Permeability and the mechanism of transport of boric acid across the plasma membrane of Xenopus laevis oocytes

Boron is an essential element for vascular plants and for diatoms, cyanobac teria, and a number of species of marine algal flagellates. Boron was recen tly established as an essential micronutrient for frogs (Xenopus laevis) an d preliminary evidence suggests that it may be essential for all animals. T he main form of B, which is available in the natural environment, is in the form of undissociated boric acid. The permeability coefficient and the mec hanism of transport of boric acid, however, have not been experimentally de termined across any animal membrane or cell. In the experiments described h ere, the permeability coefficient of boric acid in Xenopus oocytes was 1.5 X 10(-6) cm/s, which is very close with the permeability across liposomes m ade with phosphatidylcholine and cholesterol (the major lipids in the oocyt e membrane). Moreover, we investigated the mechanism of boric acid movement across the membrane of Xenopus oocytes and we compared it with the transpo rt across artificial liposomes. The transport of boric acid across Xenopus oocytes was not affected by inhibitors such as HgCl2, phloretin, or 4,4-dii sothiocyanatostilbene-2,2 ' -D-sulfonic acid (DIDS). The kinetics of B upta ke was linear with concentration changes, and the permeability remained the same at different external boric acid concentrations. These results sugges t that B transport occurs via simple passive diffusion through the lipid bi layer in Xenopus oocytes.