Chemical modification of the avian progesterone receptor by pyridoxal 5'-phosphate

Cold-induced aldimine bond cleavage by Tris in Bacillus subtilis alanine racemase

The commonly used Tris buffer acts as a surrogate substrate and deformylates pyridoxal phosphate in a bacterial alanine racemase at subzero temperatures.

Determination of pyridoxal-5'-phosphate (PLP)-bonding sites in proteins: a peptide mass fingerprinting approach based on diagnostic tandem mass spectral features of PLP-modified peptides

Peptides modified by pyridoxal-5'-phosphate (PLP), linked to a lysine residue via reductive amination, exhibit distinct spectral characteristics in the collision-induced dissociation (CID) tandem mass (MS/MS) spectra that are described here. The MS/MS spectra typically display two dominant peaks whose m/z values correspond to neutral losses of [H3PO4] (-98 Da) and the PLP moiety as [C8H10NO5P] (-231 Da) from the precursor peptide ion, respectively. Few other peaks are observed. Recognition of this distinct fragmentation behavior is imperative since determining sequences and sites of modifications relies on the formation of amide backbone cleavage products for subsequent interpretation via proteome database searching. Additionally, PLP-modified peptides exhibit suppressed precursor ionization efficiency which diminishes their detection in complex mixtures. Presented here is a protocol which describes an enrichment strategy for PLP-modified peptides combined with neutral loss screening and peptide mass fingerprinting to map the PLP-bonding site in a known PLP-dependent protein. This approach represents an efficient alternative to site-directed mutagenesis which has been the traditional method used for PLP-bonding site localization in proteins.

Risk factors for posterior vitreous detachment: a case-control study

PURPOSE

To identify possible risk factors for the development of posterior vitreous detachment (PVD).

DESIGN

Retrospective case-control study.

METHODS

A total of 138 cases with PVD and 114 age-matched controls were accrued from two different sites. Demographic, medical, ocular, and lifestyle data were obtained through chart review, questionnaires, and clinical examination. A 108-item semiquantitative food frequency questionnaire was also used to estimate macro- and micronutrient intake. Univariate and multivariate regression analyses were employed to identify variables significantly associated with the main outcome measure of PVD. Subgroup analysis of gender-specific variables was performed.

RESULTS

Among all patients, multivariate regression analysis demonstrated female gender (odds ratio [OR] = 2.01, P = .016), myopic refraction (OR = 4.32, P < .0005), and higher intake of vitamin B6 (OR = 2.61, P = .001) to be associated with PVD after controlling for age. In the subgroup analysis of women, menopause (OR = 18.2, P < .0005), myopic refraction (OR = 3.42, P = .01), and higher intake of vitamin B6 (OR = 3.92, P = .005) were associated with PVD. Specifically, there was a significant association between vitamin B6 and PVD amongst premenopausal women but not amongst postmenopausal women.

CONCLUSIONS

An association between PVD and menopause has not been documented previously. We suspect that high estrogen levels seen in premenopausal women may be protective against PVD and that hormonal changes associated with menopause may lead to changes in the vitreous, predisposing to PVD. Higher levels of intake of vitamin B6 were also associated with the development of PVD in premenopausal women possibly through an anti-estrogen effect. These findings should be investigated further with prospective studies.

Steroid (Glucocorticoid)-induced Cataract

Glucocorticoids (GC) have been widely used as a therapeutic drug for various diseases. However, there are many complications of GC therapy including cataracts. In a series of studies to elucidate the actions of GC using 15-day-old developing chick embryos, we found that GC produced hyperglycemia, hyperlipemia, osteoporosis, and cataractous lenses with a high incidence (>90%) within 48 h. Cataract formation is caused by oxidative stresses, probably derived from GC effects on the main target organ, the liver, and can be prevented by radical scavengers including ascorbic acid, and insulin. Ascorbic acid does not inhibit the inflammatory and immunosuppressive effects of GC. Therefore by analyzing and decreasing risk factors producing side effects, it will be possible to improve GC therapy without the loss of GC activity.

Protein thioacylation. 1. Reagents design and synthesis

Thioacylation is a new way for protein chemical modification. Carboxylic dithioesters and -acids react selectively and rapidly at room temperature with aliphatic amines such as lysine epsilon-amino groups leading to thioamide formation, without any other reagent or catalyst. Various thioacylating reagents were synthesized: monofunctional dithioesters bearing on the acylating end various chemical groups such as: aliphatic chains, phenyl group, mono- and dicarboxylic acids, dialkylphosphonic ester, phosphonic acid, thiol, phenol, or quaternary ammonium group. Bifunctional dithioesters containing either a polymethylene chain or an ethylene oxide oligomer as spacer group as well as some mono- and bis(dithio acids) are described. Applications of thioacylation may be involved either in enzyme chemical modifications or in the obtention of new materials from proteins. Bifunctional reagents might be used as cross-linking or coupling reagents.

Effect of glyoxylate on the function of the calcitriol receptor and vitamin D metabolism

The biological action of calcitriol is mostly mediated through the interaction of the calcitriol receptor (VDR) with vitamin D response elements (VDREs) of target genes. These interactions produce special proteins that carry out the biological activities of calcitriol. Recently, we showed that the interaction of VDRs with VDREs is inhibited by uremic toxins. We hypothesize that uremic toxins that contain aldehyde or ketone groups potentially could form Schiff bases with lysine residues of the VDR DNA binding domain and inhibit VDR interaction with VDREs. We therefore chose glyoxylate, a compound which has an aldehyde group, to test this hypothesis. In vitro glyoxylate inhibited VDR binding to the osteocalcin and osteopontin VDREs as assessed by electrophoretic mobility shift assay and the inhibition was reversed when glyoxylate was preincubated with lysine. Further, this chemical compound also blocked the induction of chloramphenicol acetyltransferase (CAT) enzyme induced by calcitriol in cells transfected with a calcitriol responsive CAT reporter gene. Since induction of 24-hydroxylase synthesis is a VDR regulated process, we also studied the effect of glyoxylate on the activity of intestinal 24-hydroxylase in rats. This enzyme activity was suppressed in rats infused with glyoxylate. Taken together, our study suggests that glyoxylate could inhibit the interaction of VDR with VDREs and alter the biological action of calcitriol.

