Substrate specificity and kinetic properties of a soluble nucleoside triphosphatase from bovine kidneys

Soluble nucleoside triphosphatase differing in its properties from all known proteins with NTPase activity was partially purified from bovine kidneys. The enzyme has pH optimum of 7.5, molecular mass of 60 kDa, as estimated by gel chromatography, and shows an absolute dependence on divalent metal ions. NTPase obeyed Michaelis-Menten kinetics in the range of substrate concentration tested from 45 to 440 microM; the apparent Km for inosine-5'-triphosphate was calculated to be 23.3 microM. The enzyme was found to possess a broad substrate specificity, being capable of hydrolyzing various nucleoside-5'-tri- as well as diphosphates.     

Assay of 1L-myo-inositol-1-phosphatase using a fluorometric method.

1L-myo-Inositol-1-phosphatase, an enzyme purified from brain tissues, catalyzes the dephosphorylation of 1L-myo-inositol 1-phosphate. This enzyme has become the subject of intense research interest, since myo-inositol is needed for the resynthesis of phosphatidylinositol. We have developed a sensitive fluorometric assay for detecting the activity of 1L-myo-inositol-1-phosphatase. The assay is based on o-aminobenzoyl beta-glycerophosphate fluorescence, according to the following principles: (I) The fluorescence yield of o-aminobenzoyl beta-glycerophosphate is increased by 2.75-fold in the presence of saturating concentrations of bovine serum albumin. (II) o-Aminobenzoyl beta-glycerophosphate has the same fluorescence yield as o-aminobenzoyl glycerol, but the latter does not bind to bovine serum albumin. (III) Dephosphorylation of the substrate, catalyzed by the monophosphatase, makes less o-aminobenzoyl beta-glycerophosphate available for binding to bovine serum albumin, thereby producing a decrease in the fluorescence intensity.     

Heterogeneity of 11β-hydroxysteroid dehydrogenase in rat tissues

Using specific antisera to purified rat liver 11β-hydroxysteroid dehydrogenase (11-HSD), we showed that the antigen is widely distributed in rat organs. Enzyme activity and immunoreactivity generally corresponded. Highest by both criteria were liver, testis, kidney and lung. In some tissues (epididymis, pancreas and duodenum) activity was found, but antigen corresponding to 11-HSD at a M_w of 34 kDa was absent. It is suggested that these tissues have alternate enzyme forms. The 11-HSD of brain and liver were compared. Brain enzyme may control selective binding of aldosterone to Type I receptors in the hippocampus and other regions. Rat brain 11-HSD resembled that of liver or kidney in most characteristics. It differed in (a) its steroid specificity: cortisol was a good substrate for liver 11-HSD, and a poor substrate for brain enzyme; (b) stability of 11-oxoreductase (11-OR) component. Brain 11-OR was not readily inactivated; 11-OR from other tissues lost activity rapidly and spontaneously. The variations in properties of 11-HSD in specific tissues may reflect aspects of its various specific functions.     

Mammalian myristoyl CoA: protein N-myristoyltransferase

Myristoyl CoA:Protein N-myristoyltransferase (NMT) is the enzyme which catalyses the covalent transfer of myristate from myristoyl CoA to the amino-terminal glycine residue of protein substrates. Although NMT is ubiquitous in eukaryotic cells, the enzyme levels and cellular distribution vary among tissues. In this article, we describe the properties of mammalian NMT(s) with reference to subcellular distribution, molecular weights, substrate specificity and the possible involvement of NMT in pathological processes. The cytosolic fraction of bovine brain contains multiple forms of NMT activity whereas bovine spleen contains only a single form. In bovine brain and spleen, the cytosol contained majority of NMT activity. In contrast, rabbit colon and rat liver NMT activity was predominantly particulate. Regional differences in NMT activity have been observed in both rabbit intestine and bovine brain. Results from our laboratory along with the existing knowledge, provide evidence for the existence of tissue specific isozymes of NMT.     

