Diabetes-associated nitration of tyrosine and inactivation of succinyl-CoA:3-oxoacid CoA-transferase

High levels of reactive species of nitrogen and oxygen in diabetes may cause modifications of proteins. Recently, an increase in protein tyrosine nitration was found in several diabetic tissues. To understand whether protein tyrosine nitration is the cause or the result of the associated diabetic complications, it is essential to identify specific proteins vulnerable to nitration with in vivo models of diabetes. In the present study, we have demonstrated that succinyl-CoA:3-oxoacid CoA-transferase (SCOT; EC 2.8.3.5) is susceptible to tyrosine nitration in hearts from streptozotocin-treated rats. After 4 and 8 wk of streptozotocin administration and diabetes progression, SCOT from rat hearts had a 24% and 39% decrease in catalytic activity, respectively. The decrease in SCOT catalytic activity is accompanied by an accumulation of nitrotyrosine in SCOT protein. SCOT is a mitochondrial matrix protein responsible for ketone body utilization. Ketone bodies provide an alternative source of energy during periods of glucose deficiency. Because diabetes results in profound derangements in myocardial substrate utilization, we suggest that SCOT tyrosine nitration is a contributing factor to this impairment in the diabetic heart.

Novel effects of nitric oxide

Nitric oxide (NO), a simple free radical gas, elicits a surprisingly wide range of physiological and pathophysiological effects. NO interacts with soluble guanylate cyclase to evoke many of these effects. However, NO can also interact with molecular oxygen and superoxide radicals to produce reactive nitrogen species that can modify a number of macromolecules including proteins, lipids, and nucleic acids. NO can also interact directly with transition metals. Here, we have reviewed the non--3',5'-cyclic-guanosine-monophosphate-mediated effects of NO including modifications of proteins, lipids, and nucleic acids.

Nitration of succinyl-CoA:3-oxoacid CoA-transferase in rats after endotoxin administration

The tyrosine nitration of proteins has been observed in diverse inflammatory conditions and has been linked to the presence of reactive nitrogen species. From many in vitro experiments, it is apparent that tyrosine nitration may alter the function of proteins. A limited number of experiments under in vivo conditions also demonstrate that protein nitration is associated with altered cellular processes. To understand the association of protein nitration with the pathogenic mechanism of the disease, it is essential to identify specific protein targets of nitration with in vivo or intact tissue models. Using anti-nitrotyrosine antibodies, we demonstrated the accumulation of nitrotyrosine in a 52-kDa protein in rat kidney after lipopolysaccharide treatment. The 52-kDa protein was purified and identified with partial sequence as succinyl-CoA:3-oxoacid CoA-transferase (SCOT; EC ). Western blot analysis revealed that the nitration of this mitochondrial enzyme increased in the kidneys and hearts of lipopolysaccharide-treated rats, whereas its catalytic activity decreased. These data suggest that tyrosine nitration may be a mechanism for the inhibition of SCOT activity in inflammatory conditions. SCOT is a key enzyme for ketone body utilization. Thus, tyrosine nitration of the enzyme with sepsis or inflammation may explain the altered metabolism of ketone bodies present in these disorders.

Cyclic nucleotide phosphodiesterases: relating structure and function

Cyclic nucleotide phosphodiesterases (PDEs) comprise a superfamily of metallophosphohydrolases that specifically cleave the 3',5'-cyclic phosphate moiety of cAMP and/or cGMP to produce the corresponding 5'-nucleotide. PDEs are critical determinants for modulation of cellular levels of cAMP and/or cGMP by many stimuli. Eleven families of PDEs with varying selectivities for cAMP or cGMP have been identified in mammalian tissues. Within these families, multiple isoforms are expressed either as products of different genes or as products of the same gene through alternative splicing. Regulation of PDEs is important for controlling myriad physiological functions, including the visual response, smooth muscle relaxation, platelet aggregation, fluid homeostasis, immune responses, and cardiac contractility. PDEs are critically involved in feedback control of cellular cAMP and cGMP levels. Activities of the various PDEs are highly regulated by a panoply of processes, including phosphorylation events, interaction with small molecules such as cGMP or phosphatidic acid, subcellular localization, and association with specific protein partners. The PDE superfamily continues to be a major target for pharmacological intervention in a number of medically important maladies.

