Lipid fractions from liver inhibit specific binding of [3H]ouabain to dog heart

Ethyl acetate extracts of bovine liver contain organic material which inhibits specific binding of ³H]ouabain in a radioreceptor binding assay. Filtration of Sephadex LH-60 resolved active material in a cholesterol-rich fraction which was further purified on a silica gel column and by TLC and found by GLC/MS to be composed mainly of long-chain fatty acids and their methyl esters. Certain authentic saturated and unsaturated fatty acids were active in the binding assay: lauric, myristic and myristoleic acids being most active. Since the amount of active saturated acids in the extracts is too small to account for the total observed activity, the presence of active unsaturated fatty acids is indicated. Furthermore, another active unidentified substance was associated with the neutral lipid fraction which also contained cholesterol and methyl esters of palmitic and stearic acids as major identifiable components, and gave a concentration-displacement curve parallel to that of ouabain. Authentic cholesterol and methyl esters of both saturated and unsaturated fatty acids were inactive in the binding assay. Thus, inhibition of specific binding of ³H]ouabain by lipid extracts of liver appears to be due, in part, to certain fatty acids and, in addition, to a potent unidentified substance associated with neutral lipids.