Improved measurement of thymidylate synthetase activity by a modified tritium-release assay

Accurate quantitation of thymidylate synthetase activity using a tritium-release assay is dependent upon measurement of only that tritium released from deoxy[5-³H]uridine monophosphate ([³H]dUMP) during the biosynthesis of thymidylate. Removal of remaining [³H]dUMP on completion of the assay by charcoal adsorption and correction for the nonenzymatic release of tritium are necessary. Although over 99% of [³H]dUMP is removed immediately following addition of charcoal, these studies demonstrate that sufficient [³H]dUMP can remain to prevent accurate measurement of low levels of thymidylate synthetase activity. By delaying measurement of radioactivity for at least 24 h following addition of charcoal, this problem is minimized. To account for nonenzymatic release of tritium, a blank containing enzyme extract with omission of $\text{±,}\text{l}\text{-5,10-}\text{methylenetetrahydrofolate}$ is demonstrated to be more effective than the commonly used blank in which water is substituted for enzyme extract. In samples containing 5-fluoro-2′-deoxyuridine monophosphate (FdUMP), a potent inhibitor of thymidylate synthetase activity, an alternative blank containing a high concentration of FdUMP (approximately 1mM) is useful in demonstrating a theoretical maximal or complete inhibition of thymidylate synthetase activity.     

5-Nitro-2'-deoxyuridine 5'-monophosphate is a potent irreversible inhibitor of Lactobacillus caesi thymidylate synthetase.

5-Nitro-2′-deoxyuridine 5′-monophosphate was found to be an active sitedirected irreversible inhibitor of thymidylate synthetase from $\text{Lactobacillus caesi}$. It's K_I was determined as 2.9 × 10^?8$\text{M}$ from a double-reciprocal plot of velocity $\text{vs}$ substrate concentration.     

Essential arginyl residues in thymidylate synthetase.

Thymidylate synthetase from amethopterin-resistant $\text{Lactobacillus}$$\text{casei}$ is rapidly and completely inactivated by 2,3-butanedione in borate buffer, a reagent that is highly selective for the modification of arginyl residues. The reversible inactivation follows pseudo-first order kinetics and is enhanced by borate buffer. dUMP and dTMP afford significant protection against inactivation while (±)-5,10-methylenetetrahydrofolate and 7,8-dihydrofolate provide little protection. Unlike native enzyme, butanedione-modified thymidylate synthetase is incapable of interacting with 5-fluoro-2′-deoxyuridylate and 5,10-(+)-methylenetetrahydrofolate to form stable ternary complex. The results suggest that arginyl residues participate in the functional binding of dUMP.     

Demonstration of separate binding sites for the folate coenzymes and deoxynucleotides with inactivated Lactobacilluscasei thymidylate synthetase

In contrast to (+)5,10-methylenetetrahydropteroylmonoglutamate which does not bind to $\text{Lactobacillus}\text{casei}$ thymidylate synthetase, the corresponding tetraglutamate analog binds to a single site with a K_D = 2 × 10^?5 M. Alkylation of one of the enzyme's four cysteines with N-ethylmaleimide or iodoacetate prevented the binding of dUMP, but did not affect the binding of the pteroyltetraglutamate. Inactivation of the synthetase with carboxypeptidase A, however, prevented the binding of (+)5,10-methylenetetrahydropteroyltetraglutamate but not that of dUMP. The binding of (+)5,10-methylenetetrahydropteroyltetraglutamate to native enzyme was associated with the appearance of a positive circular dichroic band at 303 nm ([θ] = 7 × 10⁴ deg·cm²dmol^?1). The latter effect was not impaired by the inhibition of the enzyme with N-ethylmaleimide, whereas formation of the ternary complex, coenzyme-synthetase-FdUMP, was prevented by alkylation. These studies reveal that thymidylate synthetase can be inactivated in a manner that does not prevent the binding of the substrates individually.     

Reciprocal changes in the levels of functionally related folate enzymes during the culture cycle in human fibroblasts

The activities of 6 folate enzymes were measured in extracts of human diploid skin fibroblasts during the lag, log and stationary phases of the culture cycle. The levels of 4 folate enzymes involved in nucleic acid biosynthesis, $\text{viz.}$, folate reductase, serine hydroxymethyltransferase, thymidylate synthetase and 10-formyl-THF synthetase, increased from 2–20 fold during the log phase of growth. In contrast, the levels of 2 enzymes, $\text{viz.}$, methylene-THF reductase and 5-methyl-THF: homocysteine methyltransferase, involved in regulating the levels of 5-methyl-THF, the major tissue and serum folate compound, decreased 3–4 fold during log growth, returning to high levels again only after the cells had been in the stationary phase for 5 and 20 days respectively. This reciprocal pattern of change is consistent with the known or postulated functions of these folate enzymes.     