Effect of glucose on the function of the calcitriol receptor and vitamin D metabolism

The genomic action of calcitriol is mediated through the interaction of the calcitriol receptor (VDR) with vitamin D response elements (VDREs) of the target genes. It has been proposed that chemicals capable of Schiff base formation with the VDR potentially could alter the physiological function of VDR and calcitriol metabolism. Since glucose has been shown to form Schiff bases with proteins, we tested the hypothesis that glucose could influence the function of VDR and thereby alter calcitriol metabolism. Glucose 6-phosphate inhibited VDR binding to the osteocalcin VDRE and chemically modified the DNA binding domain or the dimerization domain of the VDR in vitro. Further, glucose also blocked the production of chloramphenicol acetyltransferase (CAT) enzyme induced by calcitriol in cells transfected with a constructed VDRE attached to a CAT reporter gene. Hyperglycemia induced by glucose infusion or by streptozotocin in normal rats significantly reduced intestinal 1 alpha, 25-dihydroxyvitamin D-24-hydroxylase activity. Taken together, these findings are consistent with the hypothesis that glucose could interact with the VDR to impair its DNA binding and function within cells.

Effect of Schiff base formation on the function of the calcitriol receptor

The genomic action of calcitriol is mediated through the interaction of the calcitriol receptor (VDR) with vitamin D response elements (VDREs) of the target genes. We have shown that the interaction of VDRs with VDREs is inhibited by uremic toxins. We hypothesize that uremic toxins form Schiff bases with the lysine residues of the VDR DNA binding domain and inhibit the VDR interaction with the VDRE. In this study, pyridoxal 5'-phosphate was used as a probe to test Schiff base formation as the inhibitory mechanism, since it forms Schiff bases with steroid receptors. Pyridoxal 5'-phosphate inhibited the VDR binding to the VDREs and chemically modified the DNA binding domain of the VDR in vitro. The inhibition was reversed when pyridoxal 5'-phosphate was preincubated with lysine. Further, this chemical agent also blocked the production of chloramphenicol acetyltransferase (CAT) enzyme induced by calcitriol in cells transfected with a constructed VDRE attached to a CAT reporter gene. This finding is consistent with the hypothesis that pyridoxal 5'-phosphate could interact with the VDR and impair its DNA binding within cells. Since induction of 24-hydroxylase synthesis is a receptor mediated process, we studied the effect of pyridoxal 5'-phosphate on the synthesis of renal 24-hydroxylase in rats. When pyridoxal 5'-phosphate was infused to rats, renal 24-hydroxylase activity was suppressed, consequently, degradation of calcitriol was also reduced in these animals. Thus, chemicals capable of Schiff base formation potentially could alter the physiological function of VDR and calcitriol.

Pyridoxal 5'-phosphate related changes in retention of 1,25-dihydroxy vitamin D-receptor ligands in rat intestinal mucosa cell nuclei

After feeding rats a vitamin B-6-deficient diet, we observed a decrease in pyridoxal 5'-phosphate concentrations in intestinal mucosa cells to 32 and 48% of control in cytoplasm and cell nuclei, respectively. Correlation analysis suggested that there were two pyridoxal 5'-phosphate pools in the nuclei: a "mobile" pool (equivalent to about 5% the concentration of the cytoplasmic pyridoxal 5'-phosphate), and a "stable" pool, which was independent of cytoplasmic fluctuations of pyridoxal 5'-phosphate (about 9 pmol pyridoxal 5'-phosphate/mg DNA). Reduction in pyridoxal 5'-phosphate content in the cells of vitamin B-6-deficient animals was accompanied by a substantial increase in 1,25-dihydroxyvitamin D-receptor ligand concentration in the cell nuclei (76.6 +/- 19.7 vs 762 +/- 291 fmol/mg DNA, mean +/- SEM). The degree of 1,25-dihydroxyvitamin D accumulation in the nuclei appeared to be an exponential function of the "mobile" nuclear pyridoxal 5'-phosphate concentration. Semilogarithmic transformation of the data yielded a straight line, representing an inverse correlation between the cytoplasm-related nuclear pool of pyridoxal 5'-phosphate and the logarithm of the 1,25-dihydroxyvitamin D concentration in the nuclei (r = -0.95). These data suggest that pyridoxal 5'-phosphate may be related to 1,25-dihydroxyvitamin D retention in the nuclei, possibly through interaction of the pyridoxal 5'-phosphate with the vitamin D receptor protein in the nuclei.

