Alpha-halo [(phenylphosphinyl)methyl]phosphonates as specific inhibitors of Na(+)-gradient-dependent Na(+)-phosphate cotransport across renal brush border membrane.

ABSTRACT

Certain phosphonocarboxylate analogues of phosphate are known to inhibit Na(+)-phosphate (Pi) cotransport in renal brush border membrane (BBM), but previously tested potential inhibitors incorporating structurally versatile aryl functionality were inactive. In this work, a series of novel alpha-halogenated [(phenylphosphinyl)methyl]phosphonates [PhpXYMP X, Y = H, F (2); F, F (3); H, Cl (6); Cl, Cl (4); H, Br (7); Br, Br (5); and Cl, Br (8)] were prepared via synthesis of the corresponding triethyl esters, acid hydrolysis, and isolation as pyridine salts. The compounds were evaluated as inhibitors of Na(+)-gradient-dependent 32Pi uptake by rat renal cortex BBM vesicles (BBMV) in vitro. The PhpFMP racemate 2 had higher activity (-49% delta inhibition) than other members of the series (-22 to -39% delta inhibition). pKa values of 1.5-2.0, 2.7, and 7.1 were estimated for 2 using a 31P delta vs pH plot, indicating that in the activity assays it exists as both dianion and trianion, with the latter form predominant. PhpFMP had no significant inhibitory effect on Na(+)-gradient-dependent uptake of D-glucose or L-proline in the same BBMV, and did not inhibit BBM alkaline phosphatase. Kinetic analysis showed that PhpFMP acts as a strictly competitive inhibitor of Na(+)-Pi cotransport with Ki = 0.358 +/- 0.021 mM (n = 3). The racemate 2 was resolved as its (-)-quinine salt into enantiopure (+)-2 [Na+ salt, [alpha]25D = +6 degrees (aqueous MeOH)] and a Na+ salt of 2 enriched in (-)-2. The two compounds did not differ significantly as inhibitors of Na(+)-gradient dependent 32Pi uptake by rat renal cortex BBM vesicles (BBMV) in vitro. The results are discussed in terms of structural requirements for inhibition.