Monoamine oxidase and semicarbazide-sensitive amine oxidase in spontaneously hypertensive and in normotensive control rats

The aim of the present work was to compare monoamine oxidase (MAO) and semicarbazide sensitive amine oxidase (SSAO) activity in several tissues from spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto rats (WKY). Contribution of MAO-A, -B and SSAO to the metabolism of each substrate in each tissue was defined from experiments where the decrease of oxidative deamination of each substrate at a given concentration was measured as a function of increasing concentrations of a selective MAO-A, -B or SSAO inhibitor. In the heart, aorta and, to a lesser extent, the femoral arteries MAO-A activity was higher in SHR than in WKY. Similarly in the liver the enzyme activity was higher in SHR than in WKY but was due to the -B form of MAO. In all the other tissues studied (duodenum, brain, lungs, adrenals and kidneys) no difference in MAO-A, MAO-B or SSAO activity was found between SHR and WKY, except for the kidneys and brain, if the differences in the weights of these organs in SHR are taken into account.     

Glutathione S-Transferase from Bovine Tissues: Relationship Between Multiple Forms, Distribution and Catalytic Activity

Cytosolic functions obtained from various bovine tissues was individually subjected to column isoelectric focusing in order to resolve the glutathione S-transferase isoenzymes. The results showed a large variability in the isoenzyme pattern. All the tissues were found to have neutral-acidic forms of the enzyme, whilst liver, adrenal gland, testicle, lung and kydney contained a conspicuous amount of activity associated with the cationic forms of the enzyme. In spite of these differences, by comparison of the conjugating activity of transferases, we did not find essential inter-organ variations. Conversely, when the same tissue samples were tested for selenium independent glutathione peroxidase activity, using cumene hydroperoxide as second substrate, we observed a higher activity in the organs having the cationic form of glutathione S-transferase.     

Glutathione S-Transferase from Bovine Tissues: Relationship Between Multiple Forms,Distribution and Catalytic Activity

Cytosolic functions obtained from various bovine tissues was individually subjected to column isoelectric focusing in order to resolve the glutathione S-transferase isoenzymes. The results showed a large variability in the isoenzyme pattern. All the tissues were found to have neutral-acidic forms of the enzyme, whilst liver, adrenal gland, testicle, lung and kydney contained a conspicuous amount of activity associated with the cationic forms of the enzyme. In spite of these differences, by comparison of the conjugating activity of transferases, we did not find essential inter-organ variations. Conversely, when the same tissue samples were tested for selenium independent glutathione peroxidase activity, using cumene hydroperoxide as second substrate, we observed a higher activity in the organs having the cationic form of glutathione S-transferase.     

Studies on a peptidase acting on a synthetic collagenase substrate Developmental pattern in rat granuloma tissue and distribution of enzyme in the tissues of various animals

The developmental pattern in experimental rat granuloma tissue and the distribution in the tissues of a few animals (monkey, rabbit, guinea pig anrat) of a peptidase acting on a synthetic collagenase substrate, 4-phenylazobenzyloxycarbonyl-L-Pro-L-Leu-Gly-L-Pro-D-Arg (Pz-peptide) has been studied. Maximum enzyme activity was found in 4-month-old rats and on the fourth day of implantation of the cotton wick. Pz-peptidase appears to have a ubiquitous distribution in animal tissues; the highest enzyme activity was generally found in liver, intestine and kidney of the animals. The total activity in other organs (spleen, heart, lungs and brain) was much less compared to that of liver, intestine or kidney.     

