Specificity in the enzymic transfer of sialic acid to the oligosaccharide branches of BI- and triantennary glycopeptides of α1-acid glycoprotein

Partial $\text{in}\text{vitro}$ sialylation of biantennary and triantennary glycopeptides of α₁-acid glycoprotein using colostrum β-galactosideα(2→6) sialyltransferase followed by high resolution ¹H-NMR spectroscopic analysis of the isolated products enabled the assignment of the $\text{Galβ(1→4)GlcNAcβ(1→2)Man}\text{α(1→3)}\text{Man}$ branch as the most preferred substrate site for sialic acid attachment. The $\text{Galβ(1→4)GlcNAcβ(1→2)Man}\text{α(1→6)}\text{Man}$ branch appeared to be much less preferred and the Galβ(1→4)GlcNAcβ(1→4)Manα(1→3)Man sequence of triantennary structures was of intermediate preference for the sialyltransferase. The specificity of the β-galactoside α(2→6) sialyltransferase is thus shown to extend to structural features beyond the terminal $\text{N}$-acetyllactosamine units on the oligosaccharide chains of serum glycoproteins.     

An uncommon fucosyl linkage in surface membranes of human neuroblastoma cells

The surface membranes of human neuroblastoma cells contain a fucosyl linkage, defined by using an α-L-fucosidase from almond emulsin specific for the cleavage of Fucα1→3G1cNAc and Fucα1→4G1cNAc. These linkages are not found in significant amounts on the surface of mouse neuroblastoma cells, or human or hamster fibroblasts. The enzyme released fucose from glycoproteins as well as glycopeptides, making it particularly useful for $\text{in}$$\text{vivo}$ studies.     

A 3H-labelled trisaccharide from heparin as substrate for acetyl:CoA: 2-amino-2-deoxy-α-d-glucoside N-acetyltransferase

The tetrasaccharide fraction obtained by gel chromatography after treatment of commercially available heparin with nitrous acid was reduced with NaB³H₄ and then hydrolysed with 2m trifluoracetic acid at 70° for 3 days. By gel chromatography and electrophoresis, the ³H-labelled trisaccharide 1 bearing an unsubstituted 2-amino-2-deoxy-d-glucosyl group in the non-reducing position was obtained (18% from the ³H-labelled tetrasaccharide). By sequential, enzymic degradation, the structure $\text{α-}\text{d}\text{-GlcN}\text{-(1→4)-β-}\text{d}\text{-GlcA}\text{-(1→4)-[1-}{}^3\text{H]aManol}$ was obtained for 1, which is a substrate for acetyl-CoA: 2-amino-2-deoxy-α-d-glucoside N-acetyltransferase, an enzyme that is deficient in the Sanfilippo C syndrome. In human-skin fibroblasts, the pH optimum of acetyl transfer onto 1 was between pH 5.5 and 7.0, and dependent on the buffer. An apparent K_m for 1 of 0.14mM was found.     

The presence of two GDP-L-fucose: glycoproteine fucosyltransferases in human serum

Two soluble fucosyltransferases have been demonstrated in human serum. One enzyme transfers l-fucose from GDP-l-fucose to the terminal galactose residues of lactose, N-acetyllactosamine, and sialidase-treated α₁-acid glycoprotein, to form the blood group H determinant, α-l-fucosyl-(1 → 2)-β-d-galactosyl-R. The second enzyme transfers fucose to the terminal N-acetylglucosamine residue of sialidase-, β-galactosidase-treated α₁-acid glycoprotein. Serum from a donor with the rare “Bombay” O_h blood group (genotype hh) cannot transfer fucose to terminal galactose residues but has normal levels of the enzyme acting on sialidase-, β-galactosidase-treated α₁-acid glycoprotein. This observation, as well as mixed substrate experiments, demonstrate that the two fucosyltransferase activities are due to two separate enzymes. The GDP-l-fucose:galactoside fucosyltransferase has a pH optimum of 5.5 and the following K_m values: lactose, 31 mm; N-acetyllactosamine, 7.5 mm; sialidase-treated α₁-acid glycoprotein, 6.4 mm. The GDP-l-fucose: N-acetylglucosaminide fucosyltransferase has a pH optimum of 5.0 and a K_m for sialidase-, β-galactosidase-treated α₁-acid glycoprotein of 1.2 mm. The serum GDP-l-fucose: N-acetylglucosaminide fucosyltransferase is distinct from the blood group Lewis-dependent enzyme in milk since the serum enzyme is present in serum from Le (a-b-)donors and since the Le-dependent fucosyltransferase could not be demonstrated in serum from donors carrying the Le gene.     

