Dissociation Equilibrium Constant of /I Nerve Growth Factor*

The j3 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 1’S I-NGF at 2.5 nM; gel filtration chromatography of NGF at 5 nM, of lz51-NGF at 5 PM, and of lz51-NGF from which COOH-terminal arginyl residues, or NH,-terminal octapeptide had been cleaved, at 10 PM; and sucrose gradient centrifugation of 12”I-NGF at 10 PM. Dissociation of heavily succinylated ls51-NGF was detected by gel filtration chromatography, and the equilibrium dissociation constant for this material is estimated to be 10 PM.