Histidine-607 and histidine-643 provide important interactions for metal support of catalysis in phosphodiesterase-5

Class I cyclic nucleotide phosphodiesterases (PDEs) share a catalytic domain containing 18 invariant residues. In cGMP-binding cGMP-specific PDE (PDE5), we showed previously that point mutation of nine of these profoundly decreases k(cat) when the assay is conducted in the presence of Mg(2+); seven of these are in the prototypical metal-binding motifs A and B (HX(3)HX(n)()E) that we identified earlier. Tandem arrangement of two of these metal-binding motifs in PDEs is novel, and whether residues within these motifs are involved in metal support of catalytic activity is a fundamental question in this field. This report shows that mutation of either His-607 (A motif) or His-643 (B motif) to alanine profoundly diminishes support of PDE catalysis by Mn(2+) or Mg(2+), but mutation of His-647 in B motif or of Glu in either motif does not. H607A and H643A mutants have much greater maximum catalytic rates supported by Mn(2+) than that by Mg(2+); catalytic activity of H603A mutant is supported weakly by either. In H607A and H643A, K(a)s for Mn(2+) and Mg(2+) are increased, but the effect of Mn(2+) is 2-fold greater than that of Mg(2+) in each. Mutation of any of the other conserved residues (Asn-604, Asp-644, His-675, Asp-714, and Asp-754) causes unremarkable changes in Mn(2+) or Mg(2+) support of catalysis. This study identifies specific residues in PDE5 that contribute to interactions with catalytically relevant metals. The combined data suggest that despite a high degree of sequence similarity between each HX(3)HX(n)()E motif in PDEs and certain metallo-endopeptidases, PDEs employ a distinct complement of residues for interacting with metals involved in catalysis.

Phosphorylation of phosphodiesterase-5 by cyclic nucleotide-dependent protein kinase alters its catalytic and allosteric cGMP-binding activities

In addition to its cGMP-selective catalytic site, cGMP-binding cGMP-specific phosphodiesterase (PDE5) contains two allosteric cGMP-binding sites and at least one phosphorylation site (Ser92) on each subunit [Thomas, M.K., Francis, S.H. & Corbin, J.D. (1990) J. Biol. Chem. 265, 14971-14978]. In the present study, prior incubation of recombinant bovine PDE5 with a phosphorylation reaction mixture [cGMP-dependent protein kinase (PKG) or catalytic subunit of cAMP-dependent protein kinase (PKA), MgATP, cGMP, 3-isobutyl-1-methylxanthine], shown earlier to produce Ser92 phosphorylation, caused a 50-70% increase in enzyme activity and also increased the affinity of cGMP binding to the allosteric cGMP-binding sites. Both effects were associated with increases in its phosphate content up to 0.6 mol per PDE5 subunit. Omission of any one of the preincubation components caused loss of stimulation of catalytic activity. Addition of the phosphorylation reaction mixture to a crude bovine lung extract, which contains PDE5, also produced a significant increase in cGMP PDE catalytic activity. The increase in recombinant PDE5 catalytic activity brought about by phosphorylation was time-dependent and was obtained with 0.2-0.5 microM PKG subunit, which is approximately the cellular level of this enzyme in vascular smooth muscle. Significantly greater stimulation was observed using cGMP substrate concentrations below the Km value for PDE5, although stimulation was also seen at high cGMP concentrations. Considerably higher concentration of the catalytic subunit of PKA than of PKG was required for activation. There was no detectable difference between phosphorylated and unphosphorylated PDE5 in median inhibitory concentration for the PDE5 inhibitors, sildenafil, or zaprinast 3-isobutyl-1-methylxanthine. Phosphorylation reduced the cGMP concentration required for half-maximum binding to the allosteric cGMP-binding sites from 0.13 to 0.03 microM. The mechanism by which phosphorylation of PDE5 by PKG could be involved in physiological negative-feedback regulation of cGMP levels is discussed.

Studies of the molecular mechanism of discrimination between cGMP and cAMP in the allosteric sites of the cGMP-binding cGMP-specific phosphodiesterase (PDE5)

The regulatory domain of the cGMP-binding cGMP-specific 3':5'-cyclic nucleotide phosphodiesterase (PDE5) contains two homologous segments of amino acid sequence that encode allosteric cyclic nucleotide-binding sites, referred to as site a and site b, which are highly selective for cGMP over cAMP. The possibility that the state of protonation in these sites contributes to cyclic nucleotide selectivity was investigated. The binding of cGMP or cAMP was determined using saturation and competition kinetics at pH values between 5.2 and 9.5. The total cGMP binding by PDE5 was unchanged by variation in pH, but the relative affinity for cGMP versus cAMP progressively decreased as the pH was lowered. Using site-directed mutagenesis, a conserved residue, Asp-289, in site a of PDE5 has been identified as being important for cyclic nucleotide discrimination in this site. It is proposed that deprotonation of Asp-289 enhances the number and strength of bonds formed with cGMP, while concomitantly decreasing the interactions with cAMP.

