Photoaffinity labeling of corticotropin receptors

Adrenocorticotropin receptors: functional expression from rat adrenal mRNA in Xenopus laevis oocytes

The adrenocorticotropin (ACTH) receptor, which binds corticotropin and stimulates adenylate cyclase and steroidogenesis in adrenocortical cells, was expressed in Xenopus laevis oocytes microinjected with rat adrenal poly(A)+ RNA. Expression of the ACTH receptor in individual stage 5 and 6 oocytes was monitored by radioimmunoassay of ligand-stimulated cAMP production. Injection of 5-40 ng of adrenal mRNA caused dose-dependent increases in ACTH-responsive cAMP production. These were detected at 48 h and reached a maximum 72 h after microinjection of 20-40 ng of adrenal mRNA. In response to 1 microM ACTH, total cAMP production increased within 2.5 min and reached half-maximal and maximal levels (5-fold greater than basal) at 10 and 75 min, respectively, and then remained elevated for up to 5 h. Extracellular cAMP levels were much lower but showed prominent linear increases from almost undetectable levels, with 70- and 150-fold increases evident at 1 and 2 h, respectively. The half-maximal concentration (ED50) for stimulation of cAMP formation was 5 x 10(-8) M ACTH-(1-24); the ED50 for ACTH-(1-17) was 5 x 10(-7) M, and no response was observed with ACTH-(1-10). Size fractionation of rat adrenal poly(A)+ RNA by sucrose density-gradient centrifugation revealed that mRNA encoding the ACTH receptor was present in the 1.1- to 2.0-kilobase fraction. These data indicate that ACTH receptors can be expressed from adrenal mRNA in Xenopus oocytes and are fully functional in terms of ligand specificity and signal generation. The extracellular cAMP response to ACTH is a sensitive and convenient index of receptor expression. This system should permit more complete characterization and expression cloning of the ACTH receptor.

Characterization and regulation of angiotensin and corticotropin receptors on cultured bovine adrenal cells

Cultured bovine adrenal fasciculata cells were used to characterize angiotensin II (A-II) and corticotropin (ACTH) receptors and to study their homologous and heterologous regulation. These cells contain one type of high affinity binding sites for A-II (KD congruent to 2.4 +/- 0.3 10(-9) M) and about 100000 sites/cell. Photoaffinity labeling followed by SDS-PAGE under reducing conditions revealed a single macromolecule of apparent MR 65,000. Treatment of cells with increasing concentrations of A-II produced down-regulation of its own receptors and marked homologous and heterologous (ACTH) steroidogenic desensitization. However, the desensitization was not correlated with receptor loss and was mainly due to alterations of the steroidogenic pathway. Pretreatment of cells with ACTH also reduced A-II receptors, but this was not associated with steroidogenic desensitization. Bovine fasciculata cells contain two binding sites for ACTH: one of high affinity (KD congruent to 2.6 +/- 0.4 10(-10) M) and low capacity (2030 +/- 390 sites/cell) and the other of low affinity and high capacity. Affinity cross-linking of ACTH to plasma membranes prepared from adrenal cells revealed a labeled macromolecule of apparent MR 43000. However, cross-linking experiments to intact cells revealed, both under reducing and non-reducing conditions, two labeled macromolecules of apparent MR of 123000 and 43000. Pretreatment of cells with ACTH enhanced its receptor and the cAMP and cortisol responses to further ACTH stimulation. These effects were time- and dose-dependent. The maximal effects were observed at 10(-10) to 10(-9) M. A-II alone had no effect but it blocked partially the stimulatory action of ACTH.

Characterization of the porcine ACTH receptor with the aid of a monoclonal antibody

Monoclonal antibodies were raised against the ACTH receptor by immunization of BALB1 c mice with porcine adrenal cortex cell membranes. Competition and binding experiments confirmed that one of these antibodies, IV/14/9, reacted with the ACTH receptor in competition with ACTH and caused a definite ACTH-like effect. This antibody was used to characterize the hormone receptor of the porcine adrenal cortex. The number of antibody binding sites per cell was calculated from a Scatchard plot to be 124,100 +/- 10,000. The curve was linear indicating the existence of a single class of receptors. The finding that a high concentration of IV/14/9 totally suppressed maximally ACTH-induced steroidogenesis confirms the view that only a single class of ACTH receptors exists. The antibody IV/14/9 neither reacted with intact porcine thymus cells nor with normal porcine lymphocytes but was bound to the mouse adrenal tumor cell line Y1 and to normal rat adrenal cortex cells with a low affinity. Two dimensional electrophoresis of lysates of porcine adrenal cortex cells and subsequent blotting of the proteins on nitrocellulose, using IV/14/9 as a primary and anti-mouse Ig as a secondary reagent, permitted the detection of three forms of the receptor, two of which had an identical apparent molecular mass of about 85,000 Da (isoelectric points: 6.14 and 6.27). The third was somewhat larger (94,000 Da and pI = 6.21).

