Calcium-activated neutral protease from human placenta: purification and characterization.

Calcium-activated neutral protease (CANP) has been purified from the human placenta by chromatographic procedures. The purified enzyme is a heterodimer with one subunit of mol. wt 70 000 and another of mol.wt 32 000. It is a thiol protease, active at pH 7.5 at 30 degrees C in the presence of calcium. Half-maximal activation of the enzyme occurred with 800 microM Ca2+.Zn2+ (2 mM), ethylenediaminetetraacetic acid (EDTA)(5 mM) and ethyleneglycol-bis-N,N,N',N'-tetraacetic acid (EGTA)(2 mM) inhibited the enzyme, while Mg2+ (0.5 mM to 5 mM) had no effect on the enzyme in the presence of calcium. Mn2+ and Ca2+ activated the enzyme synergistically. CANP coexists with its endogenous inhibitor in the human placenta. The inhibitor is a protein, inactivated by trypsin and unaffected by RNase, DNase, acid and heat treatments; it inhibits the enzyme probably by interacting with the enzyme molecule itself rather than by sequestering calcium ions.