Photoaffinity labeling of human serum vitamin D binding protein and chemical cleavages of the labeled protein: identification of an 11.5-kDa peptide containing the putative 25-hydroxyvitamin D3 binding site

Photoaffinity labeling of plasma proteins

Photoaffinity labeling is a powerful technique for identifying a target protein. A high degree of labeling specificity can be achieved with this method in comparison to chemical labeling. Human serum albumin (HSA) and α₁-acid glycoprotein (AGP) are two plasma proteins that bind a variety of endogenous and exogenous substances. The ligand binding mechanism of these two proteins is complex. Fatty acids, which are known to be transported in plasma by HSA, cause conformational changes and participate in allosteric ligand binding to HSA. HSA undergoes an N-B transition, a conformational change at alkaline pH, that has been reported to result in increased ligand binding. Attempts have been made to investigate the impact of fatty acids and the N-B transition on ligand binding in HSA using ketoprofen and flunitrazepam as photolabeling agents. Meanwhile, plasma AGP is a mixture of genetic variants of the protein. The photolabeling of AGP with flunitrazepam has been utilized to shed light on the topology of the protein ligand binding site. Furthermore, a review of photoaffinity labeling performed on other major plasma proteins will also be discussed. Using a photoreactive natural ligand as a photolabeling agent to identify target protein in the plasma would reduce non-specific labeling.

Cross-talk among structural domains of human DBP upon binding 25-hydroxyvitamin D

Serum vitamin D-binding protein (DBP) is structurally very similar to serum albumin (ALB); both have three distinct structural domains and high cysteine-content. Yet, functionally they are very different. DBP possesses high affinity for vitamin D metabolites and G-actin, but ALB does not. It has been suggested that there may be cross-talk among the domains so that binding of one ligand may influence the binding of others. In this study we have employed 2-p-toluidinyl-6-sulfonate (TNS), a reporter molecule that fluoresces upon binding to hydrophobic pockets of DBP. We observed that recombinant domain III possesses strong binding for TNS, which is not influenced by 25-hydroxyvitamin D(3) (25-OH-D(3)), yet TNS fluorescence of the whole protein is quenched by 25-OH-D(3). These results provide a direct evidence of cross-talk among the structural domains of DBP.

Protein chemical characterization of Gc globulin (vitamin D-binding protein) isoforms; Gc-1f, Gc-1s and Gc-2

Gc globulin, also called vitamin D-binding protein, is a plasma protein involved in the extracellular actin-scavenger system, vitamin D transport and possibly also other biological activities. Low levels of Gc globulin have been found to correlate with multiple organ failure and non-survival of patients with fulminant hepatic failure and trauma. Here, we characterize the dominant isoforms of plasma-derived Gc globulin from Cohn fraction IV paste with respect to amino acid sequence and posttranslational modifications. Gc globulin was purified in large scale and the isoforms separated by ion exchange chromatography. The separated isoforms and several commercial preparations of individual isoforms were characterized by mass spectrometry. This revealed that the major isoforms were non-glycosylated. Compared to the Gc-1f isoform the other dominating isoforms represented an Asp/Glu substitution (Gc-1s) and a Thr/Lys substitution (Gc-2) in agreement with DNA sequencing studies. The commercial preparations were found to represent mainly one or two isoforms. An O-linked glycan with a mass of 656 Da and terminating with a sialic acid residue was detected on a minor proportion of Gc globulin molecules.

Binding of a C-terminal fragment (residues 369 to 435) of vitamin D-binding protein to actin

The vitamin D-binding protein (DBP) binds to monomeric actin with high affinity. The variation in DBP isoforms is due to genetic polymorphism and varying glycosylation. To obtain a homogeneous preparation, the cDNA for human DBP and truncations thereof were cloned and various systems were applied for heterologous bacterial and yeast expression. The full-length protein and the N- and C-terminal halves of DBP remained insoluble probably because the protein did not fold to its native three-dimensional structure due to formation of accidental intra- and inter-molecular disulfide bonds during expression in bacteria or yeast. This problem was overcome by cloning of a C-terminal fragment comprising residues 369 to 435 that did not contain disulfide bonds and was completely soluble. Binding of the C-terminal fragment to monomeric actin was demonstrated by comigration with actin during native polyacrylamide gel electrophoresis and surface plasmon resonance, however, at considerably lower affinity than full-length DBP. This suggests that in addition to the C-terminal amino acid sequence other parts (amino acid residues or sugar moieties) of DBP participate in actin binding. The C-terminal fragment was found to inhibit denaturation of actin and to decrease the rate of actin polymerisation both at the barbed and at the pointed end in a concentration-dependent manner. According to a quantitative analysis of the polymerisation kinetics, association of actin monomers to nucleate filaments was not prevented by binding of the C-terminal fragment to actin. These data suggest that the sites on the surface of actin that are involved in actin nucleation and elongation are different.

