Comparison of the biophysical characteristics of human brain-derived neurotrophic factor, neurotrophin-3, and nerve growth factor.

The structural properties of human brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) were studied using sedimentation equilibrium and circular dichroism (CD), fluorescence and Fourier-transform infrared spectroscopies, and compared with those of human nerve growth factor (NGF). Both the far UV CD and infrared spectra indicate that these three proteins have similar, but not identical, secondary structures which contain primarily beta-sheet and irregular structures. NGF appears to contain the most beta-sheet while NT-3 contains a small fraction of alpha-helix. The near UV CD spectra appear to indicate that the three proteins contain disulfide bonds in similar environments, suggesting a resemblance in tertiary structure. The fluorescent tryptophans found in the molecules are relatively solvent exposed, while Trp102 found only in NT-3 is possibly quenched. The fluorescent Trp(s) in NGF are significantly quenched relative to those in the other two neurotrophic factors. Both NT-3 and BDNF have very hydrophilic surfaces at neutral pH, as indicated by a low binding affinity to a hydrophobic probe, anilinonaphthalenesulfonate. Sedimentation equilibrium showed that BDNF, NT-3, and NGF exist as strongly associated dimers in phosphate-buffered saline, pH 7.1. Fits of the observed fringe displacements to various association models suggested that the BDNF, NT-3, and NGF samples contain, in addition to the principal dimeric species, some oligomers, and that NT-3 contains a small fraction of incompetent monomer.