Cell-type-specific and site-specific N-glycosylation of type I and type II human tissue plasminogen activator

Tissue plasminogen activator (t-PA) is an important initiator of fibrinolysis. The t-PA polypeptide has four potential N-glycosylation sites of which three are occupied in type I (Asn-117, -184, and -448) and two in type II (Asn-117 and -448). In an effort to elucidate the factors controlling the expression of N-linked oligosaccharides on this polypeptide, we have used a combination of sequential exoglycosidase digestion, methylation analysis, and controlled acetolysis to determine the oligosaccharide structures at each of the N-glycosylation sites of type I and type II t-PA when isolated from a human colon fibroblast cell strain and from a Bowes melanoma cell line. Our results suggest the following: (i) type I and type II t-PA are N-glycosylated in an identical way at Asn-117 and Asn-448, when isolated from the same cell line; (ii) Asn-117 is predominantly associated with oligomannose-type structures in all cases; (iii) Asn-184 and Asn-448 are predominantly associated with complex-type structures when t-PA is isolated from fibroblast cells, but with both complex- and oligomannose-type structures when isolated from melanoma cells; (iv) fibroblast cell derived t-PA is associated with both neutral and sialylated oligosaccharides, while melanoma cell derived t-PA is also associated with sulfated oligosaccharides, which are located exclusively at Asn-448 of type II t-PA; (v) no complex-type structures occur in common between t-PA from the two cell lines. These results indicate that the t-PA glycoprotein is secreted by each cell line as a set of glycoforms, each glycoform being unique with respect to the nature and disposition of oligosaccharides on a common polypeptide. Further, the two cell lines express no glycoform in common, despite expressing the same t-PA polypeptide. The implications of these results for both the control of oligosaccharide processing in different cell lines and the genetic engineering of mammalian glycoproteins are discussed.     

Glycosylation at Asn-184 inhibits the conversion of single-chain to two-chain tissue-type plasminogen activator by plasmin

Tissue-type plasminogen activator (tPA) is a glycosylated serine protease which is an effective thrombolytic agent. Native single-chain tPA (sc-tPA) is converted to two-chain tPA (tc-tPA) by plasmin, the product of the reaction of plasminogen with tPA. Native sc-tPA occurs as two glycoforms. Type I sc-tPA is fully glycosylated, while type II lacks glycosylation at Asn-184. The rates at which type I and type II human melanoma sc-tPA were converted to type I and type II tc-tPA by plasmin were determined by two different methods. In each case, the second-order rate constant (kcat/Km) for type II sc-tPA (approximately 8 microM-1 s-1) was about twice that for type I sc-tPA (approximately 4 microM-1 s-1). These results indicate that glycosylation at Asn-184 hinders the conversion of sc-tPA to tc-tPA and suggest that under physiological conditions type I sc-tPA may persist in the single-chain form longer than type II sc-tPA. Previous studies have shown that type I tc-tPA has a lower activity than type II tc-tPA and that sc-tPA has a lower activity and susceptibility to inhibition when compared to tc-tPA. The present work provides further evidence that tPA glycosylation serves to modulate activity. The two major glycoforms may represent more persistent but slow acting (type I) and less persistent but faster acting (type II) variants of tPA.     

Effects of N-glycosylation on in vitro activity of Bowes melanoma and human colon fibroblast derived tissue plasminogen activator

