Sialyltransferase activity in regenerating rat liver

Liver microsomal fractions catalyse the transfer of sialic acid from CMP-N-acetyl-neuraminic acid to various exogenous acceptors such as desialylated fetuin, desialylated human Tamm–Horsfall glycoprotein and desialylated bovine submaxillary-gland mucin. An increase in the rate of incorporation of sialic acid into desialylated glycoproteins was found after a lag period (7h) in regenerating liver. The increase was maximum 24h after partial hepatectomy for all acceptors tested. At later times after operation the sialyltransferase activity remained high only for desialylated fetuin. No soluble factors from liver or serum of partially hepatectomized animals influenced the activity of the sialyltransferases bound to the microsomal fraction. The sensitivity of sialyltransferases to activation by Triton X-100, added to the incubation medium, was unchanged in the microsomal preparation from animals 24h after sham operation or partial hepatectomy. The full activity of sialyltransferases towards the various desialylated acceptors showed some differences. Human Tamm–Horsfall glycoprotein was a good acceptor of sialic acid only when desialylated by mild acid hydrolysis. After this treatment, but not after enzymic hydrolysis, a decrease in molecular weight of human Tamm–Horsfall glycoprotein was observed. Further, the sialyltransferase activity as a function of incubation temperature gave different curves according to the acceptor used. The relationship between the biosynthesis of glycoproteins by regenerating liver and the sialyltransferase activity of microsomal fraction after partial hepatectomy is discussed.     

Partial deglycosylation of an anionic isoperoxidase from peach seeds – effect on enzyme activity, stability and antigenicity

An anionic isoperoxidase (EC 1.11.1.7) purified from peach seeds ( Prunus persica L. Batsch cv. Merry) was partially deglycosylated by glycopeptidase F (EC 3.2.2.18) treatment. A 40% deglycosylation resulted in an activity loss of 50% when assayed with o -dianisidine. 60% with guaiacol and 78% with 2,2'-azino-bis(3-ethyl)benzethiozoline-6-sulfonic acid (ABTS) as substrate. The indole-3-acetic acid oxidase activity loss was close to 55%. The partially deglycosylated isoperoxidase also showed a higher K_m value for H₂O₂ and higher values for Arrhenius activation energy and enthalpy of activation. There was a decrease in enzyme stability at 4°C after deglycosylation. Native and partially deglycosylated isoperoxidase reacted equally well in an enzyme-linked immunosorbent assay (ELISA) with rabbit polyclonal antibodies raised against the native enzyme. The carbohydrate moiety of this peach seed isoperoxidase appears to be important for enzyme activity and stability.     

An essential residue at the active site of aspartate transcarbamylase.

Reaction of phenylglyoxal with aspartate transcarbamylase and its isolated catalytic subunit results in complete loss of enzymatic activity. This modification reaction is markedly influenced by pH and is partially reversible upon dialysis. Carbamyl phosphate or carbamyl phosphate with succinate partially protect the catalytic subunit and the native enzyme from inactivation by phenylglyoxal. In the native enzyme complete protection from inactivation is afforded by N-(phosphonacetyl)-L-aspartate. The decrease in enzymatic activity correlates with the modification of 6 arginine residues on each aspartate transcarbamylase molecule, i.e. 1 arginine per catalytic site. The data suggest that the essential arginine is involved in the binding of carbamyl phosphate to the enzyme. Reaction of the single thiol on the catalytic chain with 2-chloromercuri-4-nitrophenol does not prevent subsequent reaction with phenylglyoxal. If N-(phosphonacetyl)-L-aspartate is used to protect the active site we find that phenylglyoxal also causes the loss of activation of ATP and inhibition by CTP. The rate of loss of heterotropic effects is exactly the same for both nucleotides indicating that the two opposite regulatory effects originate at the same location on the enzyme, or are transmitted by the same mechanism between the subunits, or both.     

