Purification of cadmium-binding proteins from related species of terrestrial Helicidae (Gastropoda, Mollusca): a comparative study.

Three species of terrestrial Helicidae (Helix pomatia, Cepaea hortensis and Arianta arbustorum) were fed cadmium-rich diet in the laboratory. The snails accumulated high amounts of the metal in their hepatopancreas. Most cadmium and some zinc were found, after centrifugation, in the soluble fractions from which a cadmium-binding protein was isolated for each species by ion exchange and gel chromatography. The proteins contained different amounts of cadmium, but little or no zinc, and showed high absorption at 254 nm indicating the presence of cadmium-mercaptide bonds. After gel filtration, a molecular weight of 12,000 was found for cadmium-binding proteins from Helix pomatia and Arianta arbustorum, whereas a molecular weight of 10,000 was found for a cadmium-binding protein from Cepaea hortensis. SDS-polyacrylamide gel electrophoresis showed one single band for each protein from Helix pomatia and Arianta arbustorum and suggested a molecular weight of 11,000 for both species. Amino acid analysis revealed, for each protein, high amounts of cysteine (12-20%), glycine (15-19%), and serine (12-14%), and moderately elevated contents of lysine (9-13%) and alanine (4-8%), but no methionine and only traces, if any, of aromatic amino acids. The ratios of cadmium to cysteine wer 1:5, 1:10 and 1:3 in the proteins from Helix pomatia, Cepaea hortensis and Arianta arbustorum, respectively. Some features of the isolated proteins resembled mammalian metallothioneins. Most characteristics, however, differed from true metallothioneins and were similar to cadmium-binding proteins found in some marine molluscs.