Inhibition of cyclic GMP-binding cyclic GMP-specific phosphodiesterase (Type 5) by sildenafil and related compounds

The cGMP-binding cGMP-specific phosphodiesterase (PDE5) degrades cGMP and regulates the intracellular level of cGMP in many tissues, including the smooth muscle of the corpus cavernosum of the penis. Sildenafil (Viagra), a specific PDE5 inhibitor, promotes penile erection by blocking the activity of PDE5, which causes cGMP to accumulate in the corpus cavernosum. In the present study, sildenafil, like other PDE5 inhibitors, stimulates cGMP binding to the allosteric sites of PDE5 by interacting at the catalytic site of this enzyme, but the drug does not compete with cGMP for binding at the allosteric sites. Both sildenafil and zaprinast are competitive inhibitors of PDE5, and double-inhibition analysis shows that these two inhibitors added together interact with the catalytic site of PDE5 in a mutually exclusive manner. After site-directed mutagenesis of each of 23 conserved amino acid residues in the catalytic domain of PDE5, the pattern of changes in the IC50 values for sildenafil or UK-122764 is similar to that found for zaprinast. However, among the three inhibitors, sildenafil exhibits the most similar pattern of changes in the IC50 to that found for the affinity of cGMP, implying similar interactions with the catalytic domain. This may explain in part the stronger inhibitory potency of sildenafil for wild-type PDE5 compared with the other inhibitors [sildenafil (Ki = 1 nM) > UK-122764 (Ki = 5 nM) > zaprinast (Ki = 130 nM)]. The affinity of each of these inhibitors for PDE5 is much higher than that of cGMP itself (Km = 2000 nM). It is concluded that residues such as Tyr602, His607, His643, and Asp754 may form important interactions for sildenafil in PDE5, but because these amino acids are conserved in all mammalian PDEs, the selectivity and potency of sildenafil is likely to be provided by a nonconserved residue or residues in the PDE5 catalytic domain.

A photoaffinity probe covalently modifies the catalytic site of the cGMP-binding cGMP-specific phosphodiesterase (PDE-5)

The cGMP-binding cGMP-specific phosphodiesterase (PDE-5) contains distinct catalytic and allosteric binding sites, and each is cGMP-specific. Cyclic nucleotide phosphodiesterase inhibitors, such as 3-isobutyl-1-methylxanthine (IBMX), are believed to compete with cyclic nucleotides at the catalytic sites of these enzymes, but the portion of PDE-5 that accounts for interaction of either of these inhibitors of the substrates themselves with the catalytic domain of the enzymes has not been identified. IBMX was derivatized to yield the photoaffinity probe 8([3-125I,-4-azido]-benzyl)-IBMX, which is referred to as 8(125IAB)-IBMX. This probe was incubated with partially purified recombinant bovine PDE-5. After UV irradiation and SDS-PAGE, a single radiolabeled band that coincided with the position of PDE-5 was visualized on the gel, and the photoaffinity labeling of PDE-5 was linear with increasing concentration of the 8(125IAB)-IBMX. Prominent Coomassie blue-stained bands other than PDE-5 were not labeled significantly. The photoaffinity labeling was progressively blocked by cGMP at concentrations higher than 10 microM, whereas cAMP or 5'-GMP exhibited only weak inhibitory effects. Other compounds that are believed to interact with the PDE-5 catalytic site, including IBMX, cIMP, and beta-phenyl-1,N2-etheno-cGMP (PET-cGMP), also inhibited the photoaffinity labeling in a concentration-dependent manner. The IC50 of PET-cGMP for inhibition of photoaffinity labeling was 10 microM, which compared favorably with an IC50 of 5 microM for inhibition of PDE-5 catalytic activity by this compound. It is concluded that the interaction of this photoaffinity probe with PDE-5 is highly specific for the catalytic site over the allosteric binding sites of PDE-5 and could prove useful in studies to map the catalytic site of PDE-5.

Hydropathic analysis and mutagenesis of the catalytic domain of the cGMP-binding cGMP-specific phosphodiesterase (PDE5). cGMP versus cAMP substrate selectivity

The mechanism of discrimination between cGMP and cAMP in the catalytic site of the cGMP-binding cGMP-specific phosphodiesterase (BTPDE5A1 or PDE5) has been investigated. A hydropathy analysis of the catalytic domains of different families of PDEs suggests that substrate selectivity of PDEs could result from the pattern of hydrophobic/hydrophilic residues in a short segment surrounding a conserved Glu that has been shown to be critical for cGMP binding in the catalytic domain of PDE5. This implies that the substrate selectivity of PDE5 could be altered by replacing the residues within this segment that are conserved in cGMP-specific PDEs with the conserved residues in the corresponding positions of cAMP-specific PDEs. The A769T/L771R, W762L/Q765Y, and W762L/Q765Y/A769T/L771R mutant PDE5s were expressed in High Five cells, and their substrate selectivities were compared with that of wild-type PDE5. The results indicate that the substrate-binding site of PDE5 contains positive elements for accommodating cGMP, as well as negative elements that discriminate against binding of cAMP, and that the cGMP/cAMP selectivity of PDE5 can be shifted 106-fold by substituting four residues of PDE5 with the residues in the corresponding positions of PDE4.

