RADIATION-INACTIVATION ANALYSIS OF THE OLIGOMERIC STRUCTURE OF THE RENAL SODIUM D-GLUCOSE SYMPORTER/

The radiation-inactivation size (RIS) of the rat renal brush-border me mbrane sodium/B-glucose cotransporter was estimated from the loss of t ransport activity in irradiated membrane vesicles. The RIS depended on the electrochemical conditions present when measuring transport activ ity. A RIS of 294 +/- 40 kDa was obtained when transport was measured in the presence of a sodium electrochemical gradient. Under sodium equ ilibrium conditions, the RIS was 84 +/- 25 kDa in the presence of a gl ucose gradient, and 92 +/- 20 kDa in its absence. In the absence of a sodium gradient, but in the presence of an electrical potential gradie nt, the RIS increased to 225 +/- 49 kDa. The 294 kDa result supports e arlier suggestions that the Na+ gradient-dependent glucose transport a ctivity is mediated by a tetramer. Individual monomers appear, however ? to carry out glucose transport under equilibrium exchange conditions or when a glucose gradient serves as the only driving force. The elec trical potential gradient-driven glucose transport RIS appears to invo lve three functional subunits. (C) 1997 Elsevier Science B.V.