Dissociation equilibrium constant of beta nerve growth factor

The beta nerve growth factor (NGF) isolated from the high molecular weight 7 S NGF complex from mouse submaxillary gland is a protein of molecular weight 26,500 which contains two noncovalently associated peptide chains. A variety of techniques were employed to determine whether the NGF dimer dissociates at the low concentrations at which it is biologically active. No dissociation was detected in 48 h at low NGF concentrations using the techniques and minimum concentrations which follow: sedimentation equilibrium of NGF at 200 nM and of 125I-NGF at 2.5 nM; gel filtration chromatography of NGF at 5 nM, of 125I-NGF at 5 pM, and of 125I-NGF from which COOH-terminal arginyl residues, or NH2-terminal octapeptide had been cleaved, at 10 pM; and sucrose gradient centrifugation of 125I-NGF at 10 pM. Dissociation of heavily succinylated 125I-NGF was detected by gel filtration chromatography, and the equilibrium dissociation constant for this material is estimated to be 10 pM.