PURIFICATION AND CHEMICAL CHARACTERIZATION OF A POTENT INHIBITOR OF THE NA-K-CL COTRANSPORT SYSTEM IN RAT URINE

A potent inhibitor of the Na-K-Cl cotransport system was purified from urines of salt-loaded rats. Mass spectroscopy revealed a molecular ma ss of 242 Da. Nuclear magnetic resonance showed a spectrum identical t o that of 4-dihydro-3-(4-hydroxyphenyl)-2H-1-benzopyran-7-ol (an ''est rogen-like'' isoflavonoid: equol). This compound inhibited cotransport fluxes at similar concentrations (IC50 = 16-24 mu M) as furosemide (I C50 approximate to 10 mu M). Cotransport inhibitory activity of urines from rats drinking tap water was fully explained by urinary equol con centrations (approximate to 27 mu M, measured by high-performance liqu id chromatography). Salt-loading increased urinary equol excretion, bu t not sufficiently high to fully explain the very important increase i n cotransport inhibitory potency. We conclude that: (i) under basal co nditions urinary equol can regulate Na-K-Cl cotransport activity in th e kidney and (ii) salt-loading should evoke the appearance of other co transport inhibitors. (C) 1996 Academic Press, Inc.