Biochemical and preliminary crystallographic characterization of the vitamin D sterol- and actin-binding by human vitamin D-binding protein

Vitamin D-binding protein (DBP), a multi-functional serum glycoprotein, has a triple-domain modular structure. Mutation of Trp145 (in Domain I) to Ser decreased 25-OH-D(3)-binding by 80%. Furthermore, recombinant Domain I (1-203) and Domain I + II (1-330) showed specific and strong binding for 25-OH-D(3), but Domain III (375-427) did not, suggesting that only Domains I and II might be required for vitamin D sterol-binding. Past studies have suggested that Domain III is independently capable of binding G-actin. We exploited this apparently independent ligand-binding property of DBP to purify DBP-actin complex from human serum and rabbit muscle actin by 25-OH-D(3) affinity chromatography. Competitive (3)H-25-OH-D(3) binding curves for native DBP and DBP-actin complex were almost identical, further suggesting that vitamin D sterol- and actin-binding activities by DBP might be largely independent of each other. Trypsin treatment of DBP produced a prominent 25 kDa band (Domain I, minus 5 amino acids in N-terminus), while actin was completely fragmented by such treatment. In contrast, tryptic digestion of purified DBP-actin complex showed two prominent bands, 52 (DBP, minus 5 amino acids in the N-terminus) and 34 kDa (actin, starting with amino acid position 69) indicating that DBP, particularly its Domains II and III were protected from trypsin cleavage upon actin-binding. Similarly, actin, except its N-terminus, was also protected from tryptic digestion when complexed with DBP. These results provided the basis for our studies to crystallize DBP-actin complex, which produced a 2.5 A crystal, primitive orthorhombic with unit cell dimensions a=80.2A, b=87.3A, and c=159.6A, P2(1)2(1)2(1) space group, V(m)=2.9. Soaking of crystals of actin-DBP in crystallization buffer containing various concentrations of 25-OH-D(3) resulted in cracking of the crystal, which was probably a reflection of a ligand-induced conformational change in the complex, disrupting crystal contacts. In conclusion, we have provided data to suggest that although binding of 25-OH-D(3) to DBP might result in discrete conformational changes in the holo-protein to influence actin-binding, these binding processes are largely independent of each other in solution.

The C(19) position of 25-hydroxyvitamin D(3) faces outward in the vitamin D sterol-binding pocket of vitamin D-binding protein

The radiolabeled affinity and photoaffinity analogues of 25-hydroxyvitamin D(3) (25-OH-D(3)) with probes at the C-19 position failed to specifically label the 25-OH-D(3)-binding pocket of vitamin D-binding protein (DBP). However, a hybrid analogue, with a bromoacetate affinity probe and a photoaffinity probe at C(3)-OH and C(19) positions, respectively, specifically labeled the ligand-binding pocket, suggesting that C(3)-OH points towards the 'inside' of the binding cavity while the C(19) position faces away from it.

Concepts for the syntheses of biotinylated steroids. Part I: testosterone derivatives as immunochemical probes

