The binding of zinc and copper ions to nerve growth factor is differentially affected by pH: implications for cerebral acidosis

It has recently been shown that transition metal cations Zn2+ and Cu2+ bind to histidine residues of nerve growth factor (NGF) and other neurotrophins (a family of proteins important for neuronal survival) leading to their in activation. Experimental data and theoretical considerations indicate that transition metal cations may destabilize the ionic form of histidine residu es within proteins, thereby decreasing their pK(a) values. Because the rele ase of transition metal cations and acidification of the local environment represent important events associated with brain injury, the ability of Zn2 + and Cu2+ to bind to neurotrophins in acidic conditions may alter neuronal death following stroke or as a result of traumatic injury. To test the hyp othesis that metal ion binding to neurotrophins is influenced by pH, the ef fects of Zn2+ and Cu2+ on NGF conformation, receptor binding and NGF tyrosi ne kinase (trkA) receptor signal transduction were examined under condition s mimicking cerebral acidosis (pH range 5.5-7.4). The inhibitory effect of Zn2+ on biological activities of NGF is lost under acidic conditions. Conve rsely, the binding of Cu2+ to NGF is relatively independent of pH changes w ithin the studied range. These data demonstrate that Cu2+ has greater bindi ng affinity to NGF than Zn2+ at reduced pH, consistent with the higher affi nity Of Cu2+ for histidine residues. These findings suggest that cerebral a cidosis associated with stroke or traumatic brain injury could neutralize t he Zn2+-mediated inactivation of NGF, whereas corresponding pH changes woul d have little or no influence on the inhibitory effects of Cu2+. The import ance of His84 of NGF for transition metal cation binding is demonstrated, c onfirming the involvement of this residue in metal ion coordination.