Urinary excretion of a novel hexasaccharide and glycopeptide analogue in fucosidosis.

Repeated Biogel P6 chromatography of the urine from a patient with fucosidosis yielded several fractions containing fucosyloligosaccharides and glycopeptides. Two of these were shown by ¹H nuclear magnetic resonance (¹H-n.m.r.) spectroscopy and permethylation analysis to have the following structures respectively: (I) αfuc (1→3) [βgal (1→4)] βglcNAc (1→2) αman ($\text{1→}\text{3}\text{6}$) βman (1→4) glcNAc and (II) αfuc (1→3) [βgal (1→4)] βglcNAc (1→2) αman ($\text{1→}\text{3}\text{6}$) βman (1→4) βglcNAc (1→4) [αfuc ($\text{1→}\text{3}\text{6}$)] βglcNAc-Asn.     

The effect of α1-acid glycoprotein (orosomucoid) on triglyceride metabolism in the nephrotic syndrome

We have identified α₁-acid glycoprotein as a new co-factor in the lipoprotein lipase reaction. We isolated an active form of the compound from nephrotic urine that is effective both $\text{in vitro}$ and $\text{in vivo}$. α₁-acid glycoprotein increased lipolysis 100% in the presence of C-II apolipoprotein in a lipoprotein lipase assay system. Rats with induced nephrotic syndrome showed a decrease in triglyceride clearance. $\text{T}\text{1}\text{2}$ was increased from 14 min to 43 min. The injection of α₁-acid glycoprotein restored the lipid clearance to normal. These findings suggest that elevated plasma triglycerides in human nephrotic patients is the direct result of excessive loss of α₁-acid glycoprotein from plasma into urine. We propose that replacement therapy may be possible.     

Comparison of isolated peanut agglutinin receptor glycoproteins from human,bovine and porcine erythrocyte membranes

Affinity chromatography has been used to isolate and compare the peanut agglutinin receptors from neuraminidase-treated human, bovine and porcine erythrocyte membranes. Passage of Triton X-100-solubilised membrane material through either Sepharose- or acrylamide-based affinity columns resulted in the reversible binding of receptor molecules to the immobilised lectin. Elution with 0.2M galactose released specifically bound glycoprotein fractions, the composition and molecular weights of which were determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate.Carbohydrate analysis by gas chromatography identified these bound glycoprotein fractions as the major sources of the $\text{O-}\text{glycosidic-linked}$ disaccharide $\text{galactosyl}\text{-β-(1 → 3)-N-}\text{acetylgalactosamine}$ in these membranes. It is suggested that these isolated fractions represent a discrete population of glycoproteins within the membranes studied, which possess both $\text{O-}\text{glycosidic}$- and $\text{N-}\text{glycosidic-linked}$ carbohydrates.     

The structure of rat liver mitochondria: a reevaluation

A β-N-acetylgalactosaminyltransferases (Ga1NAcT) that catalyzes the synthesis of a triglycosylceramide, GanglioTricer (Ga1NAcβ-Ga1β1-4G1c-cer), from lactosylceramide and UDP-Ga1NAc was isolated from guinea pig bone marrow. The enzyme was present in the supernatant solution obtained after homogenization of guinea pig bone marrow 12,000 × g pellet with 0.32 M sucrose containing 0.6% Triton X-100 and centrifugation at 129,000 × g. The enzyme that catalyzed the transfer of Ga1NAc to a tetraglycosylceramide (Lac-nTet-cer) was found in a membrane-bound fraction. The K_m values were 0.5 mM and 0.7 mM for the lactosylceramide and Lac-nTet-cer, respectively. 97.0% of the terminal [¹⁴C]Ga1NAc was cleaved by the action of pure β-hexosaminidase from [¹⁴C]triglycosylceramide.     