Potential roles of conserved amino acids in the catalytic domain of the cGMP-binding cGMP-specific phosphodiesterase

The known mammalian 3':5'-cyclic nucleotide phosphodiesterases (PDEs) contain a conserved region located toward the carboxyl terminus, which constitutes a catalytic domain. To identify amino acids that are important for catalysis, we introduced substitutions at 23 conserved residues within the catalytic domain of the cGMP-binding cGMP-specific phosphodiesterase (cGB-PDE; PDE5). Wild-type and mutant proteins were compared with respect to Km for cGMP, kcat, and IC50 for zaprinast. The most dramatic decrease in kcat was seen with H643A and D754A mutants with the decrease in free energy of binding (DeltaDeltaGT) being about 4.5 kcal/mol for each, which is within the range predicted for loss of a hydrogen bond involving a charged residue. His643 and Asp754 are conserved in all known PDEs and are strong candidates to be directly involved in catalysis. Substitutions of His603, His607, His647, Glu672, and Asp714 also produced marked changes in kcat, and these residues are likely to be important for efficient catalysis. The Y602A and E775A mutants exhibited the most dramatic increases in Km for cGMP, with calculated DeltaDeltaGT of 2.9 and 2.8 kcal/mol, respectively, that these two residues are important for cGMP binding in the catalytic site. Zaprinast is a potent competitive inhibitor of cGB-PDE, but the key residues for its binding differ significantly from those that bind cGMP.

Binding of cGMP to both allosteric sites of cGMP-binding cGMP-specific phosphodiesterase (PDE5) is required for its phosphorylation

cGMP-binding phosphodiesterases contain two homologous allosteric cGMP-binding sites (sites a and b) that are arranged in tandem; they constitute a superfamily of mammalian cyclic nucleotide receptors distinct from the cyclic nucleotide-dependent protein kinases/cation channels family. The functional role of each of these two sites in the phosphodiesterases is not known. The cGMP-binding sites of one of these phosphodiesterases, the cGMP-binding cGMP-specific phosphodiesterase (cGB-PDE, PDE5), have been analysed by using site-directed mutagenesis. Mutations that affect cGMP binding to either one or both allosteric sites do not influence cGMP hydrolysis in the catalytic site under the conditions used. However, compared with wild-type enzyme, the D289A, D478A and D289A/D478A mutants, which are defective in cGMP binding to either site a or site b, or both allosteric sites, require much higher cGMP concentrations for the allosteric stimulation of phosphorylation by the catalytic subunit of cAMP-dependent protein kinase. The cGMP effect is on the cGB-PDE rather than on the catalytic subunit of the protein kinase because the latter enzyme does not require cGMP for activity. The D289N mutant, which has higher binding affinity for cGMP than does the wild-type enzyme, is phosphorylated at lower concentrations of cGMP than is the wild-type enzyme. It is concluded that cGMP binding to the allosteric sites of cGB-PDE does not directly affect catalysis, but binding to both of these sites regulates phosphorylation of this enzyme.

Ligand-induced conformational changes in cyclic nucleotide phosphodiesterases and cyclic nucleotide-dependent protein kinases

Three methods have been used to assess the conformational effects associated with ligand binding to two unrelated cyclic nucleotide receptor proteins: the cGMP-binding, cGMP-specific phosphodiesterase (cGB-PDE or PDE5A) and the cGMP-dependent protein kinase (PKG). The methods should be applicable to other proteins and to other types of modification such as phosphorylation. The procedures use either ion-exchange chromatography, size-exclusion chromatography, or native gel electrophoresis of these proteins in the absence and presence of regulatory ligands. Measurements from these respective approaches allow documentation of changes in the quaternary structure, surface electronegativity, and relative compactness (Stokes radius) of the protein molecule. The combined data allow the changes in protein conformation to be quantitated in terms of alterations in the axial ratio or length/width dimension of the molecule. The methods can be applied to partially purified proteins and to proteins that are available in limited quantities. Conformational changes due to stable modifications of proteins can be potentially examined in crude extracts of intact cells. Each of the methods can be tailored to optimize resolution of a particular protein under a variety of conditions. Activity measurements, Coomassie brilliant blue or silver staining of gels, radioautography, or Western blot analysis can be used for detection of the protein.