Activation/inactivation of human angiotensin I converting enzyme following chemical modifications of amino groups near the active site

We have studied the effects of amidination of lysyl residues on the activity of angiotensin I converting enzyme isolated from human kidney. Anion concentration was an important reaction variable. In 4 M chloride or acetate, amidination with methyl acetimidate produced derivatives with up to a 4-fold increase in activity with hippuryl-glycyl-glycine as substrate. Modification with methyl p-hydroxybenzimidate also increased activity while treatment with methyl 4-mercaptobutyrimidate resulted in a 90% loss of activity. The effects of amidination were partially prevented when the reactions were carried out in the presence of the inhibitors, captopril or 5S-benzamido-4-oxo-6-phenyl-hexanoyl-L-proline. These results suggest that lysyl residues are present near the active site while different amino groups have a role in anion activation.     

Effects of thiols, sugars, and proteins on nitric oxide activation of guanylate cyclase.

Purification of soluble guanylate cyclase from rat liver resulted in an apparent loss of enzyme activation by nitric oxide that could be restored by dithiothreitol. methemoglobin, bovine serum albumin, or sucrose. Although hemoglobin also permitted some activation with nitric oxide, the effect of other agents to restore enzyme activation was prevented with hemoglobin. As a result of enzyme purification, there is an alteration of the dose-response relationship for nitric oxide activation. After partial enzyme purification, relatively high concentrations of nitric oxide that were stimulatory in crude enzyme preparations had no effect on enzyme activity. However, partially purified or homogeneous enzyme was activated by lower concentrations of nitric oxide. The bell-shaped dose-response curve for nitric oxide was shifted to the left with guanylate cyclase purification. The addition of dithiothreitol, methemoglobin, bovine serum albumin, or sucrose to enzyme markedly broadens the dose-response curve for nitric oxide. Thus, the apparent loss of responsiveness to nitric oxide with purification is a function of increased sensitivity of guanylate cyclase to nitric oxide. Increased sensitivity to nitric oxide with enzyme purification probably results from the removal of heme, proteins, and small molecules that can serve as scavengers or sinks for nitric oxide and prevent excessive oxidation of the enzyme.     

Characterization of serum, liver, and intestinal sialyltransferases from rats treated with colchicine

A modified high pressure liquid chromatographic method using lactose (Gal beta 1----4Glc) as an exogenous acceptor has been used to characterize the sialyltransferases known to increase in the serum of colchicine-treated rats. The results show a 10-fold increase of Gal beta 1----4GlcNAc alpha 2----6 sialyltransferase (alpha 2----6 ST), whereas the Gal beta 1----3GlcNAc alpha 2----3 sialyltransferase showed only 1.6-fold increase in the serum after 17 h of colchicine treatment. The sialyltransferase activity in serum using exogenous desialylated, alpha 1-acid glycoprotein as acceptor also showed an eightfold increase. In liver homogenate and Golgi membrane, the sialyltransferase activity when assayed with desialylated alpha 1-acid glycoprotein as acceptor showed a slight decrease after 4 h, but returned to normal level after 17 h. A similar trend was seen when the two transferases were assayed with lactose as acceptor. The antiserum to rat alpha 2----6 ST inhibited the sialyltransferase activity in serum, liver, and jejunal incubation medium. Jejunal sections from rats treated with colchicine for 4 h in presence of heated serum showed a decrease of sialyltransferase, with consequent increase of the alpha 2----6 ST enzyme activity in the medium. This result suggests that intestinal tissue could be a source of increased serum enzyme activity in colchicine treatment.     

S-Adenosylmethionine decarboxylase from human prostate. Activation by putrescine

1. The presence of S-adenosylmethionine decarboxylase in human prostate gland is reported. A satisfactory radiochemical enzymic assay was developed and the enzyme was partially characterized. 2. Putrescine stimulates the reaction rate by up to 6-fold at pH7.5: the apparent activation constant was estimated to be 0.13mm. The stimulation is pH-dependent and a maximal effect is observed at acid pH values. 3. Putrescine activation is rather specific: other polyamines, such as spermidine and spermine, did not show any appreciable effect. 4. The apparent K(m) for the substrate is 4x10(-5)m. The calculated S-adenosylmethionine content of human prostate (0.18mumol/g wet wt. of tissue) demonstrates that the cellular amounts of sulphonium compound are saturating with respect to the enzyme. 5. The enzyme is moderately stable at 0 degrees C and is rapidly inactivated at 40 degrees C. The optimum pH is about 7.5, with one-half of the maximal activity occurring at pH6.6. 6. Several carboxy-(14)C-labelled analogues and derivatives of S-adenosylmethionine were tested as substrates. The enzyme appears to be highly specific: the replacement of the 6'-amino group of the sulphonium compound alone results in a complete loss of activity. 7. Inhibition of the enzyme activity by several carbonyl reagents suggests an involvement of either pyridoxal phosphate or pyruvate in the catalytic process. 8. The inhibitory effect of thiol reagents indicates the presence of ;essential' thiol groups.     