The pH dependence of breakdown of various purified brain proteins by cathepsin D preparations

In a continuing study of control processes of cerebral protein catabolism we compared the activity of cathepsin D from three sources (rat brain, bovine brain, and bovine spleen) on purified CNS proteins (tubulin, actin, calmodulin, S-100 and glial fibrillary acidic protein). The pH optimum was 5 for hydrolysis with tubulin as substrate for all three enzyme preparations, and it was pH 4 with the other substrates. The pH dependence curve was somewhat variable, with S-100 breakdown relatively more active at an acidic pH range. The formation of initial breakdown products and the further catabolism of the breakdown products was dependent on pH; hence the pattern of peptides formed from glial fibrillary acidic protein was different in incubations at different pH's. The relative activity of the enzyme preparations differed, depending on the substrate: with tubulin and S-100 as substrates, rat brain cathepsin D was the most active and the bovine spleen enzyme was the least active. With calmodulin and glial fibrillary acidic protein as substrates, rat brain and spleen cathepsin D activities were similar, and bovine brain cathepsin D showed the lowest activity. Actin breakdown fell between these two patterns.The rates of breakdown of the substrates were different; expressed as μg of substrate split per unit enzyme per h, with rat brain cathepsin D activity was 8–9 with calmodulin and S-100, 4 with glial fibrillary acidic protein, 1.8 with actin, and 0.9 with tubulin. The results show that there are differences in the properties of a protease like cathepsin D, depending on its source; furthermore, the rate of breakdown and the characteristics of breakdown are also dependent on the substrate.We recently measured the breakdown of brain tubulin by cerebral cathepsin D in a continuing study of the mechanisms and controls of cerebral protein catabolism (Bracco et al., 1982a). We found that tubulin breakdown is heterogeneous, that membrane-bound tubulin is resistant to cathepsin D but susceptible to thrombin (Bracco et al., 1982b), and that cytoplasmic tubulin was in at least two pools, one with a higher, another with a lower, rate of breakdown. The pH optimum of tubulin breakdown by cerebral cathepsin D differed significantly from the pH optimum of hemoglobin breakdown by the same enzyme.These findings showed that the properties of breakdown by a cerebral protease depend on the substrate. To further examine this dependence of properties of breakdown on the substrate, we now report measurements of pH dependence of breakdown of several purified proteins (tubulin, actin, calmodulin, S-100, glial fibrillary acidic protein [GFA]) from brain by cathepsin D preparations from three sources, rat brain, bovine brain, and bovine spleen. We also compare the rate of breakdown of the various proteins with the rate of hemoglobin breakdown.     

OCCURRENCE AND PROPERTIES OF CATECHOL-O-METHYL TRANSFERASE IN ADRENERGIC NEURONS

—A study has been made of the catechol-O-methyl transferase activity of some peripheral tissues after sympathetic denervation. A fall in catechol-O-methyl transferase activity was found in some organs, e.g. rat and rabbit vas deferens, cat nictitating membrane and rabbit submaxillary gland but not in mouse heart and spleen. It was found that suboptimal concentrations of S-adenosylmethionine did not reveal a significant difference between normal and denervated organs but at optimal concentrations a fall was seen in some organs. Catechol-O-methyl transferase activity was present in bovine splenic nerve and in adrenal medulla. It is suggested that the fall in enzyme activity after denervation indicates a neuronal cellular localization. A kinetic study of catechol-O-methyl transferase from normal and denervated rat vas deferens suggested that the neuronal and extraneuronal catechol-O-methyl transferase had different kinetic properties and an estimation of the kinetic constants of the neuronal enzyme was made.     

Hepatitis C virus helicase/NTPase: an efficient expression system and new inhibitors