We describe synthetic strategies for the biotinylation of testosterone (T) at positions 3, 7alpha, 17alpha, and 19. These T probes are able to mimic ligand binding and may provide for a better understanding of the biospecific interaction with steroid-binding proteins such as the androgen receptor, anti-steroid antibodies, or steroid-binding serum globulins. For the 7alpha- and 17alpha-derivatives, biotinyl-N-hydroxy-succinimide esters with different types of spacer chains were used. The 3-biotin hydrazone derivative was produced using N-(epsilon-biotinyl)-caproyl hydrazide, whereas for the 19-biotinylation, a biotinyl-1-N-diamino-3, 6-dioxaoctane-amide was applied. Key reaction for the biotinylation at position 3 is the oximation of the 3-oxo function. The 17alpha-position is accessible by the reaction of the 3-protected 4-androsten-17-epoxide with oxygen in the beta-position, followed by nucleophilic ring opening with cyanide which provides the 17alpha-cyanomethyl derivative. The key step is the regioselective ketal protection of the 3-oxo function of androst-4-ene-3,17-dione using a stannoxane catalyst. An alternative pathway for the insertion of biotin at the 19-position was established by the synthesis of 17beta-hydroxy-androst-4-en-3-one-19-yl carboxymethyl ether. After activation by the carbodiimide method, the compound reacts with aminoterminal biotin derivatives. The copper(I)-catalyzed 1,6 Michael addition of 17-acetoxy-6,7-dehydro-T leads to 7alpha-derivatives by use of omega-silyl protected hydroxylalkyl-modified Grignard reagents. A functional group interconversion using the Staudinger reaction transforms the azide function into a primary omega-amino group. The absolute configurations of the different biotinylated derivatives were investigated by (1)H NMR studies. For the 7alpha-biotinylated T series, additionally, an X-ray analysis proved the axial position of the spacer group. This results in a vertical orientation of the biotin moiety toward the alpha-face of the planar tetracyclic backbone. Thus, a negligible alteration of the original structure of the upper beta-face offers the feasibility of applying the 7alpha-derivatives as optimal immunochemical tracers in competitive immunoassays. Biotinylated T derivatives should be also suitable for ligand-binding studies to the androgen receptor or to sex hormone-binding globulin.

Development of an affinity-driven cross-linker: isolation of a vitamin D receptor associated factor

A vitamin D analogue containing an affinity and a photoaffinity probe (affinity-driven cross-linker, Double Label) was synthesized. An unknown factor, associated with vitamin D receptor (VDR), was isolated from rat liver nuclear extract using a GST-VDR-ligand-binding domain fusion protein (GST-VDR-LBD), affinity labeled with Double Label, and protein-protein cross-linking by photolysis.

Probing the vitamin D sterol-binding pocket of human vitamin D-binding protein with bromoacetate affinity labeling reagents containing the affinity probe at C-3, C-6, C-11, and C-19 positions of parent vitamin D sterols

The multiple physiological properties of vitamin D-binding protein (DBP) include organ-specific transportation of vitamin D(3) and its metabolites, manifested by its ability to bind vitamin D sterols with high affinity. In the present investigation we probed the vitamin D sterol-binding pocket of human DBP with affinity labeling analogs of 25-hydroxyvitamin D(3) ¿25-OH-D(3) and 1, 25-dihydroxyvitamin D(3) ¿1,25(OH)(2)D(3) containing bromoacetate alkylating probe at C-3 (A-ring), C-6 (triene), C-11 (C-ring), and C-19 (exocyclic methylene) of the parent sterol. Competitive binding assays with DBP showed approximately 22-, 68-, and 2000-fold decrease in the binding of 1,25(OH)(2)-D(3)-11-BE, 25-OH-D(3)-3-BE, and 25-OH-D(3)-6-BE, respectively, compared to that seen with 25-OH-D(3), while there was no significant difference in the DBP-binding affinity of 25-OH-D(3)-19-BE and 25-OH-D(3). Surprisingly, ¿(14)C25-OH-D(3)-11-BE and ¿(14)C1, 25(OH)(2)-D(3)-19-BE failed to label DBP despite high-affinity DBP-binding, indicating the absence of any nucleophilic amino acid in the vicinity of their bromoacetate moiety to form a covalent bond, while these analogs are inside the binding pocket. In contrast, ¿(14)C25-OH-D(3)-6-BE and ¿(14)C25-OH-D(3)-3-BE specifically labeled DBP. BNPS-skatole digestion of DBP labeled with ¿(14)C25-OH-D(3)-3-BE or ¿(14)C25-OH-D(3)-6-BE produced two peptides (M(r) 17,400 and 33,840), with radioactivity associated with the N- and C-terminal peptides, respectively, raising the possibility that either different areas of the same vitamin D sterol-binding pocket, or different domains of DBP might be labeled by these analogs. Successful affinity labeling of recombinant domain I (1-203) of DBP with both reagents indicated that different areas of the same vitamin D-binding pocket (domain I) were labeled. These affinity analogs are potentially useful for "mapping" the vitamin D sterol-binding pocket and developing a functional model.

