The characteristics of phosphate transport across the human intestinal basolateral membrane were determined using an enriched preparation in which uphill Na+-dependent d-glucose transport could not be demonstrated but adenosine triphosphate-dependent calcium transport was present. The human basolateral membrane vesicles were oriented as follows: 64% inside-out vesicles and 36% rightside-out vesicles or sheets, or both. Phosphate transport showed a 3.4-fold transient "overshoot" phenomena in the presence of an inwardly directed sodium gradient. Computerized Michaelis-Menten kinetics for the net active transport component indicated a Vmax of 65 ± 5 pmol/mg protein per 8 s and a Km of 93 ± 15 μM. The transport process was electroneutral. Gramicidin D and transstimulation studies confirmed the presence of a Na+-phosphate carrier at the basolateral membrane. Adenosine triphosphate enhanced phosphate uptake, indicating an adenosine triphosphate-dependent phosphate transport process. These studies are the first to determine the presence of a phosphate carrier in human intestinal basolateral membrane.
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