Bombyx cysteine proteinase inhibitor (BCPI) homologous to propeptide regions of cysteine proteinases is a strong, selective inhibitor of cathepsin L-like cysteine proteinases

Bombyx cysteine proteinase inhibitor (BCPI) is a novel cysteine proteinase inhibitor. The protein sequence is homologous to the proregions of certain cysteine proteinases. Here we report the mechanism of its inhibition of sev eral cysteine proteinases. BCPI strongly inhibited Bombyx cysteine proteina se (BCP) activity with a K-i = 5.9 muM, and human cathepsin L with a K-i = 36 muM. The inhibition obeyed slow-binding kinetics. The inhibition of cath epsin H was much weaker (K-i = 82 nm), while inhibition of papain (K-i > 1 muM) and cathepsin B (K-i > 4 muM) was negligible. Following incubation wit h BCP, BCPI was first truncated at the C-terminal end, and then gradually d egraded over time. The truncation mainly involved two C-terminal amino acid residues. Recombinant BCPI lacking the two C-terminal amino acid residues still retained substantial inhibitory activity. Our results indicate that B CPI is a stable and highly selective inhibitor of cathepsin L-like cysteine proteinases.