Cd2+ transport and storage in the chloroplast of Euglena gracilis.
Euglena gracilis lacks a plant-like vacuole and, when grown in Cd2+-containing medium, 60% of the accumulated Cd2+ is located inside the chloroplast. Hence, the biochemical mechanisms involved in Cd2+ accumulation in chloroplast were examined. Percoll-purified chloroplasts showed a temperature-sensitive uptake of the free 109Cd2+ ion. Kinetics of the uptake initial rate was resolved in two components, one hyperbolic and saturable (Vmax 11 nmol 109Cd2+ min(-1) mg protein (-1), Km 13 microM) and the other, linear and non-saturable. 109Cd2+ uptake was not affected by metabolic inhibitors or illumination. Zn2+ competitively inhibited 109Cd2+ uptake (Ki 8.2 microM); internal Cd2+ slightly inhibited 109Cd2+ uptake. Cadmium was partially and rapidly released from chloroplasts. These data suggested the involvement of a cation diffusion facilitator-like protein. Chloroplasts isolated from cells grown with 50 microM CdCl2 (ZCd50 chloroplasts) showed a 1.6 times increase in the uptake Vmax, whereas the Km and the non-saturable component did not change. In addition, Cd2+ retention in chloroplasts correlated with the amount of internal sulfur compounds. ZCd50 chloroplasts, which contained 4.4 times more thiol-compounds and sulfide than control chloroplasts, retained six times more Cd2+. The Cd2+ storage-inactivation mechanism was specific for Cd2+, since Zn2+ and Fe3+ were not preferentially accumulated into chloroplasts.
Mendoza-Cózatl
David G
DG
Departamento de Bioquímica, Instituto Nacional de Cardiología, Juan Badiano #1, Col. Sección XVI, Tlalpan 14080, México, DF., Mexico. cozatl@hotmail.com
Moreno-Sánchez
Rafael
R
D000818:Animals; D001692:Biological Transport / Q000502:physiology; D002104:Cadmium / Q000378:metabolism*; D002106:Cadmium Radioisotopes; D002736:Chloroplasts / Q000378:metabolism / Q000502:physiology*; D005056:Euglena gracilis / Q000378:metabolism / Q000502:physiology*; D007501:Iron / Q000378:metabolism; D007700:Kinetics; D013696:Temperature; D015032:Zinc / Q000378:metabolism
D002106:Cadmium Radioisotopes; D002104:Cadmium; D007501:Iron; D015032:Zinc