Tissue-type plasminogen activator (t-PA), when isolated from human colon fibroblast (hcf) cells, is N-glycosylated differently than when isolated from the Bowes melanoma (m) cell line (Parekh et al., 1988). Both hcf- and m-t-PA can be separated into type I t-PA (with three occupied N-glycosylation sequons, at Asn-117, -184, and -448) and type II t-PA (with two occupied sequons, at Asn-117 and -448). Oligosaccharide analysis of each of these types of t-PA indicates that hcf-t-PA and m-t-PA have no glycoforms in common, despite having the same primary amino acid sequence. We have therefore compared in vitro the enzymatic activities and fibrin binding of type I and type II hcf- and m-t-PA with those of aglycosyl t-PA isolated from tunicamycin-treated cells. Plasminogen activation kinetics were determined by using an indirect amidolytic assay with Glu-plasminogen and a chromogenic plasmin substrate. In the absence of stimulator, there was little difference in activity between type I and type II t-PA, but the activity of aglycosyl t-PA was 2-4-fold higher than that of the corresponding glycosylated t-PA. In the presence of a fibrinogen fragment stimulator, the Kcat value of type II t-PA was approximately 5-fold that of type I t-PA from the same cell line, while the Km values for activation of Glu-plasminogen were similar (0.13-0.18 microM). The stimulated activity of glycosyl t-PA was similar to that of type II t-PA.(ABSTRACT TRUNCATED AT 250 WORDS)     

Carbohydrate structure of recombinant human uterine tissue plasminogen activator expressed in mouse epithelial cells

Recombinant human uterine tissue plasminogen activator (tPA), in part metabolically labeled with [6-3H]glucosamine or [35S]sulfate, was isolated from mouse epithelial cells (C127). Oligosaccharides present were liberated by treatment of tryptic glycopeptides with endo-beta-N-acetylglucosaminidase H or peptide-N4-(N-acetyl-beta-glucosaminyl)asparagine amidase F and fractionated by high-performance liquid chromatography. The glycans were characterized by digestion with exoglycosidases, methylation analysis and, in part, by acetolysis and 1H-NMR spectroscopy. Glycopeptides comprising individual glycosylation sites were identified by N-terminal amino acid sequencing. The results demonstrate that recombinant tPA from C127 cells carries at Asn117 oligomannosidic glycans with 5-8 mannose residues as well as small amounts of hybrid-type species. Asn184 is only partially glycosylated and substituted by fucosylated triantennary and small amounts of diantennary N-acetyllactosaminic glycans. Likewise, Asn448 carries predominantly fucosylated triantennary species, in addition to, small amounts of diantennary and tetraantennary oligosaccharides. As a characteristic feature, part of the triantennary glycans at Asn184 and Asn448 contain additional Gal(alpha 1-3) substituents and/or sulfate groups linked to position six of beta-galactosyl residues forming NeuAc(alpha 2-3)[HO3S-6]Gal(beta 1-4) units. Oligosaccharides attached to Asn448 are almost completely substituted by (alpha 2-3)- or (alpha 2-6)-linked sialic acid residues and carry the majority of sulfate groups present. Glycans at Asn184 were found to be less sialylated and sulfated.     

Carbohydrate structures of human tissue plasminogen activator expressed in Chinese hamster ovary cells

