Acid precipitation rate of 125I-FGF19 and 131I-albumin at 5, 10, and 20 min after iv delivery with or without the presence of 56-fold excess unlabeled FGF19 (one mouse for each time point). (A) In the brain, the acid precipitability of 125I-FGF19 was decreased at 10 and 20 min in comparison with the 5 min sample. The presence of excess unlabeled FGF19 helped to maintain the high acid precipitability of 125I-FGF19 at 10 min but not at 20 min. The acid precipitability of 131I-albumin remained high throughout the study. (B) In serum, the decline of acid precipitability of 125I-FGF19 was mainly seen at 20 min in the group receiving excess unlabeled FGF19.