A proposed position of a disulfide bridge in the molecules of cinnamomin and porrectin - two new type II ribosome-inactivating proteins isolated from the seeds of camphor trees

The N-terminal amino acid sequences of A-and B-chains of three isoforms of cinnamomin and porrectin-two new type II ribosome-inactivating proteins purified from the seeds of camphor trees have been compared with themselves and with that of ricin and abrin. It has completely homologous sequence of A-chain of six isoforms, and the same of six B-chain sequences. However, among 10 amino acids of the N-terminal sequence of the A-chain there is only 20% homology between cinnamomin/porrectin and abrin, no homology between cinnamomin/porrectin and ricin. In their B-chain, there are 60% homology between cinnamomin/porrectin and ricin, the fourth position is always occupied by cysteine. Compared with that of ricin/abrin, we proposed that the position of a disulfide bridge is at fourth cysteine in the B-chain of cinnamomin/porrectin and a cysteine nearby the C-terminus of A-chain of cinnamomin/porrectin.