Avidin binding of biotinylated corticotropins

Accurate measurement of avidin and streptavidin in crude biofluids with a new, optimized biotin-fluorescein conjugate

A new biotin-fluorescein conjugate with an ethylene diamine spacer was found to be the first fluorescent biotin derivative which truly mimicked d-biotin in terms of high affinity, fast association, and non-cooperative binding to avidin and streptavidin tetramers. These exceptional properties were attributed to the small size/length of the new ligand since all larger/longer biotin derivatives are known for their mutual steric hindrance and anti-cooperative binding in 4:1 complexes with avidin and streptavidin tetramers. Specific binding of the new biotin-fluorescein conjugate towards avidin and streptavidin was accompanied by 84-88% quenching of ligand fluorescence. In the accompanying study this effect was used for rapid estimation of avidin and streptavidin in a new 'single tube assay'. In the present study the strong quenching effect was utilized to accurately monitor stoichiometric titration of biotin-binding sites in samples with >/=200 pM avidin or streptavidin. The concentration was calculated from the consumption of fluorescent ligand up to the distinct breakpoint in the fluorescence titration profile which was marked by the abrupt appearance of strongly fluorescent ligands which were in excess. Due to this protocol the assay was not perturbed by background fluorescence or coloration in the unknown samples. The new fluorescence titration assay is particularly suited for quick checks on short notice because getting started only means to thaw an aliquot of a standardized stock solution of fluorescent ligand. No calibration is required for the individual assay and the ligand stock solution needs to be restandardized once per week (or once per year) when stored at -25 degrees C (or at -70 degrees C, respectively).

Radioactive probes for adrenocorticotropic hormone receptors

Our attempts to develop adrenocorticotropic hormone (ACTH) analogues that can be employed for ACTH receptor identification and isolation began with the synthesis of ACTH fragments containing N epsilon-(dethiobiotinyl)lysine (dethiobiocytin) amide in position 25 to be used for affinity chromatographic purification of hormone-receptor complexes on Sepharose-immobilized avidin resins. Because labeling ACTH or ACTH fragments by conventional iodination techniques destroys biological activity due to oxidation of Met4 and incorporation of iodine into Tyr2, we have prepared [Phe2,Nle4]ACTH1-24, [Phe2,Nle4,biocytin25]ACTH1-25 amide, and [Phe2,Nle4,dethiobiocytin25]ACTH1-25 amide by conventional synthetic techniques. The HPLC profiles and amino acid analyses of the final products indicate that the materials are of a high degree of purity. The amount of tertiary butylation of the Trp residue in the peptides was assessed by NMR and was found to be less than 0.5%. All three peptides are equipotent with the standard ACTH1-24 as concerns their ability to stimulate steroidogenesis and cAMP formation in bovine adrenal cortical cells. Iodination of [Phe2,Nle4]ACTH1-24, with iodogen as the oxidizing agent, has been accomplished without any detectable loss of biological activity. The mono- and diiodo derivatives of [Phe2,Nle4]ACTH1-24 have been prepared, separated by HPLC, and assayed for biological activity. Both peptides have the full capacity to stimulate steroidogenesis and cAMP production in bovine adrenal cortical cells.

Synthetic tools for adrenocorticotropin receptor identification

Biotinylated photoaffinity derivatives of adrenocorticotropin (ACTH) are potentially useful tools for the identification of ACTH receptors. The hormone can be attached covalently to its receptor by photoactivation, and the presence of biotin in the molecule facilitates isolation of the solubilized hormone-receptor complex on columns of immobilized succinoylavidin (Suc-avidin). Six photoprobes of ACTH1-24 have been prepared by reacting ACTH1-24, [25-biocytin]ACTH1-25 amide, and [25-dethiobiocytin]ACTH1-25 amide with either 4- or 5-azido-2-nitrophenylsulfenyl (4-NAPS and 5-NAPS, respectively) chlorides in acetic acid. The homogeneity of the photoprobes was carefully monitored by thin-layer chromatography and amino acid analyses of acid hydrolysates. The presence of underivatized starting material in the photoprobes was critically scrutinized by high-pressure liquid chromatography and was estimated to be less than 0.5%. Both the 4- and 5-NAPS derivatives stimulated maximal steroidogenesis (as compared with ACTH1-24) in calf adrenal cortical cells. However, the potencies of the two isomers differed significantly. The ED50 for steroidogenesis with 5-NAPS-ACTH1-24 was 100-fold greater than the standard (ACTH1-24) while that for 4-NAPS-ACTH1-24 was only approximately 7 times greater. Although 4-NAPS-ACTH1-24 was capable of stimulating maximal adenosine cyclic 3',5'-phosphate (cAMP) production, the 5-NAPS derivative was usually not. The level of stimulation with the 5-NAPS derivative varied considerably from cell preparation to cell preparation. ACTH1-24-induced cAMP production was inhibited by 5-NAPS-ACTH1-24 or 5-NAPS-[25-dethiobiocytin]ACTH1-25 amide.(ABSTRACT TRUNCATED AT 250 WORDS)