Subunit affinities in the 7S nerve growth factor complex

Nerve growth factor activity in extracts of the male mouse submaxillary gland is found in the 7S nerve growth factor protein. The experiments described here support the idea that the 7S protein is a naturally occurring complex in the extracts and perhaps in the gland itself. If the centrifuged extract is kept at neutral pH for 36 hr, the recovery of purified 7S nerve growth factor is 60% of the normal yield. After dissociation of the complex in the extract by dialysis to pH 10.3 and subsequent redialysis to neutral pH, again over a 36 hr time period, the recovery of purified 7S nerve growth factor is 65% of normal, showing that the α, γ and β subunits of the complex recognize each other and reform 7S nerve growth factor even in the presence of a large excess of other submaxillary gland proteins. Since little or no nerve growth factor activity is found in the low molecular weight fractions of the pH treated extracts, the lower than normal recoveries may be attributed to proteolytic degradation of the subunits rather than lack of recombination. An alternative isolation procedure is described, based on the selective subunit affinities which involves only filtration steps at different pH's. The yields of 7S nerve growth factor in this new procedure are 80-90% of that in the original method.