Structure of the ceramide octadekahexoside isolated from gastric mucosa

A fucose-containingceramide octadekahexoside exhibiting blood-group (A+H) activity has been isolated from hog gastric mucosa. Based on the results of partial acid hydrolysis, sequential degradation with specific glycosidases, oxidation with periodate and chromium trioxide, and permethylation analysis, we propose that the carbohydrate chain of this fucolipid contains four branches. Two of the branches are terminated by βGall→4βGlcNAc, one by $\text{αFucl→2βGall→}\text{3}\text{4}\text{βGlcNAc}$ and one by $\text{αGalNAcl→3(αFucl→2)βGall→}\text{3}\text{4}\text{βGlcNAc}$.     

Hydrogen bonding requirements for the insulin-sensitive sugar transport system of rat adipocytes

(1) The $\text{t}\text{1}\text{2}$ for 1.3 mM D-allose uptake and efflux in insulin-stimulated adipocytes is 1.7 ± 0.1 min. In the absence of insulin mediated uptake of D-allose is virtually eliminated and the uptake rate ($\text{t}\text{1}\text{2}\text{= 75.8 ± 4.99}\text{min}$) is near that calculated for nonmediated transport. The kinetic parameters for D-allose zero-trans uptake in insulin-treated cells are K_zt^oi = 271.3 ± 34.2 mM, V_zt^oi = 1.15 ± 0.12 mM · s^?1. (2) A kinetic analysis of the single-gate transporter (carrier) model interacting with two substrates (or substrate plus inhibitor) is presented. The analysis shows that the heteroexchange rates for two substrates interacting with the transporter are not unique and can be calculated from the kinetic parameters for each sugar acting alone with the transporter. This means that the equations for substrate analogue inhibition of the transport of a low affinity substrate such as D-allose can be simplified. It is shown that for the single gate transporter the K_i for a substrate analogue inhibitor should equal the equilibrium exchange K_m for this analogue. (3) Analogues substituted at C-1 show a fused pyranose ring is accepted by the transporter. 1-Deoxy-D-glucose is transported but has low affinity for the transporter. High affinity can be restored by replacing a fluorine in the β-position at C-1. The K_i for $\text{d}\text{-glucose}\text{= 8.62}\text{mM}$; the K_i for $\text{β-}\text{fluoro-}\text{d}\text{-glucose}\text{= 6.87}\text{mM}$. Replacing the ring oxygen also results in a marked reduction in affinity. The K_i for $\text{5-}\text{thio-}\text{d}\text{-glucose}\text{= 42.1}\text{mM}$. (4) A hydroxyl in the gluco configuration at C-2 is not required as 2-deoxy-d-galactose (K_i = 20.75 mM) has a slightly higher affinity than d-galactose (K_i = 24.49 mM). A hydroxyl in the manno configuration at C-2 interferes with transport as d-talose (K_i = 35.4 mM) has a lower affinity than d-galactose. (5) d-Allose (K_m = 271.3 mM) and 3-deoxy-d-glucose (K_i = 40.31 mM) have low affinity but high affinity is restored by substituting a fluorine in the gluco configuration at C-3. The K_i for $\text{3-}\text{fluoro-}\text{d}\text{-glucose}\text{= 7.97}\text{mM}$. (6) Analogues modified at C-4 and C-6 do not show large losses in affinity. However, 6-deoxy-d-glucose (K_i = 11.08 mM) has lower affinity than d-glucose and 6-deoxy-d-galactose K_i = 33.97 mM) has lower affinity than d-galactose. Fluorine substitution at C-6 of d-galactose restores high affinity. The K_i for $\text{6-}\text{fluoro-}\text{d}\text{-galactose}\text{= 6.67}\text{mM}$. Removal of the C-5 hydroxymethyl group results in a large affinity loss. The K_i$\text{d}\text{-xylose}\text{= 45.5}\text{mM}$. The K_i for $\text{l}\text{-arabinose}\text{= 49.69}\text{mM}$. (7) These results indicate that the important hydrogen bonding positions involved in sugar interaction with the insulin-stimulated adipocytes transporter are the ring oxygen, C-1 and C-3. There may be a weaker hydrogen bond to C-6. Sugar hydroxyls in non-gluco configurations may sterically hinder transport.     

