Characterization of the differential interaction of the microheterogeneous forms of the gamma subunit of 7 S nerve growth factor with natural and synthetic ligands.

The effect of the structural heterogeneity of the gamma subunit of 7 S nerve growth factor (NGF) on its interaction with a variety of synthetic substrates and natural ligands was studied. The major gamma species, distinguished on the basis of charge under isoelectric focusing conditions, were examined for possible differences in affinity toward basic pancreatic trypsin inhibitor, beta NGF, and 7 S NGF and differences in activity toward three synthetic arginine substrates, N alpha-benzoyl-DL-arginine-p-nitroanilide, p-tosyl-L-arginine methyl ester, N alpha-benzoyl-L-arginine ethyl ester. The most proteolytically modified form of the gamma subunit, gamma 3, was found to have an affinity for the natural ligands that was consistently an order of magnitude greater than gamma 1, the least modified form, while gamma 2 was intermediate, tending toward gamma 1. These differences were found to hold true for the activities toward the synthetic substrates as determined by the kcat/Km ratios. The nature of the affinity and activity differences among the major gamma species is discussed in light of a model where the presence or absence of certain basic residues, which along with the presence or absence of specific peptide chain cleavages forming the basis of the gamma subunit structural heterogeneity influence the access of ligand to the active site.