pH-dependent column fractionation for characterization of endogenous digoxin-like immunoreactive factors in pregnant urine.

Samples of pregnant urine were treated with three diatomaceous earth columns to fractionate organic solvent-soluble components. The first column was pre-injected with the urine sample. The second and the third columns were pre-injected with sodium bicarbonate and sodium hydroxide solutions, respectively; these played an important role in capturing acidic components in the urine extract from the first column. Neutral components could be eluted out from the columns with the organic solvent, while the basic components were retained in the first column. Once strongly and weakly acidic components were captured on the alkaline columns, they were separately treated with an acidic mobile phase and recovered. The digoxin-like activities of each fraction were examined for cross-reactivity to antidigoxin antibody and inhibitory effects on dog kidney Na+, K(+)-ATPase. Both types of reactivity were found in all three fractions and decreased in the following order: strongly acidic, neutral and weakly acidic. The neutral fraction of pregnant urine showed significantly greater cross-reactivity than that of the non-pregnant urine. Following hydrolysis of the pregnant urine with beta-D-glucuronidase, the cross-reactivity of the neutral fraction increased significantly. High performance liquid chromatographic analysis showed that several components in the neutral fraction had apparently increased after the hydrolysis, showing the possible presence of unknown digoxin-like components in the pregnant urine.