Distribution and metabolism of topically applied ethanolamine

The distribution and metabolism of topical [14C]ethanolamine was studied in vivo, using athymic nude mice, human skin grafted onto athymic nude mice, and in vitro, using excised pig skin. Ethanolamine was the only radioactive phospholipid base detected in the human skin grafts, in the mouse skin, and in the pig skin. Ethanolamine that penetrated human skin grafts or mouse skin was extensively metabolized in the animal. The liver is a major site for metabolism of ethanolamine, containing over 24% of the applied radioactive dose. The kidneys, lungs, brain, and the heart contained 2.53, 0.55, 0.27, and 0.15% of the dose, respectively. The hepatic phospholipid bases, ethanolamine, choline, and serine, were all highly radioactive. Hepatic, human skin graft, and mouse skin proteins were also highly radioactive. Over 18% of the topical radioactive dose was oxidized to 14CO2 and 4.6% was excreted in the urine over 24 hr. Urea, glycine, serine, choline, and uric acid were the urinary metabolites of ethanolamine. In vitro data showed that a relatively small percentage of the applied dose was lost from the skin by evaporation and percutaneous penetration. The bulk of the dose remained in the epidermis. Radiometric and enzymatic assays suggest that ethanolamine enhances the hydrolysis of topically applied diisopropylfluorophosphate.