A B S T R A C T Studies of the possibility that thyroxine (T4) is converted to 3,5,3'-triiodo-L-thyronine (T3) in the extrathyroidal tissues in man have been conducted in 13 patients, all but two of whom were athyreotic or hypothyroid, and all of whom were receiving at least physiological replacement doses of synthetic sodium-Lthyroxine.T3 was found in the sera of all patients, in concentrations ranging between 243 and 680 ng/100 ml (normal range 170-270 ng/100 ml). These concentrations were far in excess of those which would have been expected on the basis of the T3 contamination of the administered T4, as measured by the same technique employed in the analysis of serum. When oral medication was enriched with 'I-labeled T4 for 8 or more days, labeled T3 and tetraiodothyroacetic acid (Tetrac or TA4) were found in the serum to the extent of approximately 2-5% of total radioactivity, as assessed by unidimensional paper chromatography. The same results were obtained with a specially purified lot of radioactive T4 containing less than 0.1% T3 as a contaminant. The identities of the 'I-labeled T3 and TA4 were verified by two-dimensional chromatography as well as by specific patterns of binding in serum. The labeled T3 isolated was bound by albumin and by T4-binding globulin (TBG), but not by T4-binding prealbumin (TBPA); in contrast the labeled TA4 was bound by albumin and TBPA, but not by TBG.To exclude the possibility that the conversion of T4 to T3 was a peculiarity of the oral route of administra- There is no doubt whatsoever that at least a portion of the T3 in the blood results from direct secretion by the thyroid gland, T3 having been found in the thyroid venous effluent, and in concentrations substantially higher than in the concurrently sampled arterial blood (4, 5). What has remained uncertain, however, is what fraction, if any, of the T3 in the blood arises from the

Relatively few reports have appeared on the chromium content of biological material. Previous studies were devoted primarily to the analysis of plants and soils ( 1 ).In preparation for the present study, arc spectrography 3 was used to demonstrate the presence of trace amounts of chromium in human tissues, including the blood. Tissue analyses were performed on samples of normal human blood by means of a colorimetric method (2) with the finding of mean values of 20 y % for packed red cells and 14 y % for plasma.The present report is concerned with a new biological tracer, radioactive chromium (Cr51), which is bound by the red cells and plasma proteins. Both anionic hexavalent (Na2Cr51O4) and cationic trivalent (Cr51Cl3) states of the element have been studied.Cr51 is a soft X-ray emitter with a half-life of 26.5 days. It disintegrates by K capture with transmutation to vanadium and emission of Xrays of 4.92 Kev plus a few per cent of 0.237 Mev gamma rays (3).To prepare Cr51 from Cr50 in the pile with sufficiently high specific activity,'4 it was necessary to enrich ordinary chromic oxide (Cr2,O) electromagnetically. Naturally occurring chromium contains 4.49 %o Cr50; the enriched chromium, 41.2%o. This enriched mixture was then irradiated in the Oak Ridge pile for two months, at the end of which

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The biological importance of serum albumin makes information on its metabolism of value. In the present work the turnover rate of serum albumin in man was studied by following the disappearance from the circulation of albumin labelled with radioiodine.Previous clinical investigations (1-3) have dealt with the metabolic fate of massive intravenous infusions of concentrated albumin, employed to increase the patient's albumin stores. In contrast, the subjects in the present study received only 1 to 10 mg. of labelled protein. This minute amount of albumin could not have any significant effect upon the serum albumin concentration or upon protein metabolism, thus permitting studies on the subject in a steady state.Intravenous injection of I'll-tagged albumin was followed by a relatively rapid initial fall in the radioactivity of the circulating plasma. After approximately two days the plasma radioactivity diminished more gradually. The rate of this gradual decline was considered to represent the rate of replacement of tagged by untagged protein, or the turnover of serum albumin. METHODSIodination of albumin. The albumin molecule was labelled with an average of 2-3 iodine atoms per molecule. This ratio was selected to minimize alterations from the native state of the protein. The iodine was bound by tyrosine groups forming monoiodo-tyrosine under the conditions described by Hughes and Straessle (4) for iodination of albumin with small amounts of iodine.

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