Inhibition of nerve growth factor‐stimulated neurite outgrowth by methylamine‐modified α 2 ‐macroglobulin

α2‐Macroglobulin (α 2 M) is a rather ubiquitous protein in extracellular spaces of mammals. It is an inhibitor of endopeptidases, can be modified by aliphatic amines, and combines with a number of hormones/cytokines such as β‐nerve growth factor (NGF) [Koo PH, Stach RW (1989): J Neurosci Res 22:247]. The objective of this study is to compare the NGF‐binding properties of methylamine‐modified human α 2 M (MA‐α 2 M) versus normal α 2 M and their effects on the biological activity of NGF and neurite extension by embryonic chicken dorsal root ganglia. As determined by gel filtration, polyacrylamide gel electrophoresis, and equilibrium binding studies, these two forms of α 2 M are similar in their binding affinities, with MA‐α 2 M binding about twice as much NGF as normal α 2 M. Both normal α 2 M and MA α 2 M combine noncovalently with NGF, and prior modification of α 2 M is unnecessary for the binding to occur. In contrast to normal α 2 M, MA‐α 2 M potently inhibits the biological activity of NGF and exerts a dose‐dependent inhibition on the NGF‐stimulated neurite outgrowth by embryonic chicken dorsal root ganglia in culture. The inhibitory effect of MA‐α 2 M can be overcome by higher NGF concentrations, but is irreversible at lower NGF concentrations. Trypsin‐modified α 2 M combines covalently and noncovalently with more NGF than normal α 2 M but has very little neurite inhibitory activity. The mechanism of inhibition by MA‐α 2 M is discussed.