Pore formation by the sea anemone cytolysin equinatoxin II in red blood cells and model lipid membranes

The interaction of Actinia equina equinatoxin II (EqT-II) with human red blood cells (HRBC) and with model lipid membranes was studied. It was found that HRBC hemolysis by EqT-II is the result of a colloid-osmotic shock caused by the opening of toxin-induced ionic pores. In fact, hemolysis can be prevented by osmotic protectants of adequate size. The functional radius of the lesion was estimated to be about 1.1 nm. EqT-II increased also the permeability of calcein-loaded lipid vesicles comprised of different phospholipids. The rate of permeabilization rised when sphingomyelin was introduced into the vesicles, but it was also a function of the pH of the medium, optimum activity being between pH 8 and 9; at pH 10 the toxin became markedly less potent. From the dose-dependence of the permeabilization it was inferred that EqT-II increases membrane permeability by forming oligomeric channels comprising several copies of the cytolysin monomer. The existence of such oligomers was directly demonstrated by chemical cross-linking. Addition of EqT-II to one side of a planar lipid membrane (PLM) increases the conductivity of the film in discrete steps of defined amplitude indicating the formation of cation-selective channels. The conductance of the channel is consistent with the estimated size of the lesion formed in HRBC. High pH and sphingomyelin promoted the interaction even in this system. Chemical modification of lysine residues or carboxyl groups of this protein changed the conductance, the ion selectivity and the current-voltage characteristic of the pore, suggesting that both these groups were present in its lumen.

The role of lysine, histidine and carboxyl residues in biological activity of equinatoxin II, a pore forming polypeptide from the sea anemone Actinia equina L

Equinatoxin II, a pore forming polypeptide from the sea anemone Actinia equina L. was subjected to chemical modifications with group specific reagents. Lysine residues were modified with pyridoxal-5'-phosphate, histidine residues with diethyl pyrocarbonate and carboxyl groups with the use of a water soluble carbodiimide. Modification of charged residues had no significant influence on the toxin interaction with serum lipoproteins. Lysine 5'-phosphopyridoxylated and histidine carbethoxylated derivatives of the toxin retained lethal and hemolytic activities, but the pH profile of hemolytic activity of 5'-phospho-pyridoxylequinatoxin II was markedly altered. Modification of the toxin carboxyl groups impaired both hemolytic and lethal activities, the latter, however, to the greater extent.

Steroid hormone receptors

In the three decades since the original discovery of receptors for steroid hormones, much has been learned about the biochemical processes by which these regulatory agents exert their effects in target tissues. The intracellular receptor proteins are potential transcription factors, needed for optimal gene expression in hormone-dependent cells. They are present in an inactive form until association with the hormone converts them to a functional state that can react with target genes. Transformation of the receptor protein to the nuclear binding form appears to involve the removal of both macromolecular and micromolecular factors that act to keep the receptor form reacting with DNA. Much of the native receptor is present in the nucleus, loosely bound and readily extractable, but for some and possibly all steroid hormones, some receptor is in the cytoplasm, perhaps in equilibrium with a nuclear pool. Methods have been developed for the stabilization, purification, and characterization of receptor proteins, and through cloning and sequencing of their cDNAs, primary structures for these receptors are now known. This has led to the recognition of structural similarities among the family of receptors for the different steroid hormones and to the identification of regions in the protein molecule responsible for the various aspects of their function. Monoclonal antibodies recognizing specific molecular domains are available for most receptors. Despite the knowledge that has been acquired, many important questions remain unsolved. How does association with the steroid remove factors keeping the receptor protein in its native state, and how does binding of the transformed receptor to the response element in the promoter region enhance gene transcription? Once it has converted the receptor to the nuclear binding state, is there a further role for the steroid in modulating transcription? Still not entirely clear is the involvement of phosphorylation and/or dephosphorylation in hormone binding, receptor transformation, and transcriptional activation. Less vital to basic understanding but important in the overall picture is whether the native receptors for gonadal hormones are entirely confined to the nucleus or whether there is an intracellular distribution equilibrium. With the effort now being devoted to this field, and with the application of new experimental techniques, especially those of molecular biology, our understanding of receptor function is progressing rapidly. The precise mechanism of steroid hormone action should soon be completely established.

Pyridoxal phosphate inhibits the DNA-binding activity of the ecdysteroid receptor

The effect of pyridoxal 5'-phosphate on the binding of the ecdysteroid receptor from a nuclear extract of Drosophila melanogaster to DNA-cellulose was studied. The binding of hormone-receptor complexes to DNA-cellulose was completely blocked after a 30-min incubation with 3 mM pyridoxal 5'-phosphate at 0-4 degree C. The effect was specific for pyridoxal 5'-phosphate since related compounds (pyridoxal, pyridoxamine 5'-phosphate and pyridoxamine) were not effective or gave only 17% inhibition (pyridoxal). Under standard conditions, none of the compounds tested exerted a significant effect on the stability of [3H](20R,22R)-2 beta,3 beta, 14 alpha,20,22-pentahydroxy-5 beta-cholest-7-en-6-one ([3H]ponasterone A)-receptor complexes. The loss of DNA-binding activity caused by pyridoxal 5'-phosphate is accompanied by changes in the molecular properties of [3H]ponasterone-A-receptor complexes. A shift of [3H]ponasterone-A binding was observed from the 8.0-8.5 S to the 4.5-5.0 S region, when [3H]ponasterone-A-receptor complexes were exposed to pyridoxal 5'-phosphate during sucrose-gradient centrifugation. The inhibition of DNA-cellulose binding by pyridoxal 5'-phosphate can be reversed. Probably, pyridoxal 5'-phosphate forms a Schiff base with a critical lysine group of the ecdysteroid receptor, presumably at its DNA-binding site. The hormone-receptor complexes obtained after removal of pyridoxal 5'-phosphate had the same affinity for DNA-cellulose as 'native' complexes. DNA-cellulose-bound [3H]ponasterone-A complexes were efficiently eluted from DNA-cellulose with pyridoxal 5'-phosphate in 0.1 M KCl resulting in a 104-fold purification of the ecdysteroid receptor. The results reflect possible structural similarities between ecdysteroid and vertebrate steroid receptors.