Recombinant human tissue plasminogen activator (rt-PA), produced by expression in Chinese hamster ovary cells, is a fibrin-specific plasminogen activator which has been approved for clinical use in the treatment of myocardial infarction. In this study, the structures of the Asn-linked oligosaccharides of Chinese hamster ovary-expressed rt-PA have been elucidated. High mannose and hybrid oligosaccharides were released from the protein by endoglycosidase H digestion, whereas N-acetyllactosamine-type ("complex") oligosaccharides were released by peptide:N-glycosidase F digestion. The oligosaccharides were fractionated by gel permeation chromatography and anion exchange high performance liquid chromatography (HPLC), and their structures were analyzed by composition and methylation analysis, high pH anion exchange chromatography, fast atom bombardment-mass spectrometry (FAB-MS), and 500-MHz 1H NMR spectroscopy. High mannose oligosaccharides were found to account for 38% of the total carbohydrate content of rt-PA and consisted of Man5GlcNAc2, Man6GlcNAc2, and Man7GlcNAc2 in the ratio 1.8:1.7:1. Two hybrid oligosaccharides were identified and accounted for 3% of the carbohydrate of rt-PA. The N-acetyllactosamine-type oligosaccharides were found to comprise diantennary (34% of total carbohydrate), 2,4-branched triantennary (11%), 2,6-branched triantennary (9%), and tetraantennary (5%) structures. Sialylation of these oligosaccharides was by alpha (2----3) linkages to galactose. Most (greater than 90%) of the N-acetyllactosamine-type structures contained fucose alpha (1----6) linked to the Asn-linked N-acetylglucosamine residue. The distribution of oligosaccharide structures at individual glycosylation sites (Asn residues 117, 184, and 448) was also determined. rt-PA exists as two variants that differ by the presence (type I) or absence (type II) of carbohydrate at Asn-184. Tryptic glycopeptides were isolated by reversed phase high performance liquid chromatography and treated with peptide:N-glycosidase F. The oligosaccharides released from each glycosylation site were analyzed by high pH anion exchange chromatography. By this analysis, Asn-117 was demonstrated to carry exclusively high mannose oligosaccharides. When glycosylated, Asn-184 carried diantennary, 2,4-branched triantennary, 2,6-branched triantennary, and tetraantennary N- acetyllactosamine oligosaccharides in the ratio 9.0:4.5:1.4:1. Asn- 448 carried the same types of oligosaccharides, but in the ratio 7.5:1.6:2.1:1. The distributions of Asn-linked oligosaccharides at positions 117 and 448 were found not to be affected by the presence or absence of carbohydrate at position 184. The relevance of the     

Isolation and characterization of three different carbohydrate chains from melanoma tissue plasminogen activator

Electrophoretic analysis of endoglycosidase-treated tissue plasminogen activator obtained from human melanoma cells showed that the heterogeneity observed for the protein in these preparations is caused by an N-glycosidically linked N-acetyllactosamine type of carbohydrate chain which is present in about 50% of the molecules. An oligomannose type and an N-acetyllactosamine type of glycan is present in all molecules. Three glycopeptides were isolated and characterized by 1H-NMR, sugar determination, methylation analysis and amino acid determination. The exact attachment site for each of the three glycans could be deduced from the amino acid compositions of the glycopeptides. Asn-117 carries the oligomannose type of glycan, the structure of which was completely determined. Asn-184 is the site where the presence or absence of a biantennary N-acetyllactosamine type of glycan causes the size heterogeneity. The third N-glycosylation site, Asn-448, was found to carry a triantennary or tetraantennary N-acetyllactosamine type of carbohydrate chain.     

Differential binding of cAMP-dependent protein kinase regulatory subunit isoforms Ialpha and IIbeta to the catalytic subunit

Limited trypsin digestion of type I cAMP-dependent protein kinase holoenzyme results in a proteolytic-resistant Delta(1-72) regulatory subunit core, indicating that interaction between the regulatory and catalytic subunits extends beyond the autoinhibitory site in the R subunit at the NH(2) terminus. Sequence alignment of the two R subunit isoforms, RI and RII, reveals a significantly sequence diversity at this specific region. To determine whether this sequence diversity is functionally important for interaction with the catalytic subunit, specific mutations, R133A and D328A, are introduced into sites adjacent to the active site cleft in the catalytic subunit. While replacing Arg(133) with Ala decreases binding affinity for RII, interaction between the catalytic subunit and RI is not affected. In contrast, mutant C(D328A) showed a decrease in affinity for binding RI while maintaining similar affinities for RII as compared with the wild-type catalytic subunit. These results suggest that sequence immediately NH(2)-terminal to the consensus inhibition site in RI and RII interacts with different sites at the proximal region of the active site cleft in the catalytic subunit. These isoform-specific differences would dictate a significantly different domain organization in the type I and type II holoenzymes.     