A method has been developed for obtaining a full-length protein NS3 of hepatitis C virus with the yield of 6.5 mg/liter of cell culture, and conditions for measuring its NTPase and helicase activities have been optimized. The helicase reaction can proceed in two modes depending on the enzyme and substrate concentration ratio: it can be non-catalytic in the case of enzyme excess and catalytic in the case of tenfold substrate excess. In the latter case, helicase activity is coupled with NTPase and is stimulated by ATP. A number of NTP and inorganic pyrophosphate analogs were studied as substrates and/or inhibitors of NS3 NTPase activity, and it was found that the structure of nucleic base and ribose fragment of NTP molecule has a slight effect on its inhibitory (substrate) properties. Among the nucleotide derivatives, the most efficient inhibitor of NTPase activity is 2 -deoxythymidine 5 -phosphoryl-beta,gamma-hypophosphate, and among pyrophosphate analogs imidodiphosphate exhibited maximal inhibitory activity. These compounds were studied as inhibitors of the helicase reaction, and it was shown that imidodiphosphate efficiently inhibited the ATP-dependent helicase reaction and had almost no effect on the ATP-independent duplex unwinding. However, the inhibitory effect of 2 -deoxythymidine 5 -phosphoryl-beta,gamma-hypophosphate was insignificant in both cases, which is due to the possibility of helicase activation by this ATP analog.     

Interactions between the hepatitis C virus protein NS3 and polymethylene derivatives of nucleic bases

Nonstructural protein 3 (NS3) of hepatitis C virus plays a key role in the functioning of the virus. NS3 displays three enzymatic activities, namely, protease activity associated with the N-terminal domain, coupled nucleoside triphosphotase (NTPase), and helicase activities, localized to the C-terminal domain. In this work, we studied the effects of various polymethylene derivatives of nucleic bases on the NTPase (by the example of ATPase) and helicase activities of NS3. It was demonstrated that some tested compounds inhibited NS3 helicase activity; however, a considerable part of the compounds activated the NTPase activity of NS3 and several other proteins displaying NTPase or selective ATPase activity. Such ATPase activators have not been earlier described, suggesting an unusual activation mechanism. The activation ability of the tested compounds depended on the ratio of substrate (ATP) and activator concentrations, and reached its maximum at a 1000-fold excess of the substrate. A mechanism of ATPase activation was proposed to explain the observed effects.     

Inhibitory effects of methylxanthines on the activity of xanthine oxidase

The $\text{in vitro}$ and $\text{in vivo}$ effects of three methylxanthines caffeine, theophylline and theobromine on the activity of the enzyme xanthine oxidase (EC 1.2.3.2.) was investigated with a view to understand their biochemical action. The studies revealed all the three methylxanthines to be inhibitors of the milk xanthine oxidase activity and the inhibition was found to be competitive in nature. The preincubation studies indicated a greater inhibition of the enzyme with the methylxanthines. Excessive amount of the substrate (2.5 × 10^?4M) resulted in progressive inhibition of the enzyme activity. Low concentrations of methylxanthines exerted a definite inhibitory effect on the xanthine oxidase activity at lower substrate concentrations. At higher concentrations of the substrate, the inhibitory effect due to the same concentration of methylxanthines did not produce any added inhibition of the enzyme activity to that produced by the substrate alone. However, added inhibition by high concentrations of methylxanthines was detectable even when the enzyme activity was markedly inhibited by higher concentrations of the substrate. The $\text{in vivo}$ administration of methylxanthines caused a significant inhibition of the xanthine oxidase activity in lungs, kidneys, heart and brain of rats. Consequently, the level of uric acid in the tissues of the drug treated animals was also found to be reduced.     

Production and properties of antibody to soluble guanylate cyclase purified from bovine brain

Guanylate cyclase was purified 12,700-fold from bovine brain supernatant, and the purified enzyme exhibited essentially a single protein band on polyacrylamide gel electrophoresis. Repeated injection of the purified enzyme into rabbits produced an antibody to guanylate cyclase. The immunoglobulin G fraction from the immunized rabbit gave only one precipitin line against the purified guanylate cyclase and the crude supernatant of bovine brain on double immunodiffusion and immunoelectrophoreis. The antibody completely inhibited the soluble guanylate cyclase activity from bovine brain, various tissues of rat and mouse and neuroblastoma N1E 115 cells, whereas the Triton-dispersed particulate guanylate cyclase from these tissues was not inhibited by the antibody.     