Effect of zinc and/or pyridoxine deficiency upon oestrogen retention and oestrogen receptor distribution in the rat uterus

A deficiency of vitamin B6 has been reported to enhance oestrogen responsiveness of the uterus in rats whereas zinc deficiency provokes a syndrome suggestive of a diminution in oestrogen sensitivity. In this study [3H]oestrogen uptake by the uterus was increased in rats deficient in either nutrient and the differences were additive in the dually deficient animals. The total number of oestrogen receptors per g tissue was unaffected by either nutrient but the proportion of the receptors recovered from the nuclear fraction increased from about 6 to 74% when both nutrients were withheld. The results are consistent with the hypothesis that both zinc and pyridoxal phosphate play important metabolic roles in end-organ responsiveness to oestrogen.

Reversible dissociation of chick oviduct progesterone receptor subunits

We report here the reversible dissociation of chick oviduct progesterone receptor subunits by pyridoxal 5'-phosphate. This agent has been reported to inhibit binding of steroid hormone receptors to DNA and nuclei (Cake et al., 1978). We have found that pyridoxal 5'-phosphate inhibits binding of chick oviduct progesterone receptors to DNA-cellulose, and also dissociates 6-8S cytosolic receptor aggregates to 4S monomers. Both of these effects are shown to be reversible if pyridoxal phosphate is removed, allowing in vitro reconstitution of receptor aggregates. Fidelity of reconstitution has been assessed by testing the reconstituted aggregate for binding to DNA-cellulose, phosphocellulose, and by studies using sedimentation velocity measurements. By these three criteria, the reconstituted product is indistinguishable from the native cytosol complex from which the monomers were derived. The reconstitution reaction shows an absolute requirement for the presence of both receptor monomers A and B. Titration experiments show a molar ratio of 1:1 for A and B in the reconstituted aggregates. These reconstitution studies confirm our hypothesis (originally based upon dissociation experiments) that native receptor aggregates are composed of the A and B proteins as subunits.

Covalent modification of the solubilized rat liver vitamin K-dependent carboxylase with pyridoxal-5'-phosphate

The carboxylation of the pentapeptide substrate, Phe-Leu-Glu-Glu-Ile, by a rat microsomal vitamin K-dependent carboxylase was stimulated two- to threefold at pyridoxal-5'-P concentrations between 0.5 and 1.0 mM. This stimulation was reduced at concentrations higher than 1.0 mM. The Km for the pentapeptide was lowered twofold in the presence of 1 mM pyridoxal-5'-P. The activation by pyridoxal-5'-P is specific, as 1 mM pyridoxal, pyridoxine, pyridoxine-5'-P, pyridoxamine, pyridoxamine-5'-P, or 4-pyridoxic acid did not stimulate the pentapeptide carboxylation rate. All six analogs, as well as formaldehyde and acetaldehyde, inhibited the carboxylation reaction in a concentration-dependent manner. The activation of the carboxylase by pyridoxal-5'-P appeared to be mediated by its direct binding to the enzyme via Schiff base formation. Sodium borohydride reduction of solubilized microsomes in the presence of pyridoxal-5'-P, followed by dialysis to remove unbound material, resulted in a carboxylase preparation with a specific activity twice that of the untreated control microsomes. The derivatized enzyme was not further stimulated by added pyridoxal-5'-P. This derivatized carboxylase could be obtained in the absence of pentapeptide and divalent cations. The stimulation of the carboxylase activity by divalent cations and pyridoxal-5'-P was mediated at separate site(s) on the enzyme. Studies of the NH2-terminal pyridoxalated pentapeptide with both a normal and PLP-modified enzyme, in the presence and absence of PLP, demonstrated competition of the pentapeptide PLP moiety to a PLP site on the enzyme. It was concluded that pyridoxal-5'-P forms a covalent attachment to an epsilon-NH2 of a lysine near the active site of the carboxylase.

Localization of pyridoxal phosphate binding site on the mero-receptor domain of the glucocorticoid receptor

Previous studies have demonstrated that the vitamin pyridoxal phosphate can alter the physicochemical properties of glucocorticoid receptors. We now report the localization of a pyridoxal phosphate binding site within the mero-receptor domain of this glucocorticoid receptor. Mero-glucocorticoid receptors that are generated by trypsin (10 micrograms/ml) or chymotrypsin (100 micrograms/ml) digestion of intact receptors sediment as 2.6 S species on 5-20% sucrose gradients in the presence or absence of pyridoxal phosphate. Mero-glucocorticoid receptors prepared by exogenous proteinases are hydrophobic and show no affinity for DEAE Bio-Gel A. Treating either trypsin-generated or chymotrypsin-generated mero-receptors with pyridoxal phosphate rapidly converts the proteins (60 and 35%, respectively) into forms that bind to DEAE Bio-Gel A. Induction of DEAE binding is specific to pyridoxal phosphate, for treating mero-receptors with pyridoxal, pyridoxamine or pyridoxine phosphate is ineffective. Furthermore, DEAE binding cannot be induced by adding other pyridoxal phosphate-treated cytosols to untreated mero-receptors. High-resolution polyacrylamide gel isoelectric focussing studies indicated that treating mero-receptor generated by either proteinase with pyridoxal phosphate shifted the isoelectric points of both to lower pH values. The conversion of the mero-receptor to a more acidic form also occurred when the intact glucocorticoid receptor was treated with the vitamin prior to proteolysis. These studies localize at least one pyridoxal phosphate binding site on the mero-receptor domain of the rat thymocyte glucocorticoid receptor.

