Participation of extracellular fumarase in the utilization of malate in cultured carrot cells

Abstract

Carrot suspension cells were found to be unable to transport malate directly into the cell but utilized it as a single carbon source in a unique manner –they converted malate extracellularly to fumarate and subsequently used it instead. The uptake of fumarate proved to be inducible and sensitive to pH and protonophore. Immuno-blot experiments using an antibody raised against Arabidopsis fumarase showed that fumarase polypeptide appeared in the medium. Fumarase was not detected in medium when fumarate or glucose was used as a carbon source. The activity of fumarase, which catalyzes the reversible hydration reactions, was induced both in the medium (malate into fumarate, releasing protons) and in the cells (fumarate into malate, requiring protons) and resulted in an increase in the pH gradient across the plasma membrane. The reason for the participation of fumarase in the utilization of malate is discussed.