C-6 functionalized analogs of 25-hydroxyvitamin D3 and 1alpha,25-dihydroxyvitamin D3: synthesis and binding analysis with vitamin D-binding protein and vitamin D receptor

In this article, we describe the development of a general synthetic strategy to functionalize the C-6 position of vitamin D3 and its biologically important metabolites, i.e. 25-hydroxyvitamin D3 (25-OH-D3) and 1alpha,25-dihydroxyvitamin D3 [1,25(OH)2D3]. We employed Mazur's cyclovitamin D method to synthesize vitamin D3 analogs with several functionalities at the C-6 position. In addition, we synthesized 6-(3-hydroxypropyl) and 6-[(2-bromoacetoxy)propyl] derivatives of 25-OH-D3 15 and 16, respectively, and 6-(3-hydroxypropyl) derivative of 1,25(OH)2D3 17. Competitive binding assays of 15-17 with human serum vitamin D-binding protein showed that all these analogs specifically bound to this protein, although with significantly lower affinity than the 25-OH-D3, the strongest natural binder, but with comparable affinity with 1,25(OH)2D3, the hormone. On the other hand, 6-[3-hydroxypropyl], 1alpha,25-dihydroxyvitamin D3 17 did not show any specific binding for recombinant nuclear vitamin D receptor. These results indicated that the region containing the C-6 position of the parent seco-steroid [1,25(OH)2D3] may be an important recognition marker towards vitamin D receptor binding. Information, delineated in this article, will be important for evaluating structure-activity relationship in synthetic analogs of vitamin D and its metabolites.

Synthesis and binding-analysis of 5E-[19-(2-bromoacetoxy)methyl]25-hydroxyvitamin D3 and 5E-25-hydroxyvitamin D3-19-methyl[(4-azido-2-nitro)phenyl]glycinate: novel C19-modified affinity and photoaffinity analogs of 25-hydroxyvitamin D3

Synthesis of novel C19-modified affinity and photoaffinity analogs of vitamin D3 and 25-hydroxyvitamin D3(25-OH-D3) is described. A key step in the synthesis is a Horner-Emmons reaction between C19-nor-cyclovitamin D3-C19-ketone or C19-nor-25-hydroxy-cyclovitamin D3-C19-ketone and diethyl cyanomethylphosphonate. Competitive radioligand binding assays with human serum vitamin D-binding protein (DBP) and 5E-[19-(2-bromoacetoxy)methyl]25-hydroxyvitamin D3 and 5E-25-hydroxyvitamin D3-19-methyl[(4-azido-2-nitro)phenyl]-glycinate, 25-OH-D3-analogs containing affinity and photoaffinity probes at C19-position, demonstrated that these compounds displaced radiolabeled 25-OH-D3 from the binding pocket of DBP in a dose-dependent manner. Thus, these affinity and photoaffinity analogs are potentially useful in determining the ligand binding site topographies of DBP and possibly the vitamin D receptor.

Identification of the subdomain in the nuclear receptor for the hormonal form of vitamin D3, 1 alpha,25-dihydroxyvitamin D3, vitamin D receptor, that is covalently modified by an affinity labeling reagent

Multiple physiological actions of the hormonal form of vitamin D3, 1 alpha,25-dihydroxyvitamin D3 (1,25(OH)2D3), are mediated by a genomic pathway which is initiated by the highly specific recognition and binding by its cognate receptor (vitamin D receptor, VDR) in the target cells. Thus, knowledge of the three-dimensional geometries of the ligand, i.e., 1,25(OH)2D3, and the 1,25(OH)2D3-binding domain of VDR is crucial for a better understanding of diverse physiological roles of this hormone. Recently our laboratory has developed 1 alpha,25-dihydroxyvitamin D3-3 beta-bromoacetate (1,25(OH)2 D3-3-BE) as an affinity labeling reagent for covalently modifying the hormone binding domain of native VDRs from calf thymus and rat osteosarcoma cells and baculovirus-expressed recombinant human VDR (hVDR). In the present report, we report affinity labeling of the hormone binding domain of hVDR, expressed in Escherichia coli as a glutathione S-transferase fusion partner, site-specific cleavage of the affinity-labeled VDR with 3-bromo-3-methyl-2-(2-nitrophenylmercapto)- 3H-indole, and identification of the C-terminal subdomain of human VDR containing the putative hormone binding site.

Bacterial expression of human vitamin D-binding protein (Gc2) in functional form

In this report, we report the first expression of human vitamin D-binding protein (hDBP), a serum protein with several functions and a multidomained structure, in Escherichia coli. The recombinant protein (reDBP) was expressed as a fusion partner of glutathione S-transferase in order to facilitate proper folding of the reDBP; E. coli-expressed DBP was found to be fully functional with respect to vitamin D sterol binding, interaction with actin, and cross-reactivity with anti-DBP antibody. Furthermore, both natural DBP and reDBP were affinity-labeled with 25-hydroxyvitamin D3-3-bromo[1-14C]acetate in a similar fashion. Availability of an expression system for hDBP in functional form provides opportunity to develop mutants and truncated DBPs to study multiple ligand-binding properties of this protein in relationship with its structure.

