Renal brush border membrane bound intrinsic factor

Abstract

A highly active receptor for intrinsic factor (IF)-cobalamin (Cbl) complex has been detected and reported in mammalian kidney earlier (Seetharam, B., et al. (1988) J. Biol. Chem. 263, 4443–4449). The physiological role of this receptor in normal Cbl homeostasis is not known. In addition to binding of exogenously added IF-[ 57Co]Cbl, the renal apical membranes contain endogenous IF or IF-Cbl. Washing with pH 5 / EDTA buffer enhanced the binding of exogenously added IF-[ 57Co]Cbl to renal apical but not basolateral membranes. The pH 5 / EDTA extract from renal apical membranes bound [ 57Co]Cbl. The complex also bound to rat ileal brush border membrane and promoted ileal transport of [ 57Co]Cbl. On immunoblots using monospecific antiserum to IF a 62 kDa protein was identified in renal and intestinal apical membranes, serum and in tissue extracts of unperfused rat liver, kidney and heart. The 62 kDa band was eliminated from the renal apical membranes following pH 5 / EDTA wash. Rat urine demonstrated unsaturated [ 57Co]Cbl binding (0.2 to 0.4 pmol/day) of which only 30–40% was immunoprecipitated with anti IF and could be identified on immunoblots. The identification of IF in rat renal apical membranes (160–200 ng/mg protein) and secretion of only traces of IF in urine suggest that the renal IF-Cbl receptor may play a role in sequestering IF/IF-Cbl and prevent urinary loss of Cbl.