The amino acid sequences of isoforms of the bromelain inhibitor from pineapple stem.

Eight bromelain inhibitor (BI) isoform fractions were isolated from pineapple stem, and then submitted to conventional amino acid sequencing after performic acid oxidation and subsequent separation of the resulting light and heavy chains. The results revealed that all fractions exhibited microheterogeneity, containing at least two major components, but that all the isoinhibitors have a common double-chain structure (Mr = ca. 5,700-5,900) with five disulfide bonds and similar amino acid sequences. Notably, Fraction BI-VIII exhibited less than 40% of the specific inhibitory activity toward stem bromelain as compared with the other inhibitor fractions. This fraction was a mixture of two isoforms, BI-VIII(1) and BI-VIII(2), the latter lacking the arginine or glutamine residue at the COOH-terminus of the light chain. Furthermore, the oxidized light chain of BI-III, used as a representative normal isoinhibitor, was found to exhibit significant inhibitory activity, whereas the oxidized light chain of BI-VIII(2) lacking the COOH-terminal Arg or Gln showed only very low inhibitory activity. Therefore, the major bromelain-inhibitory site was indicated to be the COOH-terminal residue, Arg or Gln, of the light chain. This is consistent with the three-dimensional structure model constructed by computer modeling for the hypothetical complex between BI-VI and papain, a close homolog of bromelain.