Species-specific aggregation factor in sponges. Sialyltransferase associated with aggregation factor.

The sialyltransferase (= glycoprotein-sialic acid transferase) was studied in the sponge Geodia cydonium, a mesozoan organism. The experiments were performed both in intact cellular and in isolated enzyme systems. It is shown, that desialylated cells show a lower aggregation potency than the controls. During aggregation enzymic sialylation of desialylated sponge cells occurs in the presence of an aggregation factor, which is associated with a high molecular weight particle. The sialylation process is temperature-dependent and can be inhibited by N-ethylmaleimide. Sialylation occurs predominantly at a distinct cell surface component, the aggregation receptor. The sialyltransferase was isolated and purified by the following steps: Sepharose 4B, CM-cellulose, Nonidet treatment, and Sephadex G-100. By this procedure the enzyme was purified 680-fold with a 31% yield. The sialyltransferase is originally associated with the high molecular weight particle also carrying the aggregation factor. In the last step the aggregation factor was separated from the sialyltransferase. The enzyme catalyzes the transfer of sialic acid from CMP-sialic acid to the desialylated aggregation receptor. The molecular weight of the sialyltransferase has been determined to be 52,000. Kinetic studies revealed no lag phase and a dependence on enzyme concentration. The purified transferase has a pH optimum of 7.75 and requires 200 mM NaCl for activity. No requirement for Mg2+ or Ca2+ could be observed. The reaction is inhibited by 10 micronM N-ethylmaleimide.     

Role of a disulfide bond in the gamma subunit in activation of the ATPase of chloroplast coupling factor 1

The relationship between activation of the latent ATPase activity of isolated chloroplast coupling factor 1 (CF1) and reduction of a disulfide in the gamma subunit has been assessed. The sulfhydryl residues involved in the disulfide bond are distinct from residues normally accessible to maleimide modification during incubation of thylakoids in the dark or the light. Dithiothreitol-induced activation is time dependent, and correlates with reduction of the disulfide. Sulfhydryl residues exposed during activation can be reoxidized to disulfide by incubation with iodosobenzoate , with a concomitant loss of ATPase activity. Activation and deactivation are reversible, but deactivation is prevented by treatment of the reduced enzyme with N-ethylmaleimide. Heat activation does not reduce the disulfide bond unless dithiothreitol is present during activation. Prior heating of CF1, which partially activates the enzyme, renders the disulfide more susceptible to subsequent dithiol reduction. The activity obtained when heat and dithiothreitol are used together is approximately equal to the sum of the partial activations obtained with heat or dithiothreitol alone. Iodosobenzoate has no effect on heat-activated CF1. Enzyme activated by heating in the presence of dithiothreitol can be partially deactivated, consistent with reversal of the activity attributable to the dithiol effect. Fluorescence polarization of anilinonaphthylmaleimide bound to the reduced enzyme indicates that the sulfhydryl residues involved in the disulfide are in a less rigid environment than the other two sulfhydryl residues in the gamma subunit. Polarization of anilinonaphthylmaleimide bound to these sulfhydryls is reduced by heat treatment of CF1. The increased susceptibility of the disulfide to reduction upon heat treatment, and the activation of ATPase activity with or without disulfide bond cleavage are indicative of conformational changes within the gamma subunit that occur during the conversion of CF1 from a latent to an active ATPase. In addition the results are consistent with at least two distinct conformational forms of CF1 that can hydrolyze ATP.     

