Endogenous immunoreactive digitalis-like substance in neonatal serum and placental extracts

Therapeutic levels of digoxin in the serum of untreated neonates delivered to mothers who had not received the drug prenatally were detected by radio-immunoassay. Digoxin levels in neonates should be interpreted with care because of the unknown contribution by the endogenous digitalis-like substance (DLS) to the level of the drug. Three commercially available radio-immunoassay kits were compared with regard to their sensitivity and reproducibility in detecting the endogenous DLS. The kit from Clinical Assays (Cambridge, Mass., USA) was selected for further investigations. In a series of 35 paired samples of maternal and cord blood the average DLS values in terms of digoxin were 0,52 +/- 0,07 and 0,81 +/- 0,27 ng/ml respectively. This difference is statistically highly significant. In the case of infants with DLS values of 1-1,5 ng/ml in terms of digoxin, approximately 1 week was required to reach nontherapeutic digoxin levels, i.e. below 0,5 ng/ml. Gel chromatography showed that the DLS in neonatal serum was more closely associated with protein than is authentic digoxin. In placental extracts it followed the elution profile of the protein completely, but it shifted to fractions with a lower molecular weight than haemoglobin after trypsinization. The level of DLS in neonatal serum was also increased by more than half its original value by trypsinization. Proteolysis therefore seems to have a releasing effect on DLS. The molecular size of this substance is probably in the same range as that of polypeptides, since it was not dialysable from trypsinized and untreated samples through a membrane with a 22 000 dalton molecular weight cut-off point.