A RECOMBINATION ASSAY OF HIGH MOLECULAR WEIGHT NERVE GROWTH FACTOR PROTEIN COMPLEXES

Abstract— The 7S Nerve Growth Factor (NGF) found in mouse is a protein composed of three dissimilar subunits designated α, γ, and β. Mouse NGF reversibly dissociates into its component subunits at pH values lower than 5 or greater than 8. The 7S protein complex is in fast equilibrium wiih the free subunits. A quantitative determination of 7S NGF based on the reversible dissociation of the molecule into its subunits is reported here. The basis for this procedure is the addition of [¹²⁵I]α subunit which competes with the native alpha in the 7S complex. The level of NGF present in a sample can be measured in terms of the displacement of [¹²⁵I]α from the α to the NGF position in a linear sucrose gradient. Measurements are sensitive to 3.5ng of NGF, linear, and specific for the NGF molecule. This exchange is unaffccted by competing unrelated growth factors and inhibiting enzymes. Its specificity was checked by the standard bioassay and by radioimmunoassay. In addition, the assay technique overrides masking by biological inhibitors and/or by the presence of antibodies directed against the IgG contaminant in murine 75 NGF.