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 Cd-109(2+) ion. Kinetics of the uptake initial rate was resolved in two components, one hyperbolic and saturable (V-max 11 nmol Cd-109(2+) min(-1) mg protein (-1), K-m 13 muM) and the other, linear and non-saturable. Cd-109(2+) uptake was not affected by metabolic inhibitors or illumination. Zn2+ competitively inhibited Cd-109(2+) uptake (K-i 8.2 muM); internal Cd2+ slightly inhibited Cd-109(2+) 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 muM CdCl2 (ZCd(50) chloroplasts) showed a 1.6 times increase in the uptake V-max, whereas the K-m and the non-saturable component did not change. In addition, Cd2+ retention in chloroplasts correlated with the amount of internal sulfur compounds. ZCd(50) 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. (C) 2004 Elsevier B.V. All rights reserved.