Al-toxicity studies in yeast using gallium as an aluminum analogue

Aluminum (Al) is normally present in soils as the insoluble, harmless Al2O3. The highly toxic Al3+ and AlOH2+ monomeric cations are formed in acid soils but there is little consensus on the physiological basis of Al toxicity in plants. A major factor that has retarded progress in understanding aluminum toxicity in vascular plants is the lack of a convenient radioisotope for Al. Yeast and vascular plants share similar membrane transport mechanisms and so yeast (Saccharomyces cerevisiae) provides a convenient model system for studies of Al-toxicity. Al and gallium (Ga) have closely similar toxic effects on the yeast cells (Ki approximately 100 mmol m-3) and Ga3+ and Al3+, respond similarly to pH and are both reversible by a chelation agent (citric acid). We tested the feasibility of using 67Ga radioisotope as a tracer for Al transport with the view of using it to investigate the mechanism of Al uptake and toxicity in plants. The clinically available 67Ga citrate is unsuitable to use as an aluminum analogue because the chelated form is not toxic. Arrangements need to be made for it to be supplied as 67GaCl3. Large amounts of 67Ga rapidly bind to the cell wall of yeasts with a t 1/2 of approximately 1 s. There is a very slow net uptake of 67Ga into a second phase, presumably the cytoplasm. Uptake into the slow phase has a Vmax of only approximately 16 +/- 4 pmol m(-2) s(-1) (n = 16). The Km of 67Ga uptake could not be precisely determined but is below 100 mmol m(-3) (45 +/- 42 mmol m(-3), n = 16).