Kinetics of activated thrombin-activatable fibrinolysis inhibitor (TAFIa)-catalyzed cleavage of C-terminal lysine residues of fibrin degradation products and removal of plasminogen-binding sites

Partial digestion of fibrin by plasmin exposes C-terminal lysine residues, which comprise new binding sites for both plasminogen and tissue-type plasminogen activator (tPA). This binding increases the catalytic efficiency of plasminogen activation by 3000-fold compared with tPA alone. The activated thrombin-activatable fibrinolysis inhibitor (TAFIa) attenuates fibrinolysis by removing these residues, which causes a 97% reduction in tPA catalytic efficiency. The aim of this study was to determine the kinetics of TAFIa-catalyzed lysine cleavage from fibrin degradation products and the kinetics of loss of plasminogen-binding sites. We show that the k(cat) and K(m) of Glu(1)-plasminogen (Glu-Pg)-binding site removal are 2.34 s(-1) and 142.6 nm, respectively, implying a catalytic efficiency of 16.21 μm(-1) s(-1). The corresponding values of Lys(77)/Lys(78)-plasminogen (Lys-Pg)-binding site removal are 0.89 s(-1) and 96 nm implying a catalytic efficiency of 9.23 μm(-1) s(-1). These catalytic efficiencies of plasminogen-binding site removal by TAFIa are the highest of any TAFIa-catalyzed reaction with a biological substrate reported to date and suggest that plasmin-modified fibrin is a primary physiological substrate for TAFIa. We also show that the catalytic efficiency of cleavage of all C-terminal lysine residues, whether they are involved in plasminogen binding or not, is 1.10 μm(-1) s(-1). Interestingly, this value increases to 3.85 μm(-1) s(-1) in the presence of Glu-Pg. These changes are due to a decrease in K(m). This suggests that an interaction between TAFIa and plasminogen comprises a component of the reaction mechanism, the plausibility of which was established by showing that TAFIa binds both Glu-Pg and Lys-Pg.     

N-acetylimidazole inactivates renal Na,K-ATPase by disrupting ATP binding to the catalytic site

Treatment of renal Na,K-ATPase with N-acetylimidazole (NAI) results in loss of Na,K-ATPase activity. The inactivation kinetics can be described by a model in which two classes of sites are acetylated by NAI. The class I sites are rapidly reacting, the acetylation is prevented by the presence of ATP (K0.5 congruent to 8 microM), and the inactivation is reversed by incubation with hydroxylamine. These data suggest that the class I sites are tyrosine residues at the ATP binding site. The second class of sites are more slowly reacting, not protected by ATP, nor reversed by hydroxylamine treatment. These are probably lysine residues elsewhere in the protein. The associated K-stimulated p-nitrophenylphosphatase activity is inactivated by acetylation of the class II sites only; thus the tyrosine residues associated with ATP binding to the catalytic center are not essential for phosphatase activity. Inactivated enzyme no longer has high-affinity ATP binding associated with the catalytic site, although low-affinity ATP effects (inhibition of phosphatase and deocclusion of Rb) are still present. The inactivated enzyme can still be phosphorylated by Pi, occlude Rb+ ions, and undergo the major conformational transitions between the E1 Na and E2 K forms of the enzyme. Thus acetylation of the Na,K-ATPase by NAI inhibits high-affinity ATP binding to the catalytic center and produces inactivation.     

Expression of a Novel Chimeric-Truncated tPA in Pichia pastoris with Improved Biochemical Properties

Thrombolytic therapy by plasminogen activators (PAs) has been a main goal in the treatment of acute myocardial infarction. Despite improved outcomes of currently available thrombolytic therapies, all these agents have different drawbacks that may result in less than optimal outcomes. In order to make tissue plasminogen activator (tPA) more potent, while being more resistant to plasminogen activator inhibitor-1 (PAI-1) and having a higher affinity to fibrin, a new chimeric-truncated form of tPA (CT tPA) was designed and expressed in Pichia pastoris. This novel variant consists of a finger domain of Desmoteplase, an epidermal growth factor (EGF) domain, a kringle 1 (K1) domain, a kringle 2 (K2) domain, in which the lysine binding site (LBS) was deleted, and a protease domain, where the four amino acids lysine 296, arginine 298, arginine 299, and arginine 304 were substituted by aspartic acid. The chimera CT tPA showed 14-fold increase in its activity in the presence of fibrin compared to the absence of fibrin. Furthermore, CT tPA showed about 10-fold more potency than commercially available full-length tPA (Actylase^®) and provided 1.2-fold greater affinity to fibrin. A residual activity of only 68 % was observed after incubation of Actylase^® with PAI-1, however, 91 % activity remained for CT tPA. These promising findings suggest that the novel CT tPA variant might be an acceptable PA with superior characteristics and properties.     

