The physiochemical properties and physiological significance of the cadmium-binding protein (CdBP) of the algae Euglena gracilis have been studied. Following in vivo exposure of cells to 0.4 or 1.3 micrograms/mL of Cd2+, all the cytosolic Cd is bound to high molecular weight species. At 4.7 micrograms/mL, appreciable CdBP has formed in cells grown under illumination or in the dark. An analogous ZnBP could not be isolated from control or Zn-exposed (20 micrograms/mL) cells, but zinc and a trace of copper were bound to the CdBP when 2-mercaptoethanol (2-ME) is added to the homogenates of Cd-treated cells and the buffers used during isolation. The large pool of very low molecular weight zinc species previously reported is increased when cells are exposed to high cadmium levels. Two distinct species, BP-1 and BP-2 are resolved by ion-exchange chromatography on DEAE-Sephadex. Unusually high conductivities (25 and 40 mSiemen) are required to displace them, indicating that they are very negatively charged proteins at pH 8.6. The pH for half-titration of bound Cd2+ is between 5 and 6. EDTA (0.4 M) and the CdBP isolated by gel-exclusion chromatography react biphasically with pseudo-first-order rate constants of 4 +/- 3 X 10(-4) sec-1 and 7 +/- 2 X 10(-5) sec-1. Neither form of the CdBP cross-reacts with antibodies to rat liver metallothionein (MT) antibodies. The structural, chemical, and functional differences between the Euglena CdBPs and mammalian MTs are discussed.(ABSTRACT TRUNCATED AT 250 WORDS)