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 inactivation. Experimental data and theoretical considerations indicate that transition metal cations may destabilize the ionic form of histidine residues within proteins, thereby decreasing their pK(a) values. Because the release 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 hypothesis that metal ion binding to neurotrophins is influenced by pH, the effects of Zn2+ and Cu2+ on NGF conformation, receptor binding and NGF tyrosine kinase (trkA) receptor signal transduction were examined under conditions mimicking cerebral acidosis (pH range 5.5-7.4). The inhibitory effect of Zn2+ on biological activities of NGF is lost under acidic conditions. Conversely, the binding of Cu2+ to NGF is relatively independent of pH changes within the studied range. These data demonstrate that Cu2+ has greater binding affinity to NGF than Zn2+ at reduced pH, consistent with the higher affinity Of Cu2+ for histidine residues. These findings suggest that cerebral acidosis associated with stroke or traumatic brain injury could neutralize the Zn2+-mediated inactivation of NGF, whereas corresponding pH changes would have little or no influence on the inhibitory effects of Cu2+. The importance of His84 of NGF for transition metal cation binding is demonstrated, confirming the involvement of this residue in metal ion coordination.