Effect of phosphonoformic acid, dietary phosphate and the Hyp mutation on kinetically distinct phosphate transport...

We examined the kinetics of phosphate transport in mouse renal brush-border membrane vesicles under initial rate (6 s), trans zero, voltage clamp conditions. Two kinetically distinct Na+-dependent phosphate transport processes were identified: a high-affinity, low-capacity system (Km, 0.09 +/- 0.02 mM; Vmax, 539 +/- 50 pmol/mg protein per 6 s) and a low-affinity, high-capacity system (Km, 1.28 +/- 0.35 mM; Vmax, 1677 +/- 198 pmol/mg protein per 6 s). The high-affinity system was inhibited competitively by 1 mM phosphonoformic acid (PFA) (apparent Ki, 0.31 +/- 0.03 mM) and completely abolished by 20 mM PFA; the low-affinity system was insensitive to 1 mM PFA and was inhibited competitively by 20 mM PFA (apparent Ki, 9.03 +/- 1.21 mM). Dietary phosphate deprivation elicited a significant increase in Vmax of both high- and low-affinity phosphate transport systems whereas the X-linked Hyp mutation caused a 50% decrease in Vmax of the high-affinity system with no change in the low-affinity system. Phosphate deprivation of Hyp mice elicited a 3.5-fold increase in Vmax of the high-affinity system. Neither diet nor mutation significantly altered the apparent Km values of either phosphate transport process. We conclude that (1) mouse kidney brush-border membranes have two distinct Na+-dependent phosphate transport systems which differ in affinity and capacity; (2) both processes participate in the adaptive response to dietary phosphate restriction; (3) only the high-affinity system is impaired by the X-linked Hyp mutation.