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 Ca+2 in brush border membrane vesicles was analyzed. A saturable and a nonsaturable component in the uptake of Ca+2 was resolved. The saturable component is characterized by a K m much lower than the Ca+2 concentrations predicted to occur in the intestinal lumen. The nonsaturable component displays a Ca+2 permeability too high to be explained by simple diffusion. We discuss the role of the saturable component as the rate-limiting step in transmembrane Ca+2 movement, and suggest that the nonsaturable component reflects a transport mechanism operating well below its level of saturation.
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