Abstract
Napins are 1:1 disulfide-linked complexes of a smaller (ca. 4 kDa) subunit and a larger (ca. 10 kDa) subunit. The intent of the present study was to ascertain the production of napin by the seeds of a Brassica species that has not been examined previously, and also to explore new biological activities of the napin. A heterodimeric 11-kDa napin-like polypeptide has been isolated from Chinese white cabbage ( Brassica chinensis cv dwarf) seeds with a protocol comprising ion exchange chromatography on DEAE–cellulose, affinity chromatography on Affi-gel blue gel, fast protein liquid chromatography (FPLC)-ion exchange chromatography on Mono S and FPLC-gel filtration on Superdex 75. The N-terminal sequence of the 7-kDa subunit manifests striking similarity to napin large chain, albumin and trypsin inhibitor. The N-terminal sequence of the 4-kDa subunit is homologous to napin large chain and an antimicrobial peptide. The napin-like polypeptide inhibited translation in the rabbit reticulocyte system with an IC 50 of 18.5 nM. This translation-inhibitory activity was stable between pH 4 and 11, and between 10 and 40 °C. The polypeptide inhibited trypsin with a higher potency (IC 50=8.5 μM) than it inhibited chymotrypsin (IC 50=220 μM), but was devoid of ribonuclease and antifungal activities. It manifested antibacterial activity against Pseudomonas aeruginosia, Bacillus subtilis, Bacillus cereus, and Bacillus megaterium. The results revealed that the napin-like polypeptide from Chinese white cabbage seeds exhibited some potentially exploitable activities.
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