsolin levels in 43 healthy persons were 226 +/- 52 micrograms/mL. They were markedly reduced to 80 +/- 40 micrograms/mL in 14 patients with an early stage of acute hepatitis and returned to normal levels of 232 +/- 38 micrograms/mL as the disease resolved. Moreover, they showed a significant negative correlation with serum aminotransferase and bilirubin levels. In 7 patients with hepatocellular carcinoma, plasma gelsolin levels rapidly decreased from 182 +/- 42 to 87 +/- 41 micrograms/mL after transcatheter arterial embolization therapy. Because plasma gelsolin is not a hepatic protein, the decreased levels are considered to depend exclusively on the extent of actin leakage from the injured liver.

Human plasma gelsolin was specifically eluted from a Cibacron Blue F3GA column with 1 mM adenosine, guanosine, cytidine and uridine di- and triphosphates, except for cytidine 5'-diphosphate. Inorganic polyphosphates also eluted gelsolin, but neither nicotinamide adenine dinucleotide nor nicotinamide adenine dinucleotide phosphate did so. The results suggest that a terminal pyrophosphate structure might be essential for the specific elution. After elution with 1 mM adenosine 5'-triphosphate, additional gelsolin was eluted by washing the column with a high salt concentration. This indicates that human plasma gelsolin may bind to Cibacron Blue F3GA in at least two different ways.