Selective removal of alkaline phosphatase from renal brush-border membrane and sodium-dependent brush-border membrane transport

Na⁺-gradient-dependent transport of phosphate (P(i), glucose, and proline was studied in renal brush-border membranes (BBM) from which alkaline phosphatase was released by treatment with phosphatidylinositol-specific phospholipase C. BBM were prepared from rabbit kidney cortex in the form of large brush-border membrane sheets (BBMS). Incubation of BBMS with bacterial phosphatidylinositol-specific phospholipase C resulted in selective release (up to 90%) of the alkaline phosphatase from BBM; in contrast, activities of leucine aminopeptidase, γ-glutamyltranspeptidase, and maltase were not affected. Polytron homogenization of BBMS leads to the formation of brush-border membrane vesicles (BBMV) capable of concentrative uptake of solutes. BBMS and BBMV were prepared from kidneys of rabbits fed either a high (1.2% P), low (0.07% P), or normal P diet. Enzymatic deletion of alkaline phosphatase from BBMV prepared from animals fed the low P diet resulted in a marked increase in Na⁺-gradient-dependent uptake of P(i). No such effect was observed in BBMV from animals fed the normal or high P diets. These experiments indicate that the presence of alkaline phosphatase in BBM is not required for Na⁺-gradient-dependent transport of P(i), glucose, and proline. Likewise, the adaptive increase in BBM transport of P(i) elicited in response to low dietary P intake does not depend on the presence of, or increase in, alkaline phosphatase activity. Our findings argue against a direct involvement of alkaline phosphatase in Na⁺-dependent P(i) transport across the renal BBM. It is not excluded, however, that alkaline phosphatase might play a role in the modulation of P(i) transport.