5(E)-(3-azidostyryl)-2′-deoxyuridine 5′-phosphate is a photoactivated inhibitor of thymidylate synthetase

The title compound is a photoaffinity labeling reagent for thymidylate synthetase, a key enzyme for the $\text{de}\text{novo}$ biosynthesis of DNA. This compound is also a light-dependent inhibitor of murine (L-1210) and human (Namalva, Raji) tumor cell growth, and vaccinia virus replication.     

Detection by electron spin resonance of an exchange-coupled Cob(II)alamin...free radical pair species generated by anaerobic photolysis of polycrystalline adenosylcobalamin

Exposure to nitrous oxide (N₂O) $\text{in vivo}$ is accompanied by oxidation of cob[I]alamin to the inactive cob[III]alamin [1] and to loss of methionine synthetase activity [2]. There is a steady increase in thymidylate synthetase activity in marrow collected from rats exposed to N₂O and this returns to normal on restoring the animals to an air environment.     

Quenching of thymidylate synthetase fluorescence by substrate analogs.

Quenching of fluorescence occurs when $\text{Lactobacillus casei}$ thymidylate synthetase is titrated with fluorodeoxyuridylate in the presence of 1-L-methylenetetrahydrofolate to form a ternary complex. Neither fluorodeoxyuridylate nor 1-L-methylenetetrahydrofolate added singly has any effect on enzyme fluorescence but d-L-methylenetetrahydrofolate alone causes quenching. Thus ternary complex formation and interaction with d-L-methylenetetrahydrofolate alter the environment of tryptophan residues in thymidylate synthetase in a similar manner.     

5 (alpha-bromoacetyl)-2'=deoxyuridine 5'-phosphate: a mechanism based affinity label for thymidylate synthetase.

5(α-Bromoacetyl)-2′-deoxyuridine 5′-phosphate is an active site-directed irreversible inhibitor of thymidylate synthetase from $\text{Lactobacillus casei}$. The reversible inhibition (K_I4uM) is competitive with substrate and on incubation the reversible enzyme-inhibitor complex is converted to the irreversible complex with a first order rate constant (k₂) of 0.15 min^?1.     

Effects of the diastereoisomers of methylenetetrahydrofolate on the reaction catalyzed by thymidylate synthetase

Diastereoisomers of methylenetetrahydrofolate were prepared and shown to have equal and opposite CD spectra. In addition to being inactive in the spectrophotometric assay for $\text{Lactobacillus casei}$ thymidylate synthetase, the diastereoisomer having the unnatural configuration at carbon 6, does not promote removal of tritium from 5-(³H)-dUMP. It is a competitive inhibitor of the reaction with a K₁ of 5 × 10^?5$\text{M}$.     

The role of triiodothyronine in the regulation of synthesis rate and translatable mRNA level of fatty acid synthetase in a preadipocyte cell line

Triiodothyronine (T₃) effects on the activity, rate of synthesis and mRNA content of the key lipogenic enzyme, fatty acid synthetase, were studied in differentiating ob₁₇ preadipocytes cloned from ob/ob mouse epididymal adipose tissue. During differentiation in the presence of insulin, a 6–10-fold increase in both fatty acid synthetase specific activity and synthesis rate were reproducibly observed and occurred concomitantly. The relative synthesis rate exhibited a progressive elevation from 0.5% at confluence to a maximum level of 2% in the presence of insulin. The rate of the enzyme degradation determined by pulse-chase experiments was similar in differentiating cells and insulin-untreated cells of the same age ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, 40–42 h). Furthermore, the increase in the enzyme synthesis rate was preceded by a progressively elevating amount of mRNA encoding for this protein as detected by translation in a reticulocyte lysate cell-free system. It is thus suggested that the increment in total and neosynthesized fatty acid synthetase in essentially due to an increased enzyme synthesis, reflecting an increased relative content of its specific mRNA. T₃ included at a physiological concentration (1.5 nM) in the culture medium enhanced significantly both enzyme synthesis and its specific mRNA. The most important T₃ effect was an acceleration of both processes, a stimulation of the mRNA level being detected as early as day 3 post-confluence and maximum at day 5 when the effect on the synthetase synthesis rate and activity began to be enhanced. This suggests that T₃ would mainly affect fatty acid synthetase as a pretranslational level.     

