Reactivity of smooth-muscle antibodies with F- and G-actin.

The reactivity of human smooth-muscle antibodies (SMA) with F- and G-actin was investigated by means of an enzyme-linked immunosorbent assay (ELISA). Polystyrene tubes were coated with F-actin, G-actin and tropomyosin. The optimal reactivity of SMA with F-actin was achieved with tubes coated with actin concentrations in the range of 10-100 mug/ml, while both lower and higher amounts yielded lower reactivity. The reactivity with G-actin was low and only a little higher than the reactivity with tropomyosin or uncoated tubes, but much lower than the reactivity with F-actin. The differences in reactivity of SMA with F- and G-actin could not be explained by differences in binding of the two forms of actin to the tubes, although 1.7-2.5 times more F-actin than G-actin bound to the polystyrene surface after coating with equivalent amounts of protein. SMA-negative sera did not react with F-actin coated tubes indicating that the binding of antibody was not due to non-immunological binding of immunoglobulins. Human SMA thus react better with F- than with G-actin.