Heparin-mediated inactivation and transformation of mammary cytoplasmic glucocorticoid receptor

Glucocorticoid receptor of lactating mouse mammary gland cytosol was exposed to heparin when the receptor was either steroid-free or steroid-bound. Heparin caused a dose-dependent and time-dependent loss of steroid binding activity (inactivation) of the steroid-free receptor; this heparin-induced inactivation was inhibited by molybdate. In contrast, steroid-bound receptor maintained its steroid binding capacity in the presence of heparin but the heparin caused transformation of receptor as detected by increased binding to DNA-cellulose and ATP-Sepharose. Heparin also converted steroid-bound receptor from the 7-8S form to the 4S form. Molybdate inhibited both the heparin-induced transformation and associated conversion to the 4S form.

Increased target tissue uptake of, and sensitivity to, testosterone in the vitamin B6 deficient rat

Six-week old male rats were maintained for 4 weeks on a vitamin B6-free diet to cause a moderately severe degree of vitamin B6 depletion. This led to a significant reduction in the circulating concentration of testosterone in plasma (control = 8.36 +/- 1.68, deficient = 2.13 +/- 0.54 nmol/l), but had no effect on circulating concentrations of luteinizing hormone, or, in intact males, on the weight of the prostate relative to body weight. In both intact and 24-h castrated animals vitamin B6 deficiency resulted in a significant increase in the uptake of [3H]testosterone into the prostate, and both increased and prolonged the specific nuclear retention of the steroid, as assessed by the ratio of radioactivity in the nuclear pellet: the high speed supernatant fraction. The results suggest that vitamin B6 has a function in the action of testosterone (and other steroid hormones), possibly in the recycling of receptors from the nucleus back into the cytosol after initial translocation. Vitamin B6 deficient animals have either a reduced rate of synthesis of testosterone or an increased rate of metabolic clearance compared with vitamin B6 supplemented controls, and this appears to be associated with enhanced target organ response to the hormone.

Comparison of pyridoxal phosphate and 0.4 M KCl-extracted nuclear glucocorticoid receptors in HeLa S3 cells

A comparison of the physicochemical properties between pyridoxal 5'-phosphate- and 0.4 M KCl-extracted nuclear glucocorticoid receptors has been made utilizing HeLa S3 cells as a source of receptor. Both pyridoxal 5'-phosphate/NaBH4-reduced and 0.4 M KCl-extracted receptors sedimented as approximately 3.5-4.5 S species in 5-20% sucrose gradients containing 0, 0.15, and 0.4 M KCl. Under low-ionic-strength buffer conditions, pyridoxal 5'-phosphate-extracted receptor elutes close to the void volume of a Sephacryl S-300 gel-exclusion column. Increasing the [KCl] of the column to 0.4 M resulted in the elution of receptor with a Stokes radius of 58 A and calculated Mr = 96,000. Nuclear receptors extracted with 0.4 M KCl also formed a large-molecular-weight complex which eluted close to the void volume of the gel-exclusion column. Increasing the [KCl] to 0.4 M had the effect of shifting this receptor form to a species which had a Stokes radius of 62 A and calculated Mr = 89,700. Ion-exchange analysis of nuclear-extracted receptors revealed that 0.4 M KCl-extracted receptors exhibited considerable charge heterogeneity, whereas pyridoxal 5'-phosphate-extracted receptors did not. Pyridoxal 5'-phosphate-extracted receptors (approximately 86%) eluted from DEAE-cellulose at a [KCl] greater than 0.15 M; approximately 14% of the receptors had little affinity for DEAE-cellulose. Pyridoxal phosphate-treated receptors had little affinity for hydroxylapatite, phosphocellulose, and DNA-cellulose. The predominant form of 0.4 M KCl-extracted nuclear receptors (approximately 78%) eluted from DEAE-cellulose between 0.05 and 0.15 M KCl, a position coincident with "activated" glucocorticoid receptors. The remaining receptor fraction (approximately 22%) eluted from DEAE-cellulose at a [KCl] greater than 0.15 M, a position coincident with "unactivated" glucocorticoid receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Convulsant activity of pyridoxal sulphate and phosphonoethyl pyridoxal: antagonism by GABA and its synthetic analogues

Pyridoxal phosphate and its synthetic analogues--pyridoxal 5'-sulphate and the 5-phosphonoethyl analogue of pyridoxal (phosphonoethyl pyridoxal) in doses of 0.125-0.250 (mumol/10 microliters/i.c.v./rat), caused epileptic seizures characterized by running fits, vocalization, muscular fasciculation and tonic-clonic convulsions. These effects were specific and could not be demonstrated with 5'-deoxypyridoxal, N-methylpyridoxal phosphate or the 5-trans-carboxyethenyl analogue of pyridoxal phosphate (carboxyethenyl pyridoxal). Structure-activity relationships of these analogues indicated that the presence of a CHO in position 4 of the pyridine ring was essential, since its conversion to CH2NH2 or CH2OH abolished activity. The presence of an unsubstituted N was essential, since convulsions did not occur with N-methylpyridoxal phosphate. The presence of the hydroxyl group in position 5' was essential since 5'-deoxypyridoxal was inactive. The convulsive activity was potentiated in the presence of both CHO and PO4, CHO and CH2--CH2PO2-4 but especially CHO and --OSO23-- groups. This seizure activity was prevented, attenuated or reversed by intracerebroventricular administration of 20 microliter of GABA (1 mumol), muscimol (0.025 mumol), trans-4-aminocrotonic acid (0.25 mumol), isoguvacine (0.25 mumol) or THIP (0.25 mumol), but not by biogenic amines. An understanding of the mechanism of pyridoxal phosphate-related seizures may provide additional insights not only about GABA receptor sites but also about the biochemical manifestation and expression of epilepsy.

