The nerve growth factor (NGF), a protein which stimulates growth of sympathetic and embryonic sensory ganglia, was originally discovered and studied by Levi-Montalcini and her colleagues. Involved chromatography on CM-cellulose at pH4.4, NGF was obtained as a protein with an S20,w having a molecular weight of 26518 kD. When NGF performs its biological activity, it forms like a dimer. Under this situation, the NGF molecular weight rises to 53036 kD.
Subsequently, a procedure for isolating a higher molecular weight form of NGF at neutral pH was developed. The NGF molecular weight is approximately 140,000 kD. This form of NGF has been called 7S NGF. The high NGF molecular weight due to 7S NGF sedimentation properties which was found to contain three different types of subunits called α,β and γ. The βsubunit was found to have the nerve growth stimulating property, the γ subunit was shown to be an esteropeptidase and the α subunit has been found to have a protective effect on embryonic sensory ganglionic cells during dissociation by trypsin. The molecular weight of either the α- and γ- subunits is approximately 26,000 Da while the combined molecular weight of the two polypeptides of the β-subunit is 26518 Da.
This high moleculer weight nerve growth factor was mostly inactivated upon exposure to pH below 5.0 and above 8.0. Inactivation was accompanied by dissociation into acidic, basic, and neutral components. Once separated by chromatography on carboxymethyl (CM) cellulose, only the B subunit displayed nerve growth factor activity, which, however, accounted for only 25 per cent of the amount loaded on the column. Recovery of the total nerve growth factor activity and reconstitution of the 140,000-molecular-weight protein were obtained by mixing the three components at neutral pH.