The nerve growth factor zymogen. Stoichiometry of the active-site serine and role of zinc(II) in controlling autocatalytic self-activation

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Abstract
Mouse submandibular gland nerve growth factor (NGF) is a 116,000 MW protease with a high degree of specificity for certain lysyl and arginyl bonds. This protein can activate plasminogen and it is also a member of the general class of serine proteases. As isolated, NGF is an enzymically inactive zymogen. Upon dilution from high to very low protein concentrations, or upon treatment with EDTA, the zymogen undergoes autocatalytic activation. Atomic absorption spectroscopy measurements reveal that NGF contains 1 g-atom of tightly bound Zn(II)/mol. Reaction of the fully autoactivated protease with [3H] diisopropyl phosphorofluoridate yields 1 mol of labeled serine per mol of enzyme. As long as Zn(II) remains bound to the zymogen, autocatalytic activation is inhibited. Removal of this ion, by dilution of the protein or by chelation, initiates autoactivation. The physiologic purpose of this unusual reaction is not known, but Zn(II) may serve to act as a control ion which keeps the protein in an inactive form (the zymogen) until it recognizes its naturally occurring substrate.