Identification of two unique polypeptides from dog kidney outer cortex and outer medulla that exhibit different Na+/D-glucose cotransport functional properties.
The cloned Na+/D-glucose cotransporter SGLT1 and an additional recently isolated human kidney cDNA Hu14/K15 belong to a family of similar cotransport proteins including the Na(+)-dependent nucleoside and Na(+)-dependent myo-inositol carrier SMIT1. For the present study we used two different polyclonal antibodies raised against the amino acid sequence 402-420 (Ab-E) and 565-574 (Ab-P) of SGLT1 to probe brush-border membrane fractions from different regions (outer cortex-->outer medulla) of dog kidney. In Western blots both Ab-E and Ab-P react specifically (peptide blockable) with two distinct bands migrating on SDS-PAGE under reducing conditions at 75.5 kDa and 72.5 kDa. The higher molecular mass polypeptide is greatly enriched (13:1) in outer cortex and diminishes progressively towards outer medulla, whereas the lower molecular mass band is barely detectable in outer cortex but is enriched in outer medulla (4:1). Brush-border membrane vesicles (BBMV) prepared from the same outer cortical and outer medullary regions that were probed with Ab-E and Ab-P exhibit strikingly different Na+/D-glucose functional transport behavior. The Na+/D-glucose cotransport activity in outer cortical BBMV is a low-affinity system with Km = 5.98 +/- 1.01 mM, Vmax = 13.05 +/- 0.55 nmol/mg protein per min, and with 1:1 Na+:D-glucose stoichiometry. Outer medulla BBMV exhibit high-affinity Km = 0.27 +/- 0.03 mM Vmax = 0.97 +/- 0.04 nmol/mg protein per min and 2:1 Na+:D-glucose stoichiometry. Comparison of SGLT1, Hu14/K15, SNST1 and SMIT indicates that Ab-E could cross react with all four, but Ab-P would recognize SGLT1, Hu14/K15, SNST1 but not SMIT. Also SNST1 is not expressed in outer cortex. Based on currently available sequence data, and its marked enrichment in outer cortex, the 75.5 kDa band is a likely candidate protein responsible for low-affinity and 1:1 Na+:D-glucose stoichiometric Na+/D-glucose cotransport activity (Hu14/K15) while the minor 72.5 kDa band in outer cortex is probably SGLT1. In outer medulla, the predominant band recognized by both Ab-E and Ab-P is the 72.5 kDa protein and this could be either SGLT1 or SNST1.
Silverman
M
M
Department of Medicine, University of Toronto, Ontario, Canada.
Speight
P
P
Ho
L
L
D000906:Antibodies / Q000276:immunology; D002352:Carrier Proteins / Q000276:immunology / Q000378:metabolism; D003429:Cross Reactions; D005947:Glucose / Q000378:metabolism*; D006360:Heat-Shock Proteins / Q000276:immunology / Q000378:metabolism; D007672:Kidney Cortex / Q000737:chemistry* / Q000378:metabolism; D007679:Kidney Medulla / Q000737:chemistry* / Q000378:metabolism; D007700:Kinetics; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins / Q000378:metabolism*; D010455:Peptides / Q000302:isolation & purification*; D012964:Sodium / Q000378:metabolism*; D027981:Symporters
D000906:Antibodies; D002352:Carrier Proteins; D006360:Heat-Shock Proteins; D008565:Membrane Proteins; D009004:Monosaccharide Transport Proteins; D010455:Peptides; D027981:Symporters; C073034:sodium-nucleoside cotransporter; C073884:SLC5A3 protein, human; D012964:Sodium; D005947:Glucose