Triethyllead Inhibits Mitochondrial ATPase of the Endosperm and Sucrose Uptake in the Cotyledons of Ricinus communis

Abstract

The active uptake of externally supplied sucrose by isolated cotyledons of Ricinus communis L. is characterized by a K m of 17mM and a V max of 5.3 μmoles × 100 mg −1 FW × h −1 . This uptake was completely inhibited by triethyllead in a non-competitive manner, because in the presence of the toxin the cotyledons transported sucrose with a decreased V max while the K m remained unchanged. The IC 50 for isolated cotyledons was 115 μM; in situ , cotyledons were more susceptible to the toxin, as shown by an IC 50 value of 20 μM. The oligomycin-sensitive ATPase activity of mitochondria isolated from Ricinus communis endosperm was susceptible to low concentrations of triethyllead. Distinct inhibition was seen at ca. 0.1 μM triethyllead, and the IC 50 was determined to be 3 μM. Thus, triethyllead inhibits F 0 /F 1 ATPase more efficiently than oligomycin itself. Addition of glutathione (1 mM) decreased the sensitivity of the enzyme to triethyllead by a factor of 3–4 but was unable to provide full protection. It is concluded that the inhibition of ATP synthesis and the subsequent energy depletion as caused by triethyllead is the likely event leading to the inhibition of active sucrose transport. Physiological concentrations of glutathione partially protect the sucrose transport of cotyledons from the toxic effects of triethyllead.