Effects of vitamin B6 nutritional status on the uptake of [3H]-oestradiol into the uterus, liver and hypothalamus of the rat

Rats were depleted of vitamin B6 by treatment with isoniazid, and then maintained on diets providing: no vitamin B6, an adequate amount (1.2 mg/kg diet) or a very large amount (120 mg/kg) for 5 weeks. The uptake of a tracer dose of [3H]-oestradiol into the nuclei of liver and uterus was significantly greater in deficient animals than in those receiving an adequate or greater than adequate amount of the vitamin. Similarly the accumulation of oestradiol in the region of the brain corresponding to the hypothalamus, pre-optic area and septum (the major oestradiol-sensitive regions of the central nervous system) was higher in deficient animals than in other groups. There were significant inverse correlations between the uptake of oestradiol into target tissues and vitamin B6 nutritional status as determined by the concentrations of pyridoxal phosphate in plasma and liver. Uteri from deficient animals were significantly smaller than those from animals receiving an adequate or greater amount of vitamin B6, and the induction of uterine peroxidase by oestradiol was impaired. It therefore seems likely that despite the greater net accumulation of steroid in target tissues, vitamin B6 deficiency impairs biological responsiveness to oestrogens. It is suggested that this may be due to a failure of the recycling of oestradiol receptors from the nucleus.

Pyridoxal 5-phosphate protection of trypsin cleavage site on the glucocorticoid receptor protein

Pyridoxal 5-phosphate (PLP) treated rat liver triamcinolone acetonide complex was trypsin-digested followed by hydroxylamine treatment. This complex displayed a considerable fraction of DNA binding capacity of the intact complex, which was lost if trypsin digestion had not been preceded by PLP treatment. At the same time trypsin reduced the size of the complex regardless PLP pretreatment. These findings reveal that DNA binding does not require a large complex.

Pyridoxal phosphate blocks aggregation of molybdate treated glucocorticoid receptors in HeLa S3 cells

We have compared the effects of sodium molybdate and pyridoxal phosphate on the sedimentation properties of cytoplasmic glucocorticoid receptors. Whole HeLa S3 cells were incubated with [3H]-Dexamethasone (Dex) at 0-4 degrees C to produce unactivated cytoplasmic steroid receptors. When these cells are lysed in 10 mM Tris, 1 mM EDTA, 12 mM alpha-thioglycerol pH 7.4 (buffer A), [3H]-Dex receptor sediment as 7-8S species in sucrose gradients prepared in the same buffer. Sedimentation of receptors on gradients containing buffer A plus 20 mM sodium molybdate (buffer B) results in an approximately 9S species. Treatment of these receptors with pyridoxal phosphate (10 mM) followed by reduction with NaBH4 and sedimentation on sucrose gradients prepared in either buffer A or B results in the production of 3-4S species. Although [3H]-Dex labeled cells lysed in buffer B yield approximately 9S species when analyzed in gradients prepared in buffer B, centrifugation of these receptors in buffer A yield 7-8S species. Similarly when cells are lysed in buffer A labeled receptors sediment in sucrose gradients prepared in buffer B as approximately 9S species. Receptors prepared in buffer B and treated with pyridoxal phosphate and NaBH4 sediment in buffer B gradients as two discrete approximately 7S and approximately 3.5S species. Pyridoxal phosphate and NaBH4 treated receptors prepared in buffer B sediment as disperse 3-6S species when analyzed in buffer A gradients. Based on these observations we conclude that pyridoxal phosphate, NaBH4 reduced receptors are not particularly susceptible to subsequent molybdate action, but molybdate pre-treated receptors can be influenced by pyridoxal phosphate.

Pyridoxal-5-phosphate affects glucocorticoid receptors in intact lymphocytes similarly as in cell-free systems

Pyridoxal-5-phosphate markedly decreased the heat-stability of the unbound thymus cytosol receptor as well as that of the receptor-[3H]-triamcinolone acetonide complex in cell-free systems. Treatment with pyridoxal phosphate also decreased DNA-binding of the complex, being present either before or after heat- and salt-activation. 5 mM pyridoxal phosphate was required for 50% inhibition of DNA-binding. Pyridoxine, at similar concentrations, had no inhibitory effect. Pre-incubation of intact thymocytes with 10 mM pyridoxine caused also a marked decrease in the binding of [3H]-triamcinolone acetonide by the cells. Treating the cells with liposomes containing 1-100 mM pyridoxal phosphate caused an increase in intracellular pyridoxal phosphate concentration by about 0.1-10 microM and a decrease in [3H]-triamcinolone acetonide binding of the cells by about 50%. The results suggest that 1. pyridoxal phosphate acts not only on the activated but also on the unbound and non-activated forms of glucocorticoid receptor in cell-free systems; 2. pyridoxal phosphate has a similar effect in the intracellular millieu and thus, 3. pyridoxal phosphate might act as a physiologic regulator of the steroid hormone action.

The polynucleotide binding sites of estradiol receptor complexes

As a model for interaction of steroid receptors with DNA, the binding of estradiol receptor complexes (E2R) to oligodeoxynucleotides, covalently linked to cellulose, was studied in detail. Binding was optimal at concentrations of monovalent cationic salts at, or near, isotonic levels and was selective for intracellular receptors in contrast to extracellular steroid binding proteins. Among the oligomers, the order of affinity was oligo dG greater than oligo dT greater than oligo dC greater than oligo dA greater than oligo dI. The binding to oligo dG was stable to 37 degrees C exposure and the processes of adsorption and desorption, while reactivity with oligo dT, oligo dC and oligo dA was labile. The decrease in binding following purification was restored by histone 2B. Oligo dG binding was the most resistant to inhibition by cibacron blue F3GA (CB) and pyridoxal-5-phosphate. On the basis of these data, a hypothesis is proposed for the interaction of mouse uterine cytosol E2R with prevalent nonspecific and putative specific sequences of DNA.