Purification and characterization of an intracellular N-terminal exopeptidase from Streptococcus durans

An intracellular N-terminal exopeptidase isolated from cell extracts of Streptococcus durans has been purified 470-fold to homogeneity (specific activity of 12.0 μmol/min per mg). In the absence of thiol compounds, the purified aminopeptidase undergoes a slow oxidation with a 70% loss of activity, which can be restored by the addition of 2 mM β-mercaptoethanol. The purified aminopeptidase (M_r 300 000) preferred L-peptide and arylamide substrates with small nonpolar or basic side chains. SDS electrophoresis yielded a single protein band corresponding to a molecular weight of 49 400, suggesting that the native enzyme is a hexameric protein. The enzyme-catalyzed hydrolysis of $\text{L}\text{-}\text{alanyl}\text{-p-}\text{nitroanilide}$ exhibited a bell-shaped pH dependence for log V_max/K_m(pK₁ = 6.35; pK₂ = 8.50) while the log V_max versus pH profile showed only an acid limb (pK = 6.35). Methylene blue-sensitized photooxidation of the enzyme resulted in the complete loss of activity, while L-leucine, a competitive inhibitor, partially protected against this inactivation. Amino acid analysis indicated that this photooxidative loss of activity corresponded to the modification of one histidine residue per enzyme monomer. N-Ethylmaleimide (100 mM) caused a 78% reduction in enzyme activity. Treatment of the enzyme with 1.0 mM hydrogen peroxide resulted in the oxidation of two cysteine residues per enzyme monomer and caused a 70% decrease in the catalytic activity.     

Oligosaccharides accumulated in the kidney of a goat with β-mannosidosis: Mass spectrometry of intact permethylated derivatives

Two oligosaccharides accumulate in the kidney of a goat with β-mannosidosis. These oligosaccharides were isolated and purified from kidney extracts by Bio-Gel P2 gel permeation column chromatography. Their structures were characterized as Manβ1 → 4GlcNAc and Manβ1 → 4G1cNAcβ1 → 4G1cNAc by mass spectrometry of the permethylated intact oligosaccharide alcohols and permethylated native oligosaccharides. Carbohydrate composition analysis, methylation linkage studies, and enzymatic hydrolysis were also performed. Stored in 1 g of kidney were 1.6 μmol of disaccharide and 7.6 μmol of trisaccharide, which was three times that found in the brain of this affected animal (M. Z. Jones and R. A. Laine, 1981, J. Biol. Chem., 256, 5181–5184). In both the brain and kidney of the affected goat, oligosaccharide accumulation was evidently represented by membrane-bound, electron-lucent vacuoles in numerous cell types. While lesions in the brain were associated with profound neurological deficits, functional impairment of the kidney was not apparent. Similar oligosaccharides excreted in urine may be derived from those stored in the kidney. The mass spectrometric methods utilized in this investigation will facilitate comparison of oligosaccharide composition in different tissues and biological samples in β-mannosidosis and other disorders of glycoprotein catabolism.     

Heterozygosity for the IVS-I-5(G→C) mutation with a G→A change at codon 18 (Val→Met; Hb Baden) in cis and a T→G mutation at codon 126 (Val→Gly; Hb Djonburi) in trans resulting in a thalassemia intermedia

We have analyzed the hemoglobins of a young German patient with β-thalassemia intermedia and of his immediate family and included in these studies an evaluation of possible nucleotide changes in the β-globin through sequencing of amplified DNA. One chromosome of the propositus and one of his father's carried the $\text{G}\text{T}\text{G}\text{→}\text{G}\text{G}\text{G}$ mutation at codon 126 leading to the synthesis of Hb Dhoburi or α₂β₂126(H₄)Val→Gly; this variant is slightly unstable and is associated with mild thalassemic features. His second chromosome and one of his mother's had the common IVS-I-5 (G→C) mutation that leads to a rather severe β⁺-thalassemia and the $\text{G}\text{TG}\text{→}\text{A}\text{TG}$ mutation at codon 18, resulting in the replacement of a valine residue by a methionine residue. This newly discovered β-chain variant, named Hb Baden, was present for only 2–3% in both the patient and his mother. This low amount results from a decreased splicing of RNA at the donor splice-site of the first intron that is nearly completely deactivated by the IVS-I-5 (G→C) thalassemic mutation. The chromosome with the codon 18 ($\text{G}\text{TG}\text{→}\text{A}\text{TG}$) and the IVS-I-5 (G→C) mutations has thus far been found only in this German family; analysis of 51 chromosomes from patients with the IVS-I-5 (G→C) mutation living in different countries failed to detect the codon 18 ($\text{G}\text{TG}\text{→}\text{A}\text{TG}$) change.     