Regulation of a rat lung protein tyrosine kinase activity by reversible phosphorylation/dephosphorylation

Incubation of a partially purified protein tyrosine kinase from rat lung with Mg2+ and ATP resulted in about 10-15-fold activation of the enzyme activity as judged by the phosphorylation of poly(Glu:Tyr,4:1), an exogenous substrate. The activation was time dependent and was associated with the phosphorylation of a single protein band of 50 kDa. Phosphoamino acid analysis of the phosphorylated protein indicated that tyrosine was the amino acid being phosphorylated. Upon gel filtration on a Sephacryl S-200 column, the phosphorylated protein co-eluted with protein tyrosine kinase and ATP-binding activities, suggesting that all three activities are part of the same protein. In addition, pretreatment of the partially purified protein tyrosine kinase with alkaline phosphatase inhibited its enzyme activity which could be restored by reincubation with Mg2+ and ATP. These data suggest that a temporal relationship exists between the phosphorylation and the activation states of rat lung protein tyrosine kinase, and that the phospho- and dephospho- forms represent the active and inactive (or less active) forms, respectively, of the enzyme.     

Evidence for conformational changes in grape catechol oxidase

A rapid, 4–10-fold, activation of grape catechol oxidase by a short exposure to acid pH or urea is demonstrated. Activation was either reversible or irreversible, depending on length and type of treatment. The change in activity of the enzyme is due primarily to an increase in V_max, while the affinity for 4-methylcatechol decreases and that for O₂ increases. Activation occurs in intact chloroplasts as well as in a partially purified enzyme preparation. Activation was apparently due to conformational changes in the enzyme. O₂ concentration appeared to control enzyme activity, presumably by an O₂ induced conformational change. Irreversible activation was accompanied by changes in the electrophoretic mobility of the enzyme.     

Oxidation rates of some desialylated glycoproteins by galactose oxidase

Patterns of oxidation of dilute solutions of desialylated fetuin and submaxillary mucin by galactose oxidase have been examined. A significant portion (20-40%) of the terminal galactosyls exposed on the glycoproteins, which theoretically were expected to be accessible to the enzyme, was not oxidized. In comparison, galactosyls in oligosaccharides released from completely desialylated glycoproteins were oxidized more effectively with an apparently lower degree of crypticity to the enzyme. Partial desialylation usually resulted in a reduction of both the rate and the final level of substrate oxidation. A second cycle of oxidation of a desialylated substrate earlier oxidized by galactose oxidase and then reduced by NaB3H4 revealed a selectivity in the pattern of galactosyl oxidation. The same galactosyl residues oxidized in the first cycle were again the most susceptible to oxidation in the second cycle, leaving unmodified the same fraction of galactosyls throughout both cycles. The relevance of these results to the application of the galactose oxidase-NaBH4 procedure for detecting and measuring desialylated glycoconjugates in solution and in biological membranes is discussed.     

Catalytic properties of chloroplast F1-ATPase modified at catalytic or noncatalytic sites by 2-azido adenine nucleotides

When heat-activated F1-ATPase from chloroplasts was repeatedly exposed to Mg2+ and 2-azido-ATP, followed by separation from medium nucleotides and photolysis, a total of two sites per enzyme, both catalytic and noncatalytic, were labeled. In a coupled assay with pyruvate kinase about half the activity was lost when one site per enzyme was modified. However, increased modification resulted in no further loss of activity. In contrast, methanol-sulfite activation of the enzyme showed a loss of most of the catalytic capacity when one site per enzyme was modified. Predominant labeling of either one catalytic or one noncatalytic site caused a loss of most of the activity in either assay. An indication that the enzyme modified at one site retained some catalytic activity was verified by measurement of the [18O]Pi species formed when [gamma-18O]ATP was hydrolyzed by partially derivatized enzyme. With either catalytic or noncatalytic site modification, the distributions of [18O]Pi species formed showed that the modified enzyme had different catalytic characteristics. An interpretation is that with modification by azido nucleotides at either catalytic or noncatalytic sites, capacity for rapid catalysis is largely lost but the remaining sites retain weak modified catalytic properties.     

Rat testis mitochondrial adenylate cyclase. Partial purification and characterization.

