Cysteine proteinase inhibitors in human placenta.

When human placental extract was chromatographed on a Sephadex G-75 column, cysteine proteinase inhibitors with molecular weights of 80 000 and 12 300 were eluted. The high molecular weight peak (CPI-H) was identified as alpha-cysteine proteinase inhibitor. The thermostable low molecular weight peak (CPI-L) inhibited plant proteinases (papain, ficin and bromelain) as well as cathepsins B, H and L isolated from the human placenta. No cross-reactivity was observed between placental CPI-L and serum alpha-CPI.