Interaction of nerve growth factor with murine α‐macroglobulin

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Abstract
The murine nerve growth factor, when injected i.v. or, combined in vitro with plasma, was found largely associated with the mouse α-macroglobulin (a homologue of human α2-macroglobulin). The nerve growth factor α-macroglobulin complex produced is sufficiently stable to resist separation by gel filtration in 1.0 M sodium chloride, polyacrylamide gel electrophoresis, and immunoprecipitation by antibodies against α-macroglobulin. As determined by equilibrium binding studies and computer generated Scatchard analysis, α-macroglobulin apparently possesses two types of binding sites with the apparent dissociation constants of 1.2 × 10−6 and 2.9 × 10−9 M, respectively, saturable by 3.7 and 0.03 moles of nerve growth factor. Hence, about one mole of nerve growth factor is bound to each of the four subunits of α-macroglobulin. Nerve growth factor can be readily dissociated from α-macroglobulin in sodium dodecyl sulfate gel electrophoresis in the absence of a reductant. Procedures that affect the proteinase-binding or methylamine-activities of α-macroglobulin do not affect the binding of nerve growth factor, and the binding is unaffected by the presence of zinc ions or EDTA. Hence, nerve growth factor is noncovalently associated with α-macroglobulin at a site separate from that of the proteinase-, methylamine-, and zinc-binding sites of α-macroglobulin. Mouse α-macroglobulin can protect the nerve growth factor from inactivation by trypsin. Even in the presence of trypsin, α-macroglobulin-nerve growth factor complexes still can stimulate the neurite outgrowth by dorsal root ganglia of 9-day-old chicken embryos. Since α-macroglobulin can specifically and noncovalently carry nerve growth factor, one important role of this α-macroglobulin in the circulation and extracellular spaces may be to protect the nerve growth factor from proteinase inactivation.