The preparation and properties of nerve growth factor protein at alkaline pH

The nerve growth factor (NGF) subunit of 7S NGF was isolated by chromatography at high pH on QAE-Sephadex. It has the same specific NGF activity as betaNGF isolated at acid pH, showing that this activity is an intrinsic property of the subunit and is independent of the pathway of dissociation. Continued exposure of the NGF subunit to high pH resulted in an increase in the amount of the minor species beta2NGF and the formation of a new species, beta3NGF, of even lower isoelectric point. These two species and the original major species of the preparation, beta1, were isolated by isoelectric focusing. All three species had the same specific NGF activity, but differed in their ability to reform 7S NGF. The beta2 species was one-fifth as competent as beta1, while beta3 was unable to regenerate 7S NGF. Addition of alpha- and gamma-subunits to beta1NGF decreased the amount of NGF protein required to produce one Biological Unit of activity in the bioassay, but had no effect when added to beta3NGF. The interactions between the subunits in 7S NGF therefore determine, in part, the specific activity of the NGF subunit.