Functional organization and evolution of mammalian hexokinases: mutations that caused the loss of catalytic activity in N-terminal halves of type I and type III isozymes.

Mammalian hexokinases are believed to have evolved from a 100-kDa hexokinase which itself is a product of duplication and fusion of an ancestral gene encoding a 50-kDa glucose 6-phosphate-sensitive hexokinase. Type II hexokinase has been shown to possess two distinct functional active sites, one in each half, which functionally resemble the original 100-kDa hexokinase, whereas type I and III isozymes possess only one active site in the C-terminal halves. This study was conducted to identify which mutations caused the loss of catalytic activity in the N-terminal halves of type I and III isozymes. Arg 174 and Ser 447 in type I isozyme and Asp 244 in type III isozyme are speculated to be the cause, because they reside adjacent to the "catalytic" site and corresponding residues, Gly 174, Asp 447, and Gly 231, are conserved in the N-terminal half of type II isozyme as well as all other 50-kDa units that possess catalytic activity. Mutations G174R and D447S in the N-terminal half of type II isozyme reduced specific activity by approximately 79 and 57%, respectively. Therefore, neither mutation alone can account for the inactivation of the N-terminal active site in type I isozyme. Either mutation, G174R or D447S, had moderate effects on Michaelis constants, K(m), for glucose and ATP. Mg(2+). Intriguingly, mutation D447S introduced a novel inhibition by unchelated ATP (K(i) = 68 microM ATP, competitive vs ATP. Mg(2+)) to the N-terminal active site of type II isozyme. Mutation G231D caused instability to type II hexokinase and near complete loss of catalytic activity (95%), suggesting that mutation G231D not only hinders catalysis at the N-terminal active site but also leads to structural instability in type II hexokinase.     

Vampire bat salivary plasminogen activator exhibits a strict and fastidious requirement for polymeric fibrin as its cofactor, unlike human tissue-type plasminogen activator. A kinetic analysis.

The vampire bat salivary plasminogen activator (BatPA) is virtually inactive toward Glu-plasminogen in the absence of a fibrin-like cofactor, unlike human tissue-type plasminogen activator (tPA) (the kcat/Km values were 4 and 470 M-1 s-1, respectively). In the presence of fibrin II, tPA and BatPA activated Glu-plasminogen with comparable catalytic efficiencies (158,000 and 174,000 M-1 s-1, respectively). BatPA's cofactor requirement was partially satisfied by polymeric fibrin I (54,000 M-1 s-1), but monomeric fibrin I was virtually ineffective (970 M-1 s-1). By comparison, a variety of monomeric and polymeric fibrin-like species markedly enhanced tPA-mediated activation of Glu-plasminogen. Fragment X polymer was 2-fold better but 9-fold worse as cofactor for tPA and BatPA, respectively, relative to fibrin II. Fibrinogen, devoid of plasminogen, was a 10-fold better cofactor for tPA than fibrinogen rigorously depleted of plasminogen, Factor XIII, and fibronectin; the enhanced stimulatory effect of the less-purified fibrinogen was apparently due to the presence of Factor XIII. By contrast, the two fibrinogen preparations were equally poor cofactors of BatPA-mediated activation of Glu-plasminogen. BatPA possessed only 23 and 4% of the catalytic efficiencies of tPA and two-chain tPA, respectively, in hydrolyzing the chromogenic substrate Spectrozyme tPA. However in the presence of fibrin II, BatPA and tPA exhibited similar kcat/Km values for the hydrolysis of Spectrozyme tPA. Our data revealed that BatPA, unlike tPA, displayed a strict and fastidious requirement for polymeric fibrin I or II. Consequently, BatPA may preferentially promote plasmin generation during a narrow temporal window of fibrin formation and dissolution.     