Reconstitution of ATPase activity from the isolated alpha, beta, and gamma subunits of the coupling factor, F1, of Escherichia coli.

The three major subunits (α, β and γ) of the coupling factor, F₁ ATPase, of $\text{Escherichia coli}$ were separated and purified by hydrophobic column chromatography after the enzyme was dissociated by cold inactivation. The ability to hydrolyze ATP was reconstituted by dialyzing the mixture of subunits against 0.05 M Tris-succinate, pH 6.0, containing 2 mM ATP and 2 mM MgCl₂. A mixture containing α, β and γ regained ATP hydrolyzing activity. Individual subunits alone or mixtures of any two subunits did not develop ATPase activity, except for a low but significant activity with α plus β. The reconstituted ATPase had a K_m of 0.23 mM for ATP and a molecular weight by sucrose gradient density centrifugation of about 280,000.     

Receptors for glucocorticoids in lung tissue

Gas chromatography-mass spectrometric identification of partially methylated aminosugars has been developed: (a) various kinds of O-methylated 2-deoxy-2-(N-methyl)-acetamidohexitols were prepared from partially O-(1-methoxy)-ethylated 2-deoxy-2-acetamidohexoses, and their gas chromatography-mass spectrometric patterns were determined; (b) permethylated glycolipids gave a satisfactory yield of 2-deoxy-2-N-methyl-amidohexoses by acetolysis with 0.5 n sulfuric acid in 95% acetic acid, followed by aqueous hydrolysis; (c) the resulting partially methylated aminosugars and neutral sugars were analyzed after borohydride reduction and acetylation according to the procedure of Lindberg and associates (Björndal, Lindberg and Svennson, 1967; Björndal, Hellerqvist, Lindberg and Svensson, 1970).This method was successfully applied to analysis of aminosugar linkages in blood group B-active ceramide pentasaccharide from rabbit erythrocytes and in Forssman antigen of equine spleen. The structure of blood group B-active glycolipid of rabbit erythrocyte was found to be Galα1 → 3Galβ1 → 4G1cNAcβ1 → 3Ga11 → 4Glc → Cer, and that of Forssman antigen to be GaNAcα1 → 3GalNAcβ1 → 3Galα1 → 4Ga11 → 4Glc → Cer.     

Immunochemical studies on the combining site of the d-galactopyranose and 2-acetamido-2-deoxy-d-galactopyranose specific lectin isolated from Bauhinia purpurea alba seeds