Identification of key amino acids in a conserved cGMP-binding site of cGMP-binding phosphodiesterases. A putative NKXnD motif for cGMP binding

cGMP-binding phosphodiesterases contain two kinetically distinct cGMP-binding sites (a and b), and each site contains a conserved N(K/R)XnFX3DE sequence. N276A, K277A, K277R, D289A, and E290A mutants in the N276KX7FX3DE290 sequence of site a (higher affinity site) of bovine cGMP-binding, cGMP-specific phosphodiesterase (cGB-PDE or PDE5A) were expressed in High Five cells and purified. The cGMP-binding affinities of three mutants [K277A (Kd approximately 12 microM), D289A (Kd approximately 24 microM), and N276A (Kd approximately 60 microM)] were decreased in comparison with wild-type enzyme (Kd = 1.3 microM), which suggested an important role for Asn276, Lys277, and Asp289 in cGMP binding. These residues could be presented as a putative NKXnD motif, and their functions were predicted based on analogy with the canonical NKXD motif in GTP-binding proteins. No marked differences in catalytic functions such as specific activity, Km for cGMP, and IC50 for zaprinast or 3-isobutyl-1-methylxanthine were found among wild-type and mutant cGB-PDEs. This suggested that cGMP binding to site a does not influence the catalytic properties of cGB-PDE.

An essential aspartic acid at each of two allosteric cGMP-binding sites of a cGMP-specific phosphodiesterase

The amino acid sequences of all known cGMP-binding phosphodiesterases (PDEs) contain internally homologous repeats (a and b) that are 80-90 residues in length and are arranged in tandem within the putative cGMP-binding domains. In the bovine lung cGMP-binding, cGMP-specific PDE (cGB-PDE or PDE5A), these repeats span residues 228-311 (a) and 410-500 (b). An aspartic acid (residue 289 or 478) that is invariant in repeats a and b of all known cGMP-binding PDEs was changed to alanine by site-directed mutagenesis of cGB-PDE, and wild type (WT) and mutant cGB-PDEs were expressed in COS-7 cells. Purified bovine lung cGB-PDE (native) and WT cGB-PDE displayed identical cGMP-binding kinetics, with approximately 1.8 microM cGMP required for half-maximal saturation. The D289A mutant showed decreased affinity for cGMP (Kd > 10 microM) and the D478A mutant showed increased affinity for cGMP (Kd approximately 0.5 microM) as compared to WT and native cGB-PDE. WT and native cGB-PDE displayed an identical curvilinear profile of cGMP dissociation which was consistent with the presence of distinct slowly dissociating (koff = 0.26 h-1) and rapidly dissociating (koff = 1.00 h-1) sites of cGMP binding. In contrast, the D289A mutant displayed a single koff = 1.24 h-1, which was similar to the calculated koff for the fast site of WT and native cGB-PDE, and the D478A mutant displayed a single koff = 0.29 h-1, which was similar to that calculated for the slow site of WT and native cGB-PDE. These results were consistent with the loss of a slow cGMP-binding site in repeat a of the D289A mutant cGB-PDE, and the loss of a fast site in repeat b of the D478A mutant, suggesting that cGB-PDE possesses two distinct cGMP-binding sites located at repeats a and b, with the invariant aspartic acid being crucial for interaction with cGMP at each site.

Lysine-heparin interactions in antithrombin. Properties of K125M and K290M,K294M,K297M variants

Lysine residues in two different regions of antithrombin have been proposed to be involved in heparin binding and heparin-mediated acceleration of proteinase inhibition. Lysine 125 has been implicated as an essential heparin binding residue from chemical modification studies [Peterson, C. B., Noyes, C. M., Pecon, J. M., Church, F. C., & Blackburn, M. N. (1987) J. Biol. Chem. 262, 8061-8065] whereas lysines 290, 294, and 297 have been proposed from model building studies to constitute the heparin binding site [Villanueva, G. B. (1984) J. Biol. Chem. 259, 2531-2536]. To evaluate both of these proposals, we have prepared two variant human antithrombins, K125M and K290M,K294M,K297M, in which these lysines have been changed by site-directed mutagenesis to methionines. The K290M,K294M,K297M variant had properties very similar to those of wild-type recombinant antithrombin in affinity for heparin, and in rates of inhibition of thrombin and factor Xa. In contrast, K125M antithrombin had reduced affinity for both heparin pentasaccharide and full-length heparin, corresponding to delta delta Gs of 3.1 and 2.0 kcal mol-1, respectively. However, this variant was still able to inhibit both thrombin and factor Xa. Whereas the rate of thrombin inhibition was similar to that of wild-type antithrombin, the rate of factor Xa inhibition was enhanced between 2- and 3-fold, suggesting a role for lysine 125 in the allosteric coupling between the heparin binding site and the reactive center region. At saturation with either heparin pentasaccharide or full-length high-affinity heparin, the rates of inhibition of both proteinases were similar to those of wild-type antithrombin for both the K125M and K290M,K294M,K297M variants.(ABSTRACT TRUNCATED AT 250 WORDS)

