F-Actin-Depolymerizing Activity of Human Serum

Abstract

Non-heated human and animal sera contain a factor which exhibited an inhibiting activity on the staining of actin-containing structures by anti-actin antibodies in indirect immunofluoresecnce experiments. The presence of this factor lowered the viscosity of F-actin preparations and caused, as studied by electron-microscopy, a depolymerization of F-actin filaments as well as inhibition of filament formation of G-actin. The factor was, after its reaction with F-actin, liberated seemingly unaffected, indicating an enzymatic activity. The factor tentatively termed ‘F-actin depolymerizing factor’ was heat-sensitive and trypsin sensitive but resisted reduction. It was Ca²⁺ dependent and the staining inhibiting reaction was faster at 30°C and 37°C than at lower temperatures. Gel filtration experiments on Sephadex G-200 suggested a molecular size of the actin depolymerizing factor slightly higher than that of albumin. The electrophoretic mobility was that of γ₂ globulin. The physiological role of the factor might be to prevent the presence of F-actin filaments within the circulation.