Staining for Iodine in Chromatograms of Human Plasma: an Artefact due to Thiourea or Thiouracils

UNTIL the report by Werner and Block 1 , it was generally thought that at most only very small quantities of iodinated tyrosines are present in normal human serum. This conclusion was based on 131 I-chromatographic studies using plasma extracts from subjects given radioiodine. Werner and Block report, however, that when the iodine distribution in normal human serum is analysed chemically by the ceric–arsenite reaction 2 , approximately half the circulating iodine is present in the form of monoiodotyrosine or di-iodotyrosine, although the radioactivity in the same chromatogram is distributed according to the results of other workers. Hitherto these results have been very difficult to understand, and they are inconsistent with the rapid deiodination seen when iodotyrosines are injected 3 . We have recently confirmed Werner and Block's results indicating a discrepancy between the distribution of stable and radioactive iodine, when using very similar methods to estimate the distribution of iodinated amino-acids in human serum. Our extraction and chromatographic techniques differed from those of Mandl and Block 4 only in that the acidified serum was extracted three times with butanol and that the chromatogram was unidimensional and run in acid-butanol/2 N acetic acid (1 : 1). We find that when the chromatogram of serum collected 48 hr. after a 100 µc. dose of iodine-131 is scanned for radioactivity little or no counts are recordable in the mono- or di-iodotyrosine positions, whereas when the same Chromatograms are evaluated chemically for iodine by the ceric–arsenite reaction, iodotyrosines appear to be present in considerable quantity.