Phosphoglucomutase is an in vivo lithium target in yeast

Lithium is a drug frequently used in the treatment of manic depressive disorder. We have observed that the yeast Saccharomyces cerevisiae is very sensitive to lithium when growing in galactose medium. In this work we show that lithium inhibits with high affinity yeast (IC50 approximately 0.2 mm) and human (IC50 approximately 1.5 mm) phosphoglucomutase, the enzyme that catalyzes the reversible conversion of glucose 1-phosphate to glucose 6-phosphate. Lithium inhibits the rate of fermentation when yeast are grown in galactose and induces accumulation of glucose 1-phosphate and galactose 1-phosphate. Accumulation of these metabolites was also observed when a strain deleted of the two isoforms of phosphoglucomutase was incubated in galactose medium. In glucose-grown cells lithium reduces the steady state levels of UDP-glucose, resulting in a defect on trehalose and glycogen biosynthesis. Lithium acts as a competitive inhibitor of yeast phosphoglucomutase activity by competing with magnesium, a cofactor of the enzyme. High magnesium concentrations revert lithium inhibition of growth and phosphoglucomutase activity. Lithium stress causes an increase of the phosphoglucomutase activity due to an induction of transcription of the PGM2 gene, and its overexpression confers lithium tolerance in galactose medium. These results show that phosphoglucomutase is an important in vivo lithium target.