Antithrombin histidine variants 1H NMR resonance assignments and functional properties

Three variants of the 57.5 kDa human plasma proteinase inhibitor antithrombin, H1Q, H65C, and H120C, have been expressed in baby hamster kidney cells to permit assignment of the 1H NMR resonances from the three histidines and evaluation of the role of these histidines in heparin binding. The NMR assignments have enabled more definitive interpretation of previous NMR-based studies of human antithrombin to be made. Although resonances of all three histidines are perturbed by heparin binding, only histidine 120 plays a significant role in the heparin binding site. The perturbations of resonances from histidines 1 and 65 indicate proximity to the heparin binding site and consequent sensitivity to the presence of heparin.

[Direct detection of an antigen in immunoglobulin G Langmuir-Blodgett films based on a surface plasma resonance method and in a piezoelectric system]

The possibilities of direct antigen detection in unlabelled systems based on immunoglobulin G Langmuir-Blodgett films as a sensitive surface have been studied. It was shown that an increase in the monolayer number in an immunoglobulin G Langmuir-Blodgett film deposited onto a solid surface coated with a thin silver film (50 nm) resulted in the regeneration of the antigen-binding capacity of the upper antibody layer. This dependence can be used for the construction of a direct optical immunosensor based on surface plasmon resonance. Moreover, a model of a piesoelectric immunosensor on the basis of immunoglobulin G Langmuir-Blodgett films for direct ferritin detection has been proposed. The detection range of ferritin concentrations in solution is 10(-10)-10(-7) M.

Carbohydrate isoforms of antithrombin variant N135Q with different heparin affinities

We have changed one of the carbohydrate-bearing asparagine residues of human antithrombin to glutamine by site-directed mutagenesis and expressed the variant antithrombin, N135Q, in baby hamster kidney cells. Two isoforms were secreted, both of which had higher affinity for heparin than human plasma alpha antithrombin. Both forms had normal inhibitory activity toward factor Xa and showed normal heparin acceleration of proteinase inhibition. The mutation resulted in a higher production of the very high affinity form from about 30% to 60% of the total secreted antithrombin. This form should be the most useful for comparison of the effects of other mutations on heparin binding and proteinase inhibition.

Transmission of conformational change from the heparin binding site to the reactive center of antithrombin

Heparin greatly increases the rates at which antithrombin inhibits target proteinases. An important part of this rate acceleration is a heparin-induced conformational change in antithrombin. To answer the question of whether or not this change is transmitted to the reactive center, we have prepared a recombinant P1 mutant of antithrombin, R393C, labeled the cysteine with nitrobenzofuran (NBD) fluorophore, and examined the perturbation of NBD fluorescence intensity as a function of bound sulfated oligosaccharide. Two high-affinity heparins, low-affinity heparin, and dextran sulfate were used. We found (i) that binding to antithrombin of all these oligosaccharides resulted in transmission of conformational change to P1 in the reactive center, (ii) that these oligosaccharides all gave enhancements of the rate of inhibition of factor Xa beyond any contribution from surface approximation, and (iii) that the degree of perturbation of P1 correlated with the enhancement of the rate of factor Xa inhibition that was not due to surface approximation.

Heterogeneity of recombinant human antithrombin III expressed in baby hamster kidney cells. Effect of glycosylation differences on heparin binding and structure

To determine the effects of differences in glycosylation on the structure and functional properties of recombinant human antithrombin (rHAT), we have characterized the properties of the recombinant protein overexpressed by baby hamster kidney cells. Three forms of rHAT, I-III, were isolated which differed in affinity for heparin. Form I had the lowest affinity and contained a high proportion of highly branched complex carbohydrate. Form II had higher affinity and contained both complex and high mannose-type chains. Form III had the highest affinity and was similar to form II in the type of carbohydrate present, but had a lower level of glycosylation, consistent with the absence of carbohydrate at one of the four glycosylation sites. 1H NMR spectra of plasma HAT and rHAT forms I-III suggested very similar protein structures for all forms. Heparin pentasaccharide produced almost identical NMR perturbation difference spectra. The only functional difference found was in the rates of inactivation of factor Xa. Forms II and III gave second order rate constants similar to that of plasma HAT, whereas form I gave a biphasic inhibition, with the first phase having a rate about four times that of the other forms. We conclude that carbohydrate heterogeneity does not alter the structure of the HAT polypeptide or the heparin-induced conformational change, but does affect the heparin affinity and can alter the rate of proteinase inhibition.