Non-carrier-mediated uptake of the mannosidase I inhibitor 1-deoxymannojirimycin by K562 erythroleukemic cells

A 3H label was introduced at the C-1 position of the mannosidase I inhibitor 1-deoxymannojirimycin (dMM) by catalytic hydrogenolysis of benzyl-2,3-O-isopropylidene-5-N-benzyl-6-O-benzyl-alpha-D-mannofurano side with 3H2. 1-[3H]dMM as well as its precursor 1-[3H]2,3-O-isopropylidene-dMM had identical Rf as the nonradioactive compounds on TLC. Furthermore, alpha 1-antitrypsin secreted by HepG2 cells was modified indistinguishably by treatment of the cells with dMM and 1-[3H]dMM. Thus, 1-[3H]dMM had chemical and biological properties identical with authentic dMM. Uptake of [14C]mannose by K562 cells could be inhibited by glucose but not by the mannose analogue dMM. Thus, dMM does not enter the cell through hexose transporter(s). Uptake of 1-[3H]dMM by K562 cells could not be inhibited by increasing concentrations of nonradioactive dMM (from 1-32,000 microM), showing transport of dMM into cells through nonfacilitated diffusion. Furthermore, uptake of 1-[3H]dMM by K562 cells was observed at 0 degrees C.     

The consequences of introducing an autophosphorylation site into the type I regulatory subunit of cAMP-dependent protein kinase

The type I and type II regulatory subunits of cAMP-dependent protein kinase can be distinguished by autophosphorylation. The type II regulatory subunits have an autophosphorylation site at a proteolytically sensitive hinge region, while the type I regulatory subunits have a pseudophosphorylation site. Only holoenzyme formed with type I regulatory subunits has a high affinity binding site for MgATP. In order to determine the functional consequences of regulatory subunit phosphorylation on interaction with the catalytic subunit, an autophosphorylation site was introduced into the type I regulatory subunit using recombinant DNA techniques. When Ala97 at the hinge region of the type I regulatory subunit was replaced with Ser, the regulatory subunit became a good substrate for the catalytic subunit. Stoichiometric phosphorylation occurred exclusively at Ser97. Radioactivity was incorporated primarily into the recombinant regulatory subunit when catalytic subunit and [gamma-32P]ATP were added to the total bacterial extract. Phosphorylation of the mutant regulatory subunit also occurred readily following polyacrylamide gel electrophoresis and electrophoretic transfer to nitrocellulose. Phosphorylation occurred as an intramolecular event in the absence of cAMP indicating that the hinge region of the regulatory subunit occupies the substrate recognition site of the catalytic subunit in the holoenzyme complex. Holoenzyme formed with both the wild type and mutant regulatory subunits was susceptible to dissociation in the presence of high salt; however, only the native holoenzyme was stabilized by MgATP. In contrast to the wild type holoenzyme, the affinity of the mutant holoenzyme for cAMP was not reduced in the presence of MgATP. Holoenzyme formation also was not facilitated by MgATP.     

Three catalytic sites in mitochondrial ATPase

Kinetic data obtained after determining the hydrolytic activity of ATPase from rat liver in preparations where the enzyme had been purified, or in mitochondria, strongly suggest the existence of three different catalytic sites with different affinity for the substrate. The results obtained when measuring the ATPase activity at different substrate concentrations, and in the presence of the inhibitors KOCN or KSCN, or of the activators dinitrophenol and bicarbonate, show that the binding of these compounds to a regulatory site or sites affects in a different degree the hydrolytic activity of each catalytic site.     