1. Adenylate cyclase (EC 4.6.1.1) from rat testis mitochondria has been solubilized by treatment with the non-ionic detergent Lubrol PX. The soluble enzyme was further purified by DEAE-cellulose chromatography. 2. The specific activity of the adenylate cyclase eluted from the DEAE-cellulose column was found to be four times higher than that of an intact mitochondrial preparation. At this step the enzyme shows a sedimentation coefficient of 4.2 S and a diffusion coefficient (D) of 3.12 - 10- minus 7 cm-2/sec. 3. Solubilization of the adenylate cyclase resulted in loss of responsiveness to gonadotrophic hormones. Addition of phosphatidylserine to the soluble preparation partially restored the activation of adenylate cyclase by human chorionic gonadotrophin. 4. The results of this study suggest that the activity of the adenylate cyclase may be dependent on the membrane-bound phospholipids and that the enzyme attached to the mitochondrial membranes has some properties which are similar to the adenylate cyclase found to be associated with other membrane systems of the cell.?     

Conversion of skeletal tartrate-sensitive acid phosphatases into tartrate-resistant isoenzymes in vitro.

1. Chicken skeletal tartrate-sensitive (TsACP) and -resistant (TrACP) acid phosphatase isoenzymes could be separated from each other by carboxylmethyl-sepharose ion exchange chromatography. 2. Chicken skeletal TsACP showed a gradual time-dependent loss of sensitivity to tartrate inhibition when incubated at room temperature, but not at 4 degrees C. 3. The loss of sensitivity to tartrate inhibition was associated with an activation of the enzyme activity. 4. These changes were accompanied with a shift in the electrophoretic mobility of the enzyme activity from a large molecular sized form to a smaller molecular sized form that resembled the freshly prepared TrACP on the native acidic polyacrylamide electrophoresis gels, and on molecular sieve Superose-12 Fast Protein Liquid Chromatography. 5. Kinetic evaluations of the biochemical properties of the "converted" TsACP activity resembled the TrACP. 6. The apparent "conversion" was not unique to chicken TsACP, since similar "conversion" was observed with partially purified preparations of bovine bone matrix TsACP and of human osteoblastic TsACP. 7. Addition of several serine protease inhibitors did not prevent the "conversion". 8. These findings are consistent with the possibility that skeletal TsACPs are precursors of skeletal TrACPs.     

Asialoglycoprotein receptors in hepatic regeneration

The hepatic binding protein which is responsible for receptor-mediated endocytosis of desialylated glycoproteins undergoes cell-cycle dependent modulation of its expression. Endocytosis of desialylated orosomucoid by hepatocytes isolated from regenerating liver following two-thirds hepatectomy is reduced by 80% and returns to normal when regeneration is substantially complete. This decrease in endocytotic activity is due to selective loss of cell-surface associated receptor during liver regeneration.     

Requirement of an essential thiol group and ferric iron for the activity of the progesterone-induced porcine uterine purple phosphatase.

The progesterone-induced purple phosphatase isolated from the uterine flushings of pigs is activated by a variety of reagents that cleave disulfide bonds, including 2-mercaptoethanol, dithiothreitol, L-ascorbate, L-cysteine, sulfite, and cyanide. It is inhibited by various mercurials, iodoacetamide, O-iodosobenzoate, and hydrogen peroxide. Thiols increase the specific phosphatase activity from 25 to about 300 units per mg of enzyme. This activation is accompanied by a shift in the extinction maximum to higher energy to yield a protein with a pink coloration. Following maximum activation there is a gradual decrease in enzyme activity and protein color which is accompanied by loss of ferrous iron from the protein. Sodium dithionite at 10 mM or higher causes an immediate inhibition of phosphatase activity and bleaching of color, and can be used to prepare the iron-free apoprotein. The latter can be partially reactivated by Fe3+ salts but not by Fe2+. The Fe3+ restores the pink form of the enzyme with a specific activity of about 200 units/mg of protein. Cu2+ also causes some reactivation, but other metal ions were ineffective. ESR studies showed that the pink form of phosphatase contains approximately 1 atom of high spin ferric iron per molecule. It is concluded that the phosphatase requires a free thiol and Fe3+ for activity. Reduction of the iron leads to complete loss of both color and enzyme activity. The color change from purple to pink represents disulfide reduction and is not due to reduction of iron.     