[IgG, modified with coproporphyrin I and the possibility of direct measurement of the antigen-antibody complex]

Modification of rabbit IgG by coproporphyrin I activated by Woodward's reagent K as well as antigen-binding and fluorescent properties of coproporphyrin-IgG have been studied. It was shown that coproporphyrin I modified IgG retains its capacity to bind the antigen. The formation of the immune antigen-antibody complex increases the intensity of coproporphyrin-IgG fluorescence.

Chemical modification of rabbit IgG with N-dansylaziridine. Investigation of the properties of dansylated antibodies

To elucidate the effect of antigen binding fluorescent thiol reagent, N-dansylaziridine (DAZ), which is sensitive to the changes in the microenvironment, was used for modification of rabbit IgG hinge region cysteine residue. DAZ binds to hinge region Cys 226 as evidenced by the structural analysis. Labelling of IgG with DAZ does not alter its conformation, hydrodynamic behavior, nor its antigen binding properties. Upon antigen bindings, the fluorescence intensity of modified IgG increases about 80%. This result indicates that interaction of antibodies with antigen is accompanied by the conformational changes in the IgG hinge region.

[Chemical modification of rabbit immunoglobulin G by dansylaziridine and study of the properties of modified antibodies]

To elucidate the effect of the antigen binding fluorescent thiol reagent, N-dansylaziridine (DAZ) which is sensitive to microenvironmental changes, was used for modification of the rabbit IgG hinge region cystine residue. DAZ binds to the hinge region Cys 226 as could be evidenced from the structural analysis data. Labelling of IgG with DAZ does not alter either its conformation and hydrodynamic behaviour or its antigen binding properties. Upon antigen binding the fluorescence intensity of modified IgG increases up to about 80%. This finding suggests that the interaction of antibodies with the antigen is accompanied by conformational changes in the IgG hinge region.

[A new method of preparing hemin conjugates with rabbit IgG]

Stable derivatives of hemin activated by means of the Woodward reagent K were obtained, and the reaction conditions for their covalent attachment to the IgG molecule optimized. Advantages of the new method for conjugation are discussed.

Cross-linking studies of the cholesterol hydroxylation system from bovine adrenocortical mitochondria

Cytochrome P-450SCC and adrenodoxin were cross-linked with 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide. The sample containing 94% of cross-linked complex and 6% of free cytochrome P-450SCC was obtained after purification on cholate-Sepharose. Cytochrome P-450SCC in cross-linked complex completely preserves its high-spin form in the presence of Tween 20 or pregnenolone. Utilization of radioactively labelled adrenodoxin, chemical cleavage of cytochrome P-450SCC from cross-linked complex with o-iodosobenzoic acid and HPLC for separation of peptides allow us to conclude that the complex of cytochrome P-450SCC with adrenodoxin was cross-linked through two amino acid sequences of cytochrome P-450SCC-Leu-88-Thr-107 and Leu-368-Gly-416. The cross-linked complex of adrenodoxin reductase, adrenodoxin and cytochrome P-450SCC with an apparent molecular mass of 114 kDa was obtained with N-succinimidyl-6-(4'-azido-2'-nitrophenylamino)hexanoate. The composition of cross-linked complex was determined by immunoblotting and by evaluation of radioactivity using preliminary N-ethyl[2,3-14C]maleimide-modified adrenodoxin. From this data it appears that the ternary complex may exist in solution.

[Mechanism of electron transport in the cholesterol-hydroxylating system of adrenal cortex mitochondria: a triple complex of adrenodoxin reductase, adrenodoxin and cytochrome P-450]

A cross-linked ternary adrenodoxin reductase-adrenodoxin-cytochrome P-450scc complex with an apparent molecular mass of 114 kD was obtained, using N-succinimidyl-6-(4'-azido-2'-nitrophenylamino)-hexanoate. The composition of the cross-linked complex was determined by immunoblotting and radioactivity measurements, using N-ethyl [2.3-14C]maleimide-premodified adrenodoxin. The data obtained suggest that the ternary complex may exist in solution.