Drug-pyridoxal phosphate interactions

In this review it has been pointed out that vitamin B6 and its vitamers can be involved in many interactions with a number of drugs, as well as with the actions of various endocrines and neurotransmitters. Nutritional deficiencies, especially of vitamins and proteins, can affect the manner in which drugs undergo biotransformation, and thereby may also modify the therapeutic efficacy of certain drugs. The differences between nutritional vitamin B6 deficiency and the hereditary disorder producing pyridoxine dependency are discussed. In addition to a pyridoxine deficiency being able to adversely affect drug actions, the improper supplementation with vitamin B6 can in some instances also adversely affect drug efficacy. A decrease by pyridoxine in the efficacy of levodopa used in the treatment of Parkinsonism is an example. The interrelationships and enzymatic interconversions among pyridoxine vitamers, both phosphorylated and non-phosphorylated, are briefly discussed, particularly regarding their pharmacokinetic properties. The ways in which the normal biochemical functions of vitamin B6 may be interfered with by various drugs are reviewed. (1) The chronic administration of isoniazid for the prevention or treatment of tuberculosis can produce peripheral neuropathy which can be prevented by the concurrent administration of pyridoxine. An acute toxic overdose of isoniazid causes generalized convulsions, and the intravenous administration of pyridoxine hydrochloride will prevent or stop these seizures. (2) The acute ingestion of excessive monosodium glutamate will, in some individuals, cause a group of symptoms including among others headache, weakness, stiffness, and heartburn, collectively known as the 'Chinese Restaurant Syndrome.' These symptoms can be prevented by prior supplementation with vitamin B6. The beneficial effect is ascribed to the correction of a deficiency in the activity of glutamic oxaloacetic transaminase, an enzyme that is dependent on pyridoxal phosphate. Some interesting relationships are pointed out between vitamin B6, picolinic acid, and zinc. It is postulated that the intestinal absorption of zinc is facilitated by picolinic acid, a metabolite of tryptophan. The derivation of picolinic acid from tryptophan depends on the action of the enzyme kynureninase, which is dependent on pyridoxal phosphate; therefore, the adequate absorption of zinc is indirectly dependent on an adequate supply of vitamin B6. The formation of pyridoxal phosphate, on the other hand, appears to be indirectly dependent on Zn2++ which activates pyridoxal kinase.(ABSTRACT TRUNCATED AT 400 WORDS)

Extraction of nuclear androgen receptors from rat prostate with different reagents

After in vitro labelling of androgen receptors in prostate tissue from castrated rats, about 70% of the labelled androgens in nuclei could be extracted with buffer solutions with 0.4 M KCl, or 10 mM pyridoxal phosphate, or 0.4 mM Cibacron blue, or heparin (0.2 mg/ml). In the nuclear extracts, 60-80% of the steroid was recovered as steroid-receptor complex. Sedimentation values of the receptors, on sucrose gradients containing 0.4 M KCl, were 3-4 S for the KCl extract and 4-5 S for pyridoxal phosphate and heparin extracts. The Cibacron blue extract contained an aggregated form of the receptor (5-7 S). The presence of the protease inhibitor di-isopropylfluorophosphate during isolation caused a small increase in S value of the receptors. However, the differences in sedimentation values between KCl, pyridoxal phosphate and heparin extracts remained. The receptors in the KCl extracts could be precipitated with protamine sulphate only after addition of 10 mM pyridoxal phosphate to the extracts. On the basis of these results it is concluded that the androgen receptors in rat prostate can be effectively extracted from nuclei by certain reagents which have in common a strongly negatively charged group and a less polar or hydrophobic region. These reagents form complexes with nuclear receptors and influence the sedimentation values and precipitability with protamine sulphate of these receptors.

Activation of progesterone receptor by ATP

Progesterone-receptor complex from freshly prepared hen oviduct cytosol acquired the ability to bind to isolated nuclei, DNA-cellulose and ATP-Sepharose when incubated with 5-10 mM ATP at 4 degrees C. The extent of this ATP-dependent activation was higher when compared with heat-activation achieved by warming the progesterone-receptor complex at 23 degrees C. The transformation of progesterone-receptor complex which occurred in a time-dependent manner was only partially dependent on hormone presence. The ATP effect was selective in causing this transformation whereas ADP, AMP and cAMP failed to show any such effect. The non-hydrolyzable analogs of ATP, adenosine 5'-[alpha, beta-methylene]triphosphate and adenosine 5-[beta, gamma-imido]triphosphate were also found to be ineffective. Presence of 10 mM sodium molybdate blocked both the ATP and the heat-activation of progesterone-receptor complex. Mn2+ and Mg2+ had no detectable effect on the receptor activation but the presence of Ca2+ increased the extent of ATP-activation slightly. EDTA presence (greater than 5 mM) decreased the extent of receptor activation by about 40% and was, therefore, not included in the buffers used for activation studies. Divalent cations were also ineffective when tested in the presence of 1-5 mM EDTA. The steroid-binding properties of progesterone-receptor complex remained intact under the above conditions when analyzed for steroid-binding specificity and Scatchard analysis. However, the ATP-activated progesterone-receptor complex lost the ability to aggregate when tested on low-salt sucrose gradients. ATP was equally effective in activating the rat-uterine-estradiol-receptor complex at 4 degrees C and influenced the transformation of 4-S receptor form into a 5-S form when analyzed on sucrose gradients containing 0.3 M KCl. The presence of ATP also increased the rate of activation of progesterone-receptor complex at 23 degrees C. These findings suggest a role for ATP in receptor function and offer a convenient method of studying the process of receptor activation at low temperature and mild assay conditions.

