Simultaneous existence of cinnamomin (a type II RIP) and small amount of its free A- and B-chain in mature seeds of camphor tree

Abstract

Cinnamomin, a type II ribosome-inactivating protein (RIP), was isolated from the mature seeds of camphor tree ( Cinnamomum camphora). In this paper, small amount of free A- and B-chain of cinnamomin were found to be present in the mature seed cell of C. camphora besides the intact cinnamomin. Our results demonstrated that camphorin, a type I RIP previously reported to coexist with cinnamomin in the seeds of C. camphora, actually was the A-chain of cinnamomin. The percentage of free A- and B-chain in the total cinnamomin was 2.6–2.8% in the seed extract. Of these free A- and B-chain approximate 80% already existed in the seed cell, only about 20% were produced during the purification operation. As the enzymatic activity to reduce disulfide bond of cinnamomin in the seed extract of C. camphora was detected, we proposed that the free A- and B-chain were derived from the enzymatic reduction of the interchain disulfide bond of cinnamomin. It was demonstrated that the endogenous type II RIPs of several plant species, such as Cinnamomum porrectum, Cinnamomum bodinieri and Ricinus communis, could be enzymatically reduced into the free A- and B-chain in their respective seed cells. The function of the free A-chain in the seed cell and the possibility that metabolic enzymes might be involved in the reduction of the interchain disulfide bond of type II RIPs in vivo are discussed.