Cross-linking studies of steroidogenic electron transfer: covalent complex of adrenodoxin reductase with adrenodoxin

Bifunctional reagents 3,3'-dithiobis(succinimidyl propionate), 1-ethyl 3-(3-dimethylaminopropyl)carbodiimide and N-succinimidyl 3-(2-pyridyldithio)propionate have been used in an attempt to study molecular organization and covalent cross-linking of adrenodoxin reductase with adrenodoxin, the components of steroidogenic electron transfer system in bovine adrenocortical mitochondria. There was no cross-linking of individual proteins by the bifunctional reagents used, except for adrenodoxin cross-linking with water-soluble carbodiimide. Substantial cross-linking of adrenodoxin reductase with adrenodoxin was observed when water-soluble carbodiimide was used as cross-linking reagent. However, the cross-linked complex failed to transfer electrons. Significant amounts of the functional cross-linked complex (up to 42%) were observed when the proteins were cross-linked with N-succinimidyl 3-(2-pyridyldithio)propionate. Using gel filtration, ion-exchange chromatography and affinity chromatography on adrenodoxin-Sepharose, the complex was obtained in a highly purified form. In the presence of cytochrome P-450scc or cytochrome c, the cross-linked complex of adrenodoxin reductase with adrenodoxin was active in electron transfer from NADPH to heme proteins. The data obtained indicate that there are distinct binding sites on the adrenodoxin molecule responsible for the adrenodoxin reductase and cytochrome P-450scc binding, which suggests that steroidogenic electron transfer may be realized in an organized complex.

Cross-linking studies of adrenocortical cytochrome P-450scc. Evidence for a covalent complex with adrenodoxin and localization of the adrenodoxin-binding domain

A cleavable cross-linking reagent, dimethyl-3,3'-dithiobispropionimidate, was used to study the molecular organization of adrenocortical cytochrome P-450scc. Extensive cross-linking was found to occur, resulting in the formation of heterologous oligomers up to octamer. The covalently cross-linked complex of adrenocortical cytochrome P-450scc with adrenodoxin has been obtained by using dimethyl-3,3'-dithiobispropionimidate. In the presence of NADPH and adrenodoxin reductase, electron transfer to cytochrome P-450scc occurs in the complex, and, in the presence of cholesterol, the latter effectively oxidizes to pregnenolone. By using covalently immobilized adrenodoxin and heterobifunctional reagent, N-succinimidyl-3-(2-pyridyldithio)propionate, the adrenodoxin-binding site was shown to be located in the heme-containing, catalytic domain of cytochrome P-450scc. The data obtained indicate the existence of two different sites on the adrenodoxin molecule that are responsible for the interaction with adrenodoxin reductase and cytochrome P-450scc. This is consistent with the model mechanism of electron transfer in the organized complex.

[Localization of the adrenodoxin-binding fragment of cholesterol hydroxylating cytochrome P-450 from adrenal cortex mitochondria]

The interaction between cytochrome P-450scc and adrenodoxin has been studied using cleavable cross-linking reagents and limited trypsinolysis. The data obtained indicate that the site responsible for adrenodoxin binding is located on the NH2-terminal fragment F1 of cytochrome P-450scc.

[Immunochemical characteristics of cholesterol-hydroxylating cytochrome P-450 from adrenal cortex mitochondria]

Highly specific antibodies against hemeprotein were obtained by immunizing rabbits with a highly purified cholesterol-hydroxylating cytochrome P-450scc from adrenocortical mitochondria. The antibodies do not specifically interact with other components of the adrenocortical electron transport chain, e. g., adrenodoxin reductase and adrenodoxin. Using double immunodiffusion technique (Ouchterlony method), it was shown that the antibodies did not precipitate the microsomal cytochromes P-450 LM2 and LM4, cytochrome b5 and 11 beta-hydroxylating cytochrome P-450 from adrenocortical mitochondria. Antibodies against cytochrome P-450scc inhibited the cholesterol side chain cleavage activity of cytochrome P-450scc in a reconstituted system. Limited proteolysis with trypsin and immunoelectrophoresis in the presence of specific antibodies revealed that antigenic determinants are present of the heme-containing catalytic domain of cytochrome P-450scc (F1) as well as on the domain responsible for the interaction with the phospholipid membrane (F2).

[Molecular organization of cholesterol-hydroxylating cytochrome P-450 from adrenal cortex mitochondria. Chemical modification with bifunctional reagents in the study of oligomeric forms]

The molecular organization of adrenocortical cytochrome P-450scc has been investigated using chemical modification with bifunctional imidates. The oligomeric organization of cytochrome P-450scc in solution has been shown. The application of dimethyl-3,3'-dithiobispropioimidate and subsequent cleavage of the modified products by reducing agents revealed the presence of two types of intramolecular cross-links: "short" at the distance of 3,0 A between the amino groups of lysine residues and "long" ones at a distance of 11,9 A. The analysis of the products, obtained by limited proteolysis of the oligomeric forms of the cross-linked cytochrome P-450, by two-dimensional electrophoresis has shown that the cross-links are formed between the functional domain (fragment F1) and domain responsible for the interaction with the phospholipid membrane (fragment F2). A model for cytochrome P-450scc molecular organization has been suggested on the basis of the obtained results.