Prostaglandin endoperoxide synthetase-mediated metabolism of carcinogenic aromatic amines and their binding to DNA and protein

The arachidonic acid-dependent metabolism of the carcinogens, 2-naphthylamine, 4-aminobiphenyl, 2-aminofluorene, benzidine, and N-methyl-4-aminoazobenzene was mediated by a prostaglandin endoperoxide synthetase preparation. Phenacetin, a suspected carcinogen, was not a substrate but its deacetylated metabolite, $\text{p}$-phenetidine, was rapidly oxidized. For each arylamine, extensive metabolism (14–81%) was observed, resulting in high levels of products bound covalently to protein. A low level of binding to added DNA was also detected for each substrate, except $\text{p}$-phenetidine and N-methyl-4-aminoazobenzene. Chromatography of the ethyl acetate-extractable metabolites indicated that the major products were N-oxidized and/or C-oxidized derivatives.     

Cellular pH and water permeability control in frog urinary bladder a possible action on the water pathway

Mucosal acidification (from pH 8.1 to 6.0) reversibly inhibited the hydroosmotic responses to oxytocin, cyclic AMP and 8-bromo-cyclic AMP in frog urinary bladder. These inhibitory effects were only observed in the presence of a permeant buffer in the apical medium and could also be elicited by CO₂ bubbling, even when the mucosal pH was clamped at 8.1. Acid pH reduced the oxytocin-induced net water flux faster than norepinephrine or oxytocin removal and the difference was especially important at low temperature. The time course of recovery from acid pH inhibition was, at 20°C, similar to that of the hormonal action, but when the medium temperature was reduced to 6–7°C, the recovery from acid pH inhibition paradoxically became faster while the oxytocin action was markedly slowed down ($\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of changes in net water fluxes (expressed in min): oxytocin addition at 20°C, $\text{6.2 ± 0.9}$; at 6°C, $\text{24 ± 3}$; oxytocin removal at 20°C, $\text{4.7 ± 0.8}$; at 6°C, $\text{22 ± 3}$; pH inhibition at 20°C, $\text{2.6 ± 0.2}$; at 6°C $\text{2.5 ± 0.2}$; recovery from pH 6 at 20°C, $\text{6.5 ± 0.9}$; at 6°C, $\text{2.7 ± 0.3}$). These results can be explained by accepting two main loci sensitive to medium acidification: (1) the cyclase system and (2) an intracellular, temperature-independent, post-cyclic AMP site. The fact that the intramembranous particle aggregates associated with the oxytocin-induced water permeability increase did not disappear after the flow inhibition by acid pH at low temperature suggests that the second effect could be located at the water channel itself.     

Effect of pH, carbamylation and other hemoglobins on deoxyhemoglobin S aggregation inside intact erythrocytes as detected by proton relaxation rate measurements.

Measurement of the transverse water proton relaxation rate has been used to study the effect of pH, carbamylation, and other hemoglobins on the aggregation of deoxyhemoglobin S inside intact erythrocytes. Upon complete deoxygenation, cyanate-treated ($\text{S}\text{S}$) erythrocytes and erythrocytes heterozygous with respect to hemoglobin S ($\text{A}\text{S}$, $\text{C}\text{S}$, and $\text{S}\text{D}$) have high transverse water proton relaxation rates very similar to the values obtained with homozygous ($\text{S}\text{S}$) erythrocytes. These results suggest extensive intermolecular interactions between deoxyhemoglobin S molecules and a resultant increase in the correlation time for the small fraction of “irrotationally bound” water. When the transverse relaxation rate in deoxygenated ($\text{S}\text{S}$) erythrocytes was measured as a function of pH, the maximum rate was observed between pH 7.0 and 7.5. Upon increasing the pH beyond this range the observed relaxation rate decreases as does the number of sickled cells. Upon decreasing the pH, the observed transverse relaxation rate also decreases but the ratio of values from $\text{deoxy}\text{oxy}$ ($\text{S}\text{S}$) erythrocytes remains in the normal range of 4–6 and the number of sickled cells does not change. Therefore, the deoxyhemoglobin S aggregate inside sickled erythrocytes, as observed by water proton relaxation rates, is not altered by carbamylation or by the presