The combination of encephalopathy, high serum antithyroid antibody concentrations, and responsiveness to glucocorticoid therapy seems unlikely to be due to chance. However, there is no evidence of a pathogenic role for the antibodies, which are probably markers of some other autoimmune disorder affecting the brain.

Abstract.-A sensitive and specific radioimmunoassay for adenosine 3',5'-cyclic phosphate (cyclic AMP) has been developed which allows measurement of the nucleotide in extracts of 5-10 mg of tissue. The radioimmunoassay is sufficiently specific for cyclic AMP to eliminate the need for prior chromatographic separation of the cyclic nucleotide from other tissue nucleotides. The radioimmunoassay system is based upon competition of cyclic AMP with a labeled cyclic AMP derivative of high specific activity for binding sites on an antibody specific for the cyclic nucleotide. Antibody to cyclic Ai\IP was obtained by immunizing rabbits with an antigen prepared by conjugating succinyl cyclic AMP with human serum albumin. A high specific activity derivative of cyclic AAMP was prepared by synthesizing succinyl cyclic AMP tyrosine methyl ester (SCAMP-TME) and iodinating the phenolic hydroxyl group of the tyrosine moiety with "25I. Free and antibody-bound 125I-SCAMP-TME were separated by precipitation of the antibody-bound fraction with a second antibody (goat anti-rabbit gamma globulin). Displacement of 1251-SCAMP-T]NIE by unlabeled cyclic AMP when plotted as a semilogarithmic function was linear over a concentration range of 2-100 picomoles. The specificity of the antibody was tested against structurally related nucleotides, nucleosides, and purine bases. All had less than 0.005 per cent of the potency of cyclic AMP in inhibiting 125IJ SCAMIP-TME binding. The marked differences in affinity of the various cyclic nucleotides to cyclic AMP antibody would suggest that antibodies can be developed for each of the cyclic nucleotides by the principles used in this work. Adenosine 3',5'-cyclic phosphate (cyclic AMP) mediates a remarkable array of physiological phenomena' 2 and has been identified as a "secondary messenger" in the concept of hormone action proposed by Sutherland and his associates.' The methods available for the measurement of this nucleotide,3 -7 which is present in mammalian tissues in very low concentrations (0.2-1.5 m,4moles/ gm), involve a variety of complex enzymatic procedures36 and require the initial separation of cyclic AMP from other nucleotides by chromatographic techniques. We have developed a radioimmunoassay sensitive to 1-2 picomoles of cyclic AMP which permits rapid measurement of large numbers of samples using as little as 10-20 mg of tissue. The radioimmunoassay is highly specific for cyclic AMP, and therefore eliminates the need for prior chromatographic separation from other tissue nucleotides. This immunoassay is similar to the radioimmunoassay techniques developed for peptide hormones8 I and involves the competition of labeled and unlabeled cyclic AMP for antibody. In the assay to be described, free and antibody-bound cyclic AMP are then separated by precipitation of the latter with a second antibody.

A B S T R A C T To investigate mechanisms of extrathyroidal thyroid hormone metabolism, conversion of thyroxine (T4) to 3,5,3'-triiodothyronine (T3) and degradation of 3,3',5'-triiodothyronine (rT3) were studied in rat liver homogenates. Both reactions were enzymatic. For conversion of T4 to T3, the Km of T4 was 7.7 ,uM, and the Fasting resulted in inhibition of T4 to T3 conversion and of rT3 degradation by rat liver homogenates which was reversible after refeeding. Serum T4, T3, and thyrotropin concentrations fell during fasting, with no decrease in serum protein binding as assessed by a T3-charcoal uptake. There was no consistent change in serum rT3 concentrations. Dexamethasone had no effect in vitro. In vivo dexamethasone administration resulted in elevated serum rT3 concentrations after 1 day, and after 5 days, in inhibition of T4 to T3 conversion and rT3 degradation without altering serum T4, T3, or thyrotropin concentrations. Endotoxin treatment had no effect of iodothyronine metabolism in liver homogenates. In kidney homogenates the reaction rates and response to propylthiouracil in vitro were similar to

No abstract

To determine the implication of decreased T3 production during fasting, seven normal men were fasted for 80 hours on two occasions; they received 5 microgram of T3 every three hours durnig the second fast. The mean serum T3 concentration declined during the control fast from 120 to 73 ng per deciliter (P less than 0.01), but remained slightly above base-line values during the T3 fast. Mean serum T4 concentrations did not change, and mean serum rT3 concentrations increased, during both fasts. The peak serum TSH increment after TRH was 11.1 micromicron per milliliter before fasting, 8.9 (not significant) after the control fast and 2.2 (P less than 0.01) after the T3 fast. Urea excretion was 9.1 per cent higher during the T3 fast; there were no differences in the changes in blood glucose, plasma fatty acids or other substrates during the two fasts. Pretreatment with potassium iodide lowered serum T4 concentrations and increased the serum TSH response to TRH after fasting. We conclude that the decrease in serum T3 concentrations during fasting spares muscle protein. Fasting is accompanied by a lower set point of TSH secretion, which remains sensitive to changes in serum thyroid hormone concentrations.