The combining site of the Bauhinia purpurea alba lectin was studied by quantitative precipitin and precipitin inhibition assays. Of 45 blood group substances, glycoproteins, and polysaccharides tested, 35 precipitated over 75% of the lectin. Precursor blood group substances with I activity (Cyst OG 10% from 20% and Cyst OG 20% from 10%), desialized fetuin, and desialized ovine salivary glycoprotein, in which more than 75% of the carbohydrate side chains have dGalN Ac linked through α1 → to the OH group of Ser or Thr of a protein core, completely precipitated the lectin. The poorly reactive blood group substances after mild acid hydrolysis or Smith degradation, as well as sialic acid-containing glycoproteins after removal of sialic acid, had substantially increased activity so that more than 80% of the lectin was precipitated. Precipitability with various blood group substances and glycoproteins is ascribable to the terminal nonreducing dGalNAc, dGalβ1 → 3dGalNAc, dGalβ1 → 3 or 4dGlcNAc, and dGalβ1 → 3 or 4dGlcNAcβ1 → 3dGal determinants on the carbohydrate moiety. Of the monosaccharides tested for inhibition of precipitation, dGalNAc and its p-nitrophenyl and methyl α-glycosides were best. These compounds were four to five times better than the corresponding dGal compounds but methyl βDGalNAcp was only about 40% more active than methyl βdGalp. The α-anomers of p-nitrophenyl DGalNAcp and dGalp, were twice as active as the corresponding β-anomers. Methyl αDGalNAcp was four times as active as the β-anomer but the inhibitory power of the methyl α- and β-anomers of dGal were about equal. Among the oligosaccharides tested, dGalβ1 → 3dGalNAc and its tosyl derivatives were most active, the tosyl glycosides being about twice as active as dGalβ1 → 3dGalNAc, which was somewhat more active than dGalNAcα1 → 6dGal and dGalNAc, and 2.5 and 5 times as active as dGalNAcα1 → 3dGalβ1 → 3dGlcNAc and dGalNAcαl → 3dGa1, respectively (blood group A specific). These findings suggest that a subterminal dGalNAc β-linked and substituted on carbon 3 plays an important role in binding. Consistent with this inference are the findings that dGalβ1 → 3dGlcNAc and dGalβ1 → 6dGal were poorer inhibitors although dGalβ1 → 3dGlcNAc was two to three times as active as glycosides of dGal. Oligosaccharides with terminal nonreducing dGal and subterminal α-linked dGal were as active or less active than dGal. dGalβ1 → 3dGlcNAcβ1 → 3dGalβ1 → 4dGlc (lacto-N-tetraose) and dGalβ1 → 3dGlcNAcβ1 → 3dGal-β1-O-(CH₂)₈COOCH₃ were equally active and 1.5 times as potent as dGalβ1 → 3dGlcNAc whereas dGalβ1 → 3dGlcNAcβ1 → 6dGal was only 40% as potent as dGalβ1 → 3dGlcNAc suggesting that a third sugar may be part of the determinant. Substitution of dGalβ1 → 3dGlcNAcβ1 → 3dGalβ1 → 4dGlc on the subterminal dGlcNAc by lFucα1 → 4 in lacto-N-fucopentaose II reduced activity fourfold; if the nonreducing dGal is substituted by lFucα1 → 3 as in lacto-N-fucopentaose I its activity is almost completely abolished. This suggests that a terminal nonreducing dGal as well as subterminal dGlcNAc are contributing to binding. The β → 3 linkage of the terminal dGal to the subterminal amino sugar is significant since dGalβ1 → 4dGlcNAc is a poorer inhibitor. Although the available data suggest that the combining site of the lectin Bauhinia purpurea alba may be most complementary to the structure dGalβ1 → 3dGalNAcβ1 → 3dGal, several other possibilities remain to be tested when suitable oligosaccharides become available.     

Endo-β-N-acetylglucosaminidase from fig latex

Commercially available fig latex contains several endo-β-N-acetylglucosaminidases which catalyze the reaction: (Man)_nG1cNAcβ₁→4G1cNAcAsn → (Man)_nG1cNAc + G1cNAcAsn. Using (NH₄)₂SO₄ fractionation followed by chromatography on Sephadex G-100 and DEAE-Sephadex A-50, two distinct types of endo-β-N-acetylglucosaminidases have been partially purified and characterized. One, called F-I, hydrolyzes the di-N-acetylchitobiosyl linkage in the glycopeptide, (Man)₃(G1cNAc)₂Asn prepared from human IgG, much faster than that linkage in the glycopeptides, (Man)₅(G1cNAc)₂Asn and (Man)₆(G1cNAc)₂Asn both from ovalbumin. The other, called F-II, hydrolyzes the same linkage in (Man)₅(G1cNAc)₂-Asn and (Man)₆(G1cNAc)₂Asn, but not that in (Man)₃(G1cNAc)₂Asn.     

Thiophosphorylation of (Na + K+)-ATPase yields an ADP-sensitive phosphointermediate