Identification of the reactive sulfhydryl groups of S-adenosylmethionine synthetase

S-Adenosylmethionine synthetase from Escherichia coli is rapidly inactivated by N-ethylmaleimide. In the presence of excess N-ethylmaleimide inactivation follows pseudo first-order kinetics, and loss of enzyme activity correlates with the incorporation of 2 eq of N-[ethyl-2-3H]maleimide/subunit. Preincubation of the enzyme with methionine and the ATP analog adenylylimidodiphosphate reduced the rate of N-ethylmaleimide incorporation more than 30-fold. Two N-[ethyl-2-3H]maleimide-labeled tryptic peptides were purified from the modified enzyme by reverse phase high performance liquid chromatography. The modified residues were identified as cysteine 90 and cysteine 240 by comparison of the amino acid compositions of these peptides with the protein sequence. These are the first residues to be implicated in the activity and/or structure of the enzyme. N-Ethylmaleimide-modified S-adenosylmethionine synthetase exists mainly as a dimer in conditions where the native enzyme is a tetramer. Accumulation of the dimer parallels the loss of the enzyme activity. When an enzyme sample was partially inactivated, separation of tetrameric and dimeric enzyme forms by gel filtration revealed that the residual enzyme activity was solely present in the tetramer and N-[ethyl-2-3H] maleimide was present predominantly in the dimer. Gel filtration studies of the tetramer-dimer equilibrium for the native enzyme indicated that the dissociation constant between the tetramer and dimers is less than 6 x 10(-11) M. Similar studies for the N-ethylmaleimide-modified protein indicated that the dissociation constant of the tetramer is approximately 4 x 10(-4) M. Upon modification the strength of dimer-dimer interactions is diminished by at least 9 kcal/mol.     

Activation of liver succinate dehydrogenase in rats exposed to hypobaric conditions

1. On brief exposure of rats to hypobaric conditions, the activity of hepatic mitochondrial succinate dehydrogenase was raised from the basal state to a ;partially activated state'. This was further raised to ;fully activated state' by preincubation of mitochondria with succinate, as was the activity in mitochondria from normal rats. 2. On washing mitochondria with the homogenizing sucrose medium the activity excess obtained on preincubation with succinate was lost in mitochondria from both normal and treated rats. 3. The enzyme in the ;partially activated state' from animals exposed to hypobaric conditions was stable to the washing procedure but was labilized and reverted to a low basal state of activity on freezing and thawing of the isolated mitochondria. 4. The results suggest that activation of succinate dehydrogenase under hypobaric conditions represents a conformational change leading to a stable, partially activated, form of the enzyme system: this is the first evidence of physiological modulation of this rate-limiting step in the control of the rate of oxidation of succinate.     

RAT BRAIN STEM TRYPTOPHAN HYDROXYLASE: MECHANISM OF ACTIVATION BY CALCIUM

Abstract— The activity of soluble tryptophan hydroxylase from rat brain stem was increased in presence of mm concentrations of calcium. Similarly to that observed by treating the enzyme with sodium dodecyl sulphate or trypsin, this activation resulted mainly from an increased affinity of tryptophan hydroxylase for both its substrate, tryptophan, and the cofactor 2-amino-4-hydroxy-6-methyl-5,6,7,8-tetrahydropteridine (6-MPH₄). In addition, the optimal pH for the enzymic activity was shifted from 7.6 to 7.9 following activation by calcium, sodium dodecyl sulphate or trypsin.
Under the assay conditions used for measuring tryptophan hydroxylase activity, calcium also stimulated a neutral proteinase. This latter enzyme could be eliminated from the solution of tryptophan hydroxylase by filtration through Sephadex G 200. The resulting partially purified tryptophan hydroxylase could be activated by calcium only when the neutral proteinase was included in the assay mixture. In support of this conclusion, the effect of calcium on tryptophan hydroxylase was very small in the new born rat when the activity of the neutral proteinase was low. In addition, the activating effect of Ca²⁺ could be antagonized not only by a chelating agent like EGTA but also (partially) by specific inhibitors of proteinases such as benzethonium and PMSF.
These results strongly suggest that the activation of tryptophan hydroxylase by calcium is the consequence of a partial proteolysis of the enzyme by the calcium-dependent neutral proteinase. Therefore, the physiological significance of this irreversible effect is doubtful.