Photoreactive, active derivatives of trypsin and chymotrypsin inhibitors from soybeans and chickpeas

The photoreactive arylsufenyl chloride 2-nitro-4-azidophenylsulfenyl chloride (2,4-NAPS-Cl) has been used for the selective modification of tryptophan in Kunitz's soybean trypsin inhibitor (STI). The ultraviolet absorption spectrum and amino acid analysis of 2,4-NAPS-STI indicated that only one of the two tryptophans, 93 or 117, present in STI was modified. Amino acid analysis of the two separated CNBr-cleavage products of 2,4-NAPS-STI showed that only tryptophan 93 underwent modification. 2,4-NAPS-STI fully retained its inhibitory activity against trypsin. The covalent attachment of 2,4-NAPS-STI to tritiated trypsin after photolysis was demonstrated by exclusion chromatography on Sephadex G-50 in the presence of guanidine hydrochloride. Photoreactive derivatives of the Bowman-Birk trypsin-chymotrypsin inhibitor (BBI) from soybeans and of CI, the trypsin-chymotrypsin inhibitor from chick peas were prepared by selective modification of the epsilon-amino groups of 2,4(5)-NAPS-Cl. The ultraviolet absorption spectra of the photolabeled inhibitors indicated that three out of the five lysines of BBI and one of the seven lysines of CI were modified. The inhibitory activity of the modified inhibitors towards trypsin and chymotrypsin was not reduced even after photolysis. The specific lysine residues that constitute the trypsin-inhibitory sites of BBI and CI did not react with the photoreactive reagents. Further modification of the photoreactive derivatives of BBI and CI with maleic anhydride, directed towards the trypsin-reactive sites, resulted in almost complete loss of trypsin-inhibiting activity without reducing the ability to inhibit chymotrypsin. A pronounced potentiation effect (approximately 2x) of the chymotrypsin inhibiting activity was noted for 2,5-NAPS-CI and it was retained even after maleylation followed by photolysis, raising the possibility of exposure of an additional chymotrypsin inhibitory site in CI.

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)

Similarity between the corticotropin (ACTH) receptor and a peptide encoded by an RNA that is complementary to ACTH mRNA

An interesting pattern in the genetic code was recently observed: Codons for hydrophilic and hydrophobic amino acids on one strand of nucleic acid are complemented by codons for hydrophobic and hydrophilic amino acids on the other strand, respectively. The average tendency of codons for "uncharged" (slightly hydrophilic) amino acids is to be complemented by codons for "uncharged" (slightly hydrophilic) amino acids. We have postulated that this pattern can result in the binding of peptides that are encoded by complementary RNA strands and we have presented supporting evidence. In this report we demonstrate the specific and high-affinity binding of naturally occurring peptides [corticotropin (ACTH) and gamma-endorphin] to synthetically derived counterparts that were specified by RNA sequences complementary to the mRNA for ACTH and gamma-endorphin, respectively. That this binding might result from one peptide being an "internal image" of the other was strongly suggested by the observation that antibody to the peptide that was encoded by the complementary RNA for ACTH recognized the adrenal cell ACTH receptor. Based on these findings, a theory on the evolution of peptides and their receptors is suggested.

Photoaffinity labelling of peptide hormone receptors

Photoreactive peptide derivatives for the labelling of hormone receptors are usually prepared by inserting a chemically stable aryl azide or nitroaryl azide into a specific site of the molecule, such as an alpha or omega amino or carboxyl group, or into the side-chain of an Arg, Cys, His, Trp or Tyr. With p-azidophenylalanine (Pap), a more or less isosteric replacement of Tyr or Phe can be achieved when other alterations would impair the biological activity of the hormone. Reversible attachment of photoreactive groups via S-S linkage to, e.g., SH-Trp makes it possible to release the covalently bound hormone from the receptor. This is an advantage when photolabelling is used for the isolation of receptors. Photoaffinity labelling of intact cell systems may reveal insights into dynamic aspects, such as receptor inactivation and turnover or the temporal involvement of second messengers, and may be particularly useful for the study of peptide action on target cells occurring only in small numbers. The Xenopus MSH-melanophore system which is stimulated irreversibly upon UV-irradiation of photoreactive alpha-MSH represents such an example.

Preparation of photoreactive derivatives of glutathione and [9-(2-mercaptotryptophan)]corticotropin by selective modification of the sulfhydryl group

The photoreactive arylsulfenyl chlorides 2-nitro-4-azidophenylsulfenyl chloride (2,4-NAPS-Cl) and 2-nitro-5-azidophenylsulfenyl chloride (2,5-NAPS-Cl) have been used for the selective modification of thiol groups in glutathione and [Trp(SH)9]corticotropin (ACTH). Both reagents reacted rapidly with both types of thiol groups to form unsymmetrical disulfides. The photoreactive derivatives of glutathione and [Trp(SH)9]ACTH were stable to neutral and acidic conditions but were readily cleaved above pH 9 and by beta-mercaptoethanol. Photolysis of the NAPS derivatives of [Trp(SH)9]-ACTH at neutral pH resulted in the formation of covalently liked polymers and dimers which yielded monomer upon treatment with beta-mercaptoethanol. Analysis of the amino acid composition of acid hydrolysates of photolysed monomeric and dimeric products indicated a decrease in proline, valine, tyrosine, and phenylalanine.