The binding of human acidic fibroblast growth factor (aFGF) to heparin
has been analyzed by a variety of different approaches to better eluc
idate the nature of this protein/sulfated polysaccharide interaction.
Static and dynamic light scattering as well as analytical ultracentrif
ugation analyses indicates that 14-15 molecules of aFGF can bind to a
16-kDa heparin chain, with approximately 10 of these bound relatively
uniformly to high-affinity sites. The dissociation constants of these
latter sites are estimated to be approximately 50-140 nM on the basis
of surface plasmon resonance experiments in which the association and
dissociation rates of aFGF interaction with immobilized heparin were m
easured. The size of the binding site of aFGF on heparin was also dete
rmined by heparin lyase digestion of aFGF/heparin complexes followed b
y isolation and characterization of protected oligosaccharides. The sm
allest aFGF-protected oligosaccharide comigrated with S(1-->4)-alpha-L
-IdoAp-2S(1-->4)-alpha-D-GlcNp2S6S (where DELTAUA represents 4-deoxy-a
lpha-L-threo-hex-4-enopyranosyluronic acid and S is sulfate). Thus, a
FGF appears to bind at high density (one molecule every 4-5 polysaccha
ride units) and with high affinity to heparin. This potentially provid
es a concentrated, stabilized storage form of the growth factor that c
an be released for receptor-mediated cellular activation in response t
o the proper stimuli. It is also possible that close proximity of aFGF
molecules on the highly sulfated regions of heparan chains may be inv
olved in the induction of receptor aggregation as suggested by Ornitz
et al.