Cleavage of disulfide bonds leads to inactivation and degradation of the type IIa, but not type IIb sodium phosphate cotransporter expressed in Xenopus laevis oocytes

Tris(2-carboxyethyl)phosphine (TCEP) reduces (cleaves) disulfide bonds of t he renal proximal tubule type IIa Na/Pi- cotransporter (rat NaPi IIa) and t hereby inhibits its function, We tested the effect of TCEP on the murine ty pe IIa Na/P-i-cotransporter and the corresponding IIb intestinal isoform bo th expressed in Xenopus laevis oocytes. After incubation with TCEP the func tion of NaPi IIa was inhibited and protein amount was decreased, Injection of the lysosomal inhibitor leupeptin prevented degradation of the protein. Exposure of oocytes to TCEP at 0 degrees C led to a reduction in transport function without concomitant loss in Na/Pi IIa protein. In contrast to NaPi type IIa, the type IIb isoform was neither inhibited, nor degraded after i ncubation with TCEP, These results suggest that cleavage of disulfide bonds led to changes within the confirmation of the type IIa transporter that re sult in (i) inhibition of the transport activity and (ii) internalization a nd subsequent lysosomal degradation of transporter protein. Sequence compar isons suggest the involvement/presence of different disulfide bonds in type IIa and type IIb Na/P-i-cotransporters.