Abstract
BACKGROUND: Metallothionein (MT) is a group of proteins with low molecular masses and high cysteine contents, and it is classified into different types, which generally contains two domains with typical amino acid sequences. RESULTS: In this report, two full-length cDNAs (MT-1 and MT-II) encoding MT-like proteins were isolated from the roots of sweet potato (Ipomoea batatas [L.] Lam. 'Tainong 57'). Their open reading frames contained 642 and 519 nucleotides (66 and 81 amino acids) for MT-1 and MT-II, respectively, and exhibited a relatively low amino acid sequence similarity. On the basis of the amino acid sequence similarity and conserved residues, it is suggested that MT-I is a member of the plant MT Type-I family, and MT-II is a member of the plant MT Type-II family. The corresponding mRNA levels of MT-1 and MT-II were the highest found in the storage roots. Recombinant MT-1 and MT-II protein overproduced in E. coli (M15) was purified by Ni(2+)-chelated affinity chromatography. MT-1 and MT-II reduced dehydroascorbate (DHA) in the presence of glutathione (GSH) to regenerate L-ascorbic acid (AsA). However, without GSH, MT-1 and MT-II has very low DHA reductase activity. And AsA was oxidized by AsA oxidase to generate monodehydroascorbate (MDA) free radical. MDA was also reduced by MT-1 and MT-II to AsA in the presence of NADH mimicking the MDA reductase catalyzed reaction. CONCLUSIONS: These data suggest that MT-1 and MT-II have both DHA reductase and MDA reductase activities. MT-1 and MT-II are apparently the first reported plant MTs exhibiting both DHA and MDA activities in vitro.