Measurement of serum free thyroxine by RIA in various clinical states.

A radioimmunoassay for quantitatively measuring the serum concentration of free thyroxine is described. This method does not require equilibrium dialysis, and is rapid and reproducible. The serum values obtained by this radioimmunoassay and by equilibrium dialysis are similar in normal subjects, hyperthyroid and hypothyroid patients, pregnant women, "sick euthyroid" patients, and euthyroid patients with hereditary TBG abnormalities. The method also provides a total serum thyroxine concentration in the same assay procedure.

Direct solid-phase 125I radioimmunoassay of serum cortisol.

We report a solid-phase iodine- 125 radioimmunoassay for serum cortisol in which the interfering binding proteins are inactivated by a combination of reaction pH and 8-anilino-1-naphthalenesulfonic acid. The procedure is easy to perform and gives accurate and reliable results. The shift in pH for optimum binding of an antibody resulting from immobilization on a solid support, used to decrease cortisol binding competition in the present test, is potentially exploitable in other antibody and enzyme systems.

Isotope labeling of free and aminoacyl transfer RNA synthetase-bound transfer RNA.

The structural organization of the complex of Escherichia coli Ile-tRNA synthetase and tRNA Ile has been studied by isotope labeling of purine units in the tRNA. Free or bound tRNA is incubated in tritiated water for 5 to 15 h at 37 degrees in order to incorporate tritium into the C-8 positions of purine units. (Previous work has shown that the labeling rate of a purine is very sensitive to its microenvironment.) Under conditions where exchange-out does not occur, the nucleic acid is digested with nucleases and purines are subsequently isolated from known locations in the structure. Four purines are substantially perturbed by bound Ile-tRNA synthetase; in each case, the rate of labeling is retarded in the presence of the synthetase. The four purines occur at or near the 3' terminus and at the interface of the dihydrouridine stem and loop. These bases occur in segments of the tRNA that previous photochemical cross-linking studies have identified as important for synthetase-tRNA interactions. It appears that the effects observed on these sites are caused by their direct interaction with or shielding by the bound synthetase. In addition, two other sites, one in the anticodon and one in the amino acid acceptor-T psi C helix, appear to be perturbed (retarded labeling rates) by the bound enzyme. The data also suggest there is no significant conformational change in the tRNA upon binding to the synthetase.

Comparison of isotope labeling patterns of purines in three specific transfer RNAs.

Purine C-8 tritium-labeling rates have been measured at specific sites in Escherichia coli tRNAIle and tRNA2Tyr. The results are compared with those obtained for yeast tRNAPhe (preceding paper(Gamble et al., 1976)). The tRNAIle and tRNAPhe fall into the same general class of tRNA structures, while tRNA2Tyr is in a differint class; in particular, the latter is characterized by a large extra loop. In each of the three tRNAs the 3'-terminal A has the same labeling rate and, on a relative basis, appears to be the most rapidly labeled site. Bases in cloverleaf helical sections have markedly retarded labeling rates that collectively fall within an approximately threefold range of time constants. At some of the common purines, believed to be essential for the construction of a general system of tertiary interactions, exchange rates for yeast tRNAPhe are significantly different than those for the two Escherichia coli tRNAs. these differences may arise from variations among the tRNAs in the relative stabilities of specific tertiary interactions, or from other factors as well. In the case of tRNA2Tyr, labeling rates for bases in the large variable region are sufficiently retarded to suggest some structural organization for this part of the molecule. In addition, since exchange rates are similar for some of the bases common to Escherichia coli tRNAIle and tRNA2Tyr, it is likely that the large variable loop of tRNA2Tyr does not interact with or perturb these common sites. Finally, for all three tRNAs, structure formation (e.g., base pairing, base stacking) invariably decreases the labeling rate, even though the variety of base environments in the three-dimensional structures of these tRNAs might be expected to affect the acidity of C-8 and other chemical properties in diverse ways. Although these chemical effects no doubt bear influence, in these studies the dominant influence on exchange may be the effect of structure on the accessibility of solvent molecules, i.e. water.

Two photo-cross-linked complexes of isoleucine specific transfer ribonucleic acid with aminoacyl transfer ribonucleic acid synthetases.

Escherichia coli Ile-tRNA synthetase and tRNA-Ile have been cross-linked photochemically by the direct action of ultraviolet light. In addition, photo-induced joining of tRNA-Ile E. coli to Val-tRNA synthetase from yeast has also been achieved. This yeast enzyme is known to mischarge E. coli tRNA-Ile with valine. Regions on tRNA-Ile involved in cross-linking have been determined for both complexes. In each case, three distinct parts of the nucleic acid are found cross-linked. Two of these are the same in both complexes and involve the dihydrouridine stem and loop region. The third part is unique for each complex. It involves the 3' terminus in the cognate one and the 3' side of the anticodon in the non-cognate case. When the cross-linked regions are projected onto a model of the three-dimensional structure of tRNA, it is clear that these and other data are consistent with having each enzyme bound in a similar orientation across tRNA-Ile. The enzymes are viewed as spanning the distance from the anticodon to the 3' terminus and making extensive contact with the area in which the two helical branches of the L-shape tRNA structure come together.

Three photo-cross-linked complexes of yeast phenylalanine specific transfer ribonucleic acid with aminoacyl transfer ribonucleic acid synthetases.

Yeast tRNA-Phe has been cross-linked photochemically to three aminoacyl-tRNA synthetases, yeast phenylalanyl-tRNA synthetase, Escherichia coli isoleucyl-tRNA synthetase, and E. coli valyl-tRNA synthetase. The two non-cognate enzymes are known to interact with tRNA-Phe. In each complex, three regions on the tRNA are found to cross-link. Two of these are common to all of the complexes, while the third is unique to each. Thus, the cognate and non-cognate complexes bear considerable similarity to each other in the way in which the respective enzyme orients on tRNA-Phe, a result which was also established for the complexes of E. coli tRNA-Ile (BUDZIK, G.P., LAM, S.M., SCHOEMAKER, H.J.P., and SCHIMMEL, P.R. (1975) J. Biol. Chem. 250, 4433-4439). The common regions include a piece extending from the 5'-side of the acceptor stem to the beginning of the dihydrouridine helix, and a segment running from the 3' side of the extra loop into the TpsiC helix. These two regions overlap with and include some of the homologous bases found in eight tRNAs aminoacylated by yeast phenylalanyl-tRNA synthetase (ROE, B., SIROVER, M., and DUDOCK, B. (1973) Biochemistry 12, 4146-4153). Although well separated in the primary and secondary structure, these two segments are in close proximity in the crystallographic tertiary structure. In two of the complexes, the third cross-linked fragment is near to the two common ones. The picture which emerges is that the enzymes all interact with the general area in which the two helical branches of the L-shaped tertiary structure fuse together, with additional interactions on other parts of the tRNAas well.