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Brush border membranes were isolated from tilapia (Oreochromis mossambicus) intestine by the use of magnesium precipitation and differential centrifugation. The membrane preparation was enriched 17-fold in alkaline phosphatase. The membranes were 99% right-side-out oriented as indicated by the unmasking of latent glyceraldehyde-3-phosphate dehydrogenase and acetylcholine esterase activity by detergent treatment. The transport of Ca2+ in brush border membrane vesicles was analyzed. A saturable and a nonsaturable component in the uptake of Ca2+ was resolved. The saturable component is characterized by a Km much lower than the Ca2+ concentrations predicted to occur in the intestinal lumen. The nonsaturable component displays a Ca2+ permeability too high to be explained by simple diffusion. We discuss the role of the saturable component as the rate-limiting step in transmembrane Ca2+ movement, and suggest that the nonsaturable component reflects a transport mechanism operating well below its level of saturation.

Ca+2 transport across intestinal brush border membranes of the cichlid teleost Oreochromis mossambicus