(1) Treatment of $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-ATPase}$ from rabbit kidney outer medulla with the $\text{γ-}{}^{35}\text{S}$ labeled thio-analogue of ATP in the presence of $\text{Na}{}^{\mathrm{+}}\text{+ Mg}{}^{\mathrm{2+}}$ and the absence of K⁺ leads to thiophosphorylation of the enzyme. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ value for [γ-S]ATP is 2.2 μM and for Na⁺ 4.2 mM at 22°C. Thiophosphorylation is a sigmoidal function of the Na⁺ concentration, yielding a Hill coefficient $\text{n}\text{H}\text{= 2.6}$. (2) The thio-analogue ($\text{K}{}_{\mathrm{<mtext>m</mtext><mtext>\; =\; 35\; \mu M</mtext>}}$) can also support overall $\text{(Na}{}^{\mathrm{+}}\text{+ K}{}^{\mathrm{+}}\text{)-ATPase}$ activity, but $\text{V}{}_{\mathrm{<mtext>max</mtext>}}$ at 37°C is only $\text{1.3 γ}\text{mol}\text{· (mg protein)}{}^{\mathrm{?}}\text{·}$ h^?1 or 0.09% of the specific activity for ATP ($\text{K}{}_{\mathrm{<mtext>m</mtext><mtext>\; =\; 0.43\; mM</mtext>}}$). (3) The thiophosphoenzyme intermediate, like the natural phosphoenzyme, is sensitive to hydroxylamine, indicating that it also is an acylphosphate. However, the thiophosphoenzyme, unlike the phosphoenzyme, is acid labile at temperatures as low as 0°C. The acid-denatured thiophosphoenzyme has optimal stability at pH 5–6. (4) The thiophosphorylation capacity of the enzyme is equal to its phosphorylation capacity, indicating the same number of sites. Phosphorylation by ATP excludes thiophosphorylation, suggesting that the two substrates compete for the same phosphorylation site. (5) The (apparent) rate constants of thiophosphorylation (0.4 s^?1 vs. 180 s^?1), spontaneous dethiophosphorylation (0.04 s^?1 vs. 0.5 s^?1) and K⁺-stimulated dethiophosphorylation (0.54 s^?1 vs. 230 s^?1) are much lower than those for the corresponding reactions based on ATP. (6) In contrast to the phosphoenzyme, the thiophosphoenzyme is ADP-sensitive (with an apparent rate constant in ADP-induced dethiophosphorylation of 0.35 s^?1, $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$$\text{ADP = 48 μM at 0.1 mM ATP}$) and is relatively K⁺-insensitve. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for K⁺ in dethiophosphorylation is 0.9 mM and in dephosphorylation 0.09 mM. The thiophosphoenzyme appears to be for 75–90% in the ADP-sensitive E₁-conformation.     

Purification of a water-soluble Mg2+-ATPase from human erythrocyte membranes

A water-soluble Mg²⁺-ATPase previously reported (White, M.D. and Ralston, G.B. (1976) Biochim. Biophys. Acta 436, 567–576) has been purified from human erythrocyte membranes. The purified enzyme has a molecular weight of 575 000; the apparent minimum molecular weight was 100 000, corresponding to a soluble protein of the component 3 region. The $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ value for ATP was 1 mM and apparent $\text{K}{}_{\mathrm{<mtext>m</mtext>}}$ for Mg²⁺ was 3.6 mM. By means of histochemical activity staining in acrylamide gels it was shown that the purified ATPase preparation could be inhibited by Cd²⁺ and Zn²⁺ salts, $\text{p-}\text{chloromercuribenzoate}$ and $\text{N-}\text{ethylmaleimide}$, known inhibitors of membrane endocytosis.     

Reaction of peptide thiobenzyl esters with mammalian chymotrypsinlike enzymes: A sensitive assay method

Sensitive methods for the determination of rat mast cell protease I, rat mast cell protease II, human skin chymotrypsinlike enzyme, dog skin chymotrypsinlike enzyme, human leukocyte cathepsin G, and bovine chymotrypsin A_α with peptide thiobenzyl ester substrates are reported. Kinetic constants as well as the maximum sensitivity for the hydrolysis of the peptide substrates succinyl-phenylalanyl-leucyl-phenylalanine thiobenzyl ester and succinyl-alanyl-alanyl-prolyl-phenylalanine thiobenzyl ester were determined. Hydrolysis rates were followed spectrophotometrically at 324 nm by the formation of 4-thiopyridone (? = 19,800 m^?1 cm^?1), the product of the reaction between benzylthiol, released during hydrolysis of the peptide thiobenzyl esters, and 4,4′-dithiodipyridine present in the assay mixture. Peptide thiobenzyl ester substrates were shown to be very sensitive substrates, predominantly because of the large extinction coefficient of 4-thiopyridone and the high $\text{k}{}_{\mathrm{<mtext>cat</mtext>}}\text{K}{}_{\mathrm{m}}$ values for these compounds.