Halogenated benzimidazoles and benzotriazoles as inhibitors of the NTPase/helicase activities of hepatitis C and related viruses.

A search has been initiated for lead inhibitors of the nonstructural protein 3 (NS3)-associated NTPase/helicase activities of hepatitis C virus, the related West Nile virus, Japanese encephalitis virus and the human mitochondrial Suv3 enzyme. Random screening of a broad range of unrelated low-molecular mass compounds, employing both RNA and DNA substrates, revealed that 4,5,6,7-tetrabromobenzotriazole (TBBT) hitherto known as a potent highly selective inhibitor of protein kinase 2, is a good inhibitor of the helicase, but not NTPase, activity of hepatitis C virus NTPase/helicase. The IC50 is approximately 20 micro m with a DNA substrate, but only 60 micro m with an RNA substrate. Several related analogues of TBBT were enzyme- and/or substrate-specific inhibitors. For example, 5,6-dichloro-1-(beta-d-ribofuranosyl)benzotriazole (DRBT) was a good, and selective, inhibitor of the West Nile virus enzyme with an RNA substrate (IC50 approximately 0.3 micro m), but much weaker with a DNA substrate (IC50 approximately 3 micro m). Preincubation of the enzymes, but not substrates, with DRBT enhanced inhibitory potency, e.g. the IC50 vs the hepatitis C virus helicase activity was reduced from 1.5 to 0.1 micro m. No effect of preincubation was noted with TBBT, suggesting a different mode of interaction with the enzyme. The tetrachloro congener of TBBT, 4,5,6,7,-tetrachlorobenzotriazole (TCBT; a much weaker inhibitor of casein kinase 2) is also a much weaker inhibitor than TBBT of all four helicases. Kinetic studies, supplemented by comparison of ATP-binding sites, indicated that, unlike the case with casein kinase 2, the mode of action of the inhibitors vs the helicases is not by interaction with the catalytic ATP-binding site, but rather by occupation of an allosteric nucleoside/nucleotide binding site. The halogeno benzimidazoles and benzotriazoles included in this study are excellent lead compounds for the development of more potent inhibitors of hepatitis C virus and other viral NTPase/helicases.     

Characterization of Ca(2+)-ATPase of Setaria cervi (Nematoda: Filarioidea): effect of phenothiazines and anthelmintics.

1. A Mg2+ independent, Ca(2+)-ATPase requiring high concentrations of Ca2+ (5 mM) for the activation, equally distributed in cuticle-muscular-hypodermis, genital organs and gastrointestinal tissues and mainly localized in 10,000 g pellet fraction, was identified in Setaria cervi, a bovine filarial parasite. 2. Filarial enzyme showed Km value of 3.33 mM for ATP as computed from the double reciprocal Lineweaver-Burk plot. 3. The enzyme could be completely solubilized by sonication with about 4-fold increase in specific activity of the enzyme. 4. The enzyme showed about 2-fold activation by the calmodulin fractions isolated from S. cervi and rat brain homogenates. 5. The enzyme was highly sensitive to inhibition with some phenothiazine derivatives. Trifluoperazine was observed to be the most potent inhibitor followed by promethazine and chlorpromazine. 6. Some anthelmintics viz. diethycarbamazine and centperazine were found to be highly potent inhibitors of the enzyme, significant inhibition of filarial Ca(2+)-ATPase was also observed with levamisole and suramine. 7. Studies indicate Ca(2+)-ATPase of S. cervi as a potential chemotherapeutic target.     

