Proto-14C was administered through a duodenal tube in a normal subject. Serial isolations of crystalline 2-COOH porphyrin and stercobilin from the feces during the next 12 days, and radioactivity measurements provided evidence that some fraction of the administered Protounderwent an enterohepatic circulation. A small proportion (1.5-3%) was converted to bile pigment. The data point to a continuous partial enterohepatic circulation of Proto-with loss of a constant fraction on each circulation.
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Ibrahim et al. (1968) studied this question.