Cytosolic Type II Estrogen Binding Site in Rat Uterus: Specific Photolabeling with Estrone

Abstract

A low affinity (Kd = 30 nM), large capacity (Bmax = 2.6 pmol/g tissue) estrogen binding site was photolabeled from estradiol-stimulated rat uterus cytosol. To maximize levels of this binding site and reduce those of the type I binding site, ovariectomized rats were injected with high doses of estradiol (10μg per day) for four days with the last injection two hours before sacrifice. This treatment depleted type I estrogen receptors from the cytosol (by 90%) and raised levels of type II sites in the nucleus without affecting cytosolic type II levels. The type II estradiol binding sites were distinguished from the type I sites on the basis of their dissociation kinetics, pH-sensitivity and their behavior towards potassium chloride, somatostatin, sodium thiocyanate, sulfhydryl reagents and ammonium sulfate precipitation. These type II binding sites could be covalently photolabeled with tritiated estrone. A molecular weight of 43 kDa was found on SDS PAGE.     

Purification and characterization of seven distinct forms of liver microsomal cytochrome P-450 from untreated and inducer-treated male Wistar rats

A total of nine forms of cytochrome P-450 were purified to homogeneity from liver microsomes of male Wistar rats. They were P-451 I and P-451 II from untreated rats, P-450 II and P-450 III from phenobarbital-treated rats, MC-P-448 L and MC-P-448 H from 3-methylcholanthrene-treated rats, and P-452, P-448 L, and P-448 H from 3,4,5,3',4'-pentachlorobiphenyl-treated rats. Among them, MC-P-448 L and MC-P-448 H were indistinguishable from P-448 L and P-448 H, respectively, with regard to electrophoretic, spectral, catalytic and immunochemical properties, and thus seven forms were distinct hemoproteins. The minimal molecular weight of each form was as follows: P-451 I (49,000), P-451 II (52,000), P-450 II (52,000), P-450 III (53,500), P-452 (48,000), P-448 L (56,000), P-448 H (54,000). Judging from the oxidized absolute spectra, P-448 H was a high-spin form and the others were of low-spin type. In a reconstituted system, N-demethylations of benzphetamine and aminopyrine were catalyzed by most of the forms at comparable rates. On the other hand, the activities for the oxidations of benzo[a]pyrene, 7-ethoxycoumarin, biphenyl, and estradiol-17 beta varied greatly among the forms of cytochrome P-450. The most efficient catalysts were as follows: P-448 L and P-451 II for benzo[a]pyrene 3-hydroxylation; P-448 L for 7-ethoxycoumarin O-deethylation; P-448 L, P-451 II, and P-448 H for biphenyl 4-hydroxylation; P-448 L and P-448 H for biphenyl 2-hydroxylation; and P-451 II and P-448 H for estradiol 2-hydroxylation. P-451 I, P-450 II, and P-450 III were somewhat poorer catalysts in metabolizing all the substrates except for benzphetamine and aminopyrine, but their substrate specificities were still distinguishable from one another. Of all the purified cytochrome P-450's, P-452 showed the least ability to metabolize all the substrates. Judging from the properties, it appears that six forms in male Wistar rats correspond to the distinct forms of cytochrome P-450 in Long-Evans and/or Sprague-Dawley rats reported by other workers, but P-451 I is a new constitutive isozyme in Wistar rats.     

Characterization and partial purification of an estrogen type II binding site in chick oviduct cytosol