Characterization of the mutant α-mannosidase in bovine mannosidosis

Residual acidic α-mannosidase, varying in amount up to approx. 15% of normal values, can be measured in various organs of a calf with mannosidosis. The highest specific activity and relative proportion of residual activity were found in the liver. Chromatography on DEAE-cellulose showed that the residual activity was associated with two components, which were eluted at comparable positions with those found in normal tissues. The residual activity had a lower thermal stability and a higher K_m value for a synthetic substrate than did the normal enzyme. No differences in molecular weight or electrophoretic mobility between normal acidic α-mannosidase and the residual activity were observed by gel filtration and electrophoresis on cellulose acetate respectively. The isoelectric focusing profiles for the α-mannosidase in the normal and pathological livers were very similar. It is suggested that a mutant enzyme, resulting from a mutation in a structural gene, accounts for the residual acidic α-mannosidase in mannosidosis. The mutant enzyme, which cross-reacts with antiserum raised against normal bovine acidic α-mannosidase, is present at a decreased concentration compared with the normal enzyme. There is a correlation between the concentrations of residual activity and cross-reacting material in mannosidosis. α-Mannosidase with a pH optimum of 5.75 and which is activated by Zn²⁺ was also detected in the liver of the calf with mannosidosis. However, it is probably not a product of the defective gene because addition of Zn²⁺ indicated that it was also present in normal tissues.     

Studies on the formation by rat brain preparations of CDP-diglyceride from CTP and phosphatidic acids of varying fatty acid compositions.

The enzyme, CTP:phosphatidate cytidylyltransferase (EC2.7.7.41) which catalyses formation of CDP-diglyceride from CTP and phosphatidic acid has been studied in rat brain preparations and other tissues. Improvement, as judged by the higher tissue activities obtained, in the assay method for this enzyme was achieved through use of phosphatidic acids sonicated in buffer-detergent solution saturated with ether and containing bovine serum albumin and use of short incubation times which essentially provided a measure of initial rates. The enzyme of rat brain microsomes yielded with 1,2-dioleolphosphatidic acid as substrate a pH optimum of 6.8 with maleate buffer and optimal concentrations of 60mM for MG2+, 6MM for CTP and 250 mug per 0.8 ml for phosphatidic acid. Enzyme activity was mainly located in the 90,000 X g fraction (microsomal) with small but significant activity in the 12,000 X g fraction. Comparison of activities (nanomoles CTP incorporated per milligram protein per minute) amongst tissues showed the following order: brain, 1.87; liver, 1.32; lung, 1.19; small intestine, 1.00; kidney, 0.69; heart, 0.41; diaphragm, 0.07; skeletal muscle, 0.02. Examination of the effect of varying the fatty acid composition in the phosphatidic acids added exogenously gave the following order (activities in parentheses); 1-stearoyl-2-oleoyl- (5.58), 1-oleoyl-2-stearoyl- (5.37), 1,2-dioleoyl- (4.49) 1-palmitoyl-2-oleoyl-(3.85), 1-stearoyl-2-arachidonoyl-(3.31), 1-arachidonoyl-2-stearoyl-(3.16), 1,2-diarachidonoyl-(0.72), 1,2-dicaproyl-(0.67), 1,2-dipalmitoyl-(0.67) and 1,2-distearoyl-(0.18). The single bis- and lysophosphatidic acids tested were inactive as substrates. Apart from a possible preference for one or more unsaturated fatty acids the transferase enzyme showed no selectivity in respect to the fatty acid distribution of phosphatidic acids.     

ARGINASES OF MOUSE BRAIN AND LIVER

The arginase present in mouse brain and liver was studied in order to determine if the activity in both tissues was due to the same enzyme. Mouse liver contains only one arginase enzyme whereas mouse brain contains two. One of the arginases in the brain is distinct from the liver enzyme as determined by fractionation on DEAE-cellulose, CM-cellulose and disc gel electrophoresis. The second enzyme from brain tissue has the same properties as the liver enzyme when subjected to these same fractionation techniques. However this arginase can be distinguished from the liver enzyme by its K_m for arginine heat lability and MnCl₂ activation curve. Thus both arginases in brain differ from the liver enzyme. No interconversion of the brain enzymes was detected, and the molecular weight of all the arginases appears to be the same. The observation of multiple distinct brain and liver arginases in mouse brain and liver was confirmed with bovine tissues.