Aminopropylation of vitamin D hormone (1 alpha,25-dihydroxyvitamin D3), its biological precursors, and other steroidal alcohols: an anchoring moiety for affinity studies of sterols

In this communication we describe a simple two-step procedure for the conversion of several steroidal alcohols to their aminopropyl ether derivatives. To demonstrate the usefulness of this procedure we synthesized a second-generation photoaffinity labeling analog of 1 alpha,25-dihydroxyvitamin D3, and a 25-hydroxyvitamin D3 derivative containing a long and chemically stable tether. Utilities of these aminopropyl ether derivatives of steroids in various affinity studies of receptor proteins are discussed.

Trp-145 is essential for the binding of 25-hydroxyvitamin D3 to human serum vitamin D-binding protein

Chemical modification of specific amino acid residues in a protein has been a valuable tool in identifying amino acid residues that are responsible for ligand binding of a protein. In the present investigation, we targeted Trp and His residues in human serum vitamin D-binding protein (hDBP) by modifying them with specific chemical modifiers. We also evaluated the results of these modifications in the binding of 25-hydroxy[26(27)-3H]vitamin D3 ([3H]25-OH-D3) to hDBP. We observed a dose-dependent loss of binding activity by N-bromosuccinimide (specific for Trp). Similar results were observed with diethylpyrocarbonate (specific for His). Furthermore, loss of [3H]25-OH-D3-binding was protected by preincubation of hDBP samples with an excess of 25-hydroxyvitamin D3. These results strongly emphasized the importance of Trp (single residue at position 145) and 1 His residue (out of a total of 6) in the vitamin D sterol-binding by vitamin D-binding protein.

Profile of ligand specificity of the vitamin D binding protein for 1 alpha,25-dihydroxyvitamin D3 and its analogs

The profile of structural preference for the ligand binding domain of the human vitamin D binding protein (DBP) was determined by steroid competition assay of 71 analogs of 1 alpha,25-dihydroxyvitamin D3 [1 alpha,25(OH)2D3]. The following categories of structural modification were evaluated [values represent fold change; R = reduction, I = increase in binding to the DBP from the reference 1 alpha, 25(OH)2D3]: (1) deletion in the A ring of the 1 alpha-hydroxyl-(20-1600I); (2) conversion of the triene system to the previtamin form (6-40R); (3) addition of substituents to carbon 11 of the C ring (4-14R); (4) inversion of the C/D ring junction (8-20R); (5) unsaturation of the D ring (16-ene; 4-140R); (6) replacement of hydrogen with deuterium atoms (no effect); alteration of the side chain by (7) adding or deleting carbon atoms (5-12R); (8) addition of fluorines (0.2-10R); (9) presence of unsaturation (22-ene, 0-5R; 23-ene, 3R-10I; 23-yne, 5-20R); (10) addition of hydroxyls (2-100R); and (11) addition of an aromatic ring (0-20I). Thus the DBP ligand binding domain could tolerate only modest changes to the structure of 1 alpha,25(OH)2D3 without a reduction in binding of the analog. The increases in binding seen in the aromatic side chain and with a triple bond at carbon-23 may be indicative of a preferred conformation of the flexible 1 alpha, 25(OH)2D3 side chain. In addition, a comparison was made of the DBP ligand binding domain with that of the human HL-60 cell 1 alpha, 25(OH)2D3 nuclear receptor. Both ligand binding domains could equivalently accommodate to the presence of (1) a side-chain cyclopropyl group, (2) 22-ene or 23-yne, (3) lengthening the side chain by two carbons, (4) presence of four to six fluorine atoms, (5) substitution of an oxygen for carbon 22, and (6) presence of a 22-[m-(dimethylhydroxymethyl)phenyl] aromatic group in the side chain. The DPB could tolerate better than the HL-60 cell receptor the presence of a 22-(p-hydroxyphenyl) aromatic group in the side chain and the absence of the 1 alpha-hydroxyl. In contrast, the HL-60 cell receptor could tolerate better than the DBP the following structural modifications: presence of a 16-ene, or 16-ene plus 23-yne unsaturation, and presence of an 11 beta-hydroxyl.