In vitro binding to an in vivo effect on the cytosol and nuclear progesterone receptors of various progestins, and their relationship to synthesis of uteroglobin in rabbit uterus

In vitro binding affinities of various progestins to cytosol and nuclear progesterone receptors of rabbit uterus were determined and correlated with the biological potency of these steroids. In addition, cytosol and nuclear progesterone receptor levels were measured after a 5-day administration of different progestins (0.5 mg/kg daily) with variable biologic activities. The receptor levels were compared with the biological response; the induction of uteroglobin synthesis. Cytosol and nuclear progesterone receptors had identical steroid binding properties (r = 0.98). The correlation between the in vitro binding affinity (cytosol or nuclear) and the in vivo biologic activity of the steroids was good (r = 0.73). After a 5-day treatment with progestins, the nuclear receptor concentration correlated n an inverse manner (r = -0.84) with the uterine fluid uteroglobin concentration. A similar, but slightly weaker correlation (r = -0.81) was also found for the cytosol receptor content and uteroglobin secretion. These data indicate that not only nuclear, but also cytosol progesterone receptor levels decrease in the rabbit uterus during chronic hormone action. Decline in the nuclear progesterone receptor content seemed to occur during treatment with all progestational steroids, while only progestins with high biological potency were capable of decreasing the cytosol receptor content.

Uterine progesterone receptor: stabilization and physicochemical alterations produced by sodium molybdate

Incubation of hamster uterine cytosol with millimolar concentrations of sodium molybdate prior to addition of labeled steroid increased recovery of progesterone receptor 2-fold. This stabilizing effect of molybdate was also manifest on gel electrophoresis of the receptor. In the absence of molybdate, no specific [3H]progesterone binding was detectable on polyacrylamide gel electrophoresis. But, in the presence of 5 mM sodium molybdate, a [3H]progesterone-binding species was clearly evident on the gels. The radioactivity associated with this binding was displaceable by unlabeled progesterone but not by cortisol and depended on the concentration of [3H]progesterone employed, suggesting that this binding species is a progesterone receptor. Molybdate treatment produced a small increase in receptor size on low-salt sucrose gradients (from 6-7 to S to 7.5 S). There was no effect of molybdate of receptor sedimentation in the presence of high salt (0.3 M KCl). Further analysis of this phenomenon by gel filtration suggested that this molybdate-mediated increase in receptor size was due to receptor aggregation. In low-salt buffers, molybdate treatment markedly increased the proportion of receptors contained in large aggregates (Stokes radius greater than 8.0 nm). Again, this effect was abolished in the presence of high salt. In conjunction with receptor stabilization, molybdate prevented binding of uterine progesterone receptor to DNA--cellulose. These findings suggest that sodium molybdate stabilizes the unliganded, unactivated form of the receptor. Moreover, these effects seem to be mediated through a direct interaction of molybdate with the receptor, one which results in receptor aggregation.

Diethylpyrocarbonate inhibition of estradiol receptor binding to oligonucleotides

Exposure of calf uterine estradiol-receptor complexes to diethylpyrocarbonate (ethoxyformic anhydride) at pH 6.3-6.5 results in a decrease in the ability of the receptor to bind to oligodeoxyribonucleotides. The inhibition of binding to oligodeoxypyrimidines is greater than the inhibition of binding to oligodeoxyguanylate. The inhibition by 6.6 mM diethylpyrocarbonate is complete within 10 min at 4 degrees C. Addition of equimolar quantities of histidine or imidazole prior to exposure to diethylpyrocarbonate prevents subsequent inhibition of oligodeoxyribonucleotide binding. In comparison to histidine, other amino acids tested were deficient in this ability. Diethylpyrocarbonate modification of the receptor causes complete loss of oligodeoxyribonucleotide binding activity at times when there is a loss of less than 20% of bound steroid. Pyridoxal 5'-phosphate treatment of receptor does not prevent subsequent modification by diethylpyrocarbonate, suggesting that the site of reaction is not an essential lysine of the DNA-binding domain. Treatment of the ethoxyformylated receptor with 0.45 M hydroxylamine results in recovery of 70% of the receptor's oligonucleotide-binding ability. The time course of the reaction of diethylpyrocarbonate with the estradiol receptor and the demonstration of hydroxylamine reversal of inhibition suggest that histidine is involved in the binding of estradiol receptor to oligodeoxyribonucleotides.

Effect of quinoline-type antimalarial drugs on the binding of oestradiol-17 beta and progesterone by rabbit and human uterine cytosols

Rabbit and human uterine cytosol, prepared and tested in phosphate buffer, bound less oestradiol-17 beta or progesterone than cytosol from the same source prepared and tested in Tris-HCl buffer. Dissociation constants were the same in both buffer systems, and the difference in binding was due to a difference in the number of binding sites. Three quinoline-type antimalarial drugs, chloroquine, quinine and mefloquine, and the quinoline derivative, 4-(4'-hydroxy-1'-methylbutylamino)-7-chloroquinoline, increased the steroid binding capacity of phosphate-buffered cytosol to that of Tris-buffered cytosol, the optimal concentration of quinoline derivative being 1.4-1.6 mM. Tris (50 mM) increased the binding capacity of phosphate-buffered cytosol to that of Tris-buffered cytosol. The effects of Tris and quinoline derivatives were not additive. By gel chromatography and sucrose density gradient centrifugation it was shown that the molecular size and sedimentation behaviour of the oestradiol and progesterone receptors were not affected by the quinoline derivatives. Two types of binding site are proposed, one requiring the presence of low molecular weight, basic compounds. The uterine levels of chloroquine attained by normal pharmacological doses of the drug are potentially capable of influencing the binding of oestradiol-17 beta and progesterone in the uterine cytosol.