An estrogen binding site of moderate affinity (Kd approximately 10 nM) and high capacity (approximately 25-70 pmol/g of tissue) was measured in DES-stimulated chick oviduct cytosol. Saturation analysis by [3H]estradiol exchange demonstrated that this binding site displayed sigmoidal binding characteristics suggesting a cooperative binding mechanism. Competition analysis with a number of compounds demonstrated that the bioflavonoid luteolin was a better competitor for binding to type II sites in chick than either estradiol or DES. Steroid specificity was demonstrated by the inability of 17 alpha-estradiol, progesterone, testosterone, corticosterone, and the triphenylethylene antiestrogen nafoxidine (U-1100A) to compete for [3H]-17 beta-estradiol binding to chick oviduct cytosol preparations. In addition, the binding site appeared to be sensitive to sulfhydryl reducing reagents as evidenced by a 75% reduction in binding activity in the presence of dithiothreitol. Both prelabeling and postlabeling procedures used in conjunction with Sephacryl S-300 chromatography resulted in a single major peak of type II binding activity representing a molecular weight in the 40,000 range. Type II binding activity was recoverable after precipitation with ammonium sulfate, and this material was subjected to a variety of column chromatography procedures in order to achieve further purification of the type II site. Significant purification of the site was achieved with a bioflavonoid-Sepharose (quercetin-Sepharose) affinity matrix. The purified type II sites eluted from quercetin-Sepharose displayed the same sigmoidal binding curves characteristic of native cytosol.     

Steady-state kinetic properties of FoF1-ATPase: the pH effect.

1. The kinetic properties of FoF1-ATPase from submitochondrial particles isolated from rat heart were studied, with emphasis to the pH effect. The velocity data were treated according to the Hill equation, and the results were discussed on the basis of the knowledge on the soluble F1-ATPase properties. 2. Three kinetic phases were observed in the range of pH 6.0-8.5, with apparent dissociation constant values (K0.5) of 0.001, 0.04 and 1.5 mM (respectively sites I, II and III) at pH 7.0. Their contribution to the total activity of the enzyme were pH-dependent on the range of 6.0-7.0, but not from 7.0 to 8.5, where the maximal velocity (V) for site III was some 4-fold larger than for site II, and the total V of sites II and III was some 40-fold larger than V assumed for site I. Therefore, two catalytic sites seem to participate significantly in the catalysis at steady-state condition. 3. Azide increased the sites II and III K0.5 values as well as decreased the site III V. In the presence of bicarbonate these two sites were not distinguishable, and the kinetic parameters at pH 7.0 were similar to those for sites II and III combined. Both azide and bicarbonate did not have a significant effect on site I, and this behavior was not pH-dependent. 4. The studies on the effect of pH on the kinetic parameters showed the following results: (1) the optimum pH for V was around 8.5; (2) decrease in the K0.5 values at pH below 7.0 for site II, and increase at pH over 7.0 for sites II and III; (3) in the pH range of 6.0-8.5 the Hill coefficient increased for site II, decreased for site III, and an intermediary effect was observed for the sites II and III combined, with a Michaelis-Menten behavior in the highest affinity pH, which was found in the physiological range.     

Low Dose Ru486 Prevents Progesterone Antagonism on Uterine Growth and Concomitant Type II Oestradiol Binding Inducton by Oestradiol in Rats

Abstract

The effect of RU486, a synthetic antisteroid, on the antagonism of progesterone (Pg) and dexamethasone (Dex) against oestradiol (Oe) induced uterine growth, and on uterine oestradiol binding (type I and type II sites) was studied in ovariectomised CFY rats. Changes of hypothalamic low affinity [3H]Oe binding have also been evaluated. Inhibitory effects of Pg but not of Dex on uterine growth and type II Oe binding site induction were prevented by RU486. Antiprogestin effect of RU486 could also be demonstrated on low affinity [3H]Oe binding in hypothalami. The inhibitory effect of dexamethasone on type II Qe binding was not opposed by antisteroid, on the contrary, RU486 seemed to potentiate this effect of Dex. Evaluation of type I binding was complicated by the distorting effect of type II binding at the saturation curve. Changes of type I binding seemed to parallel those of type II binding except after Oe+Dex treatment where an increased level of uterine cytoplasmic type I sites and a simultaneous decrease of type II sites were found. Blockage of [3H]Oe binding at high RU486 concentrations was found in vitro in the uterine cytoplasmic fraction. A less pronounced effect was observed in the nuclear fraction. Possible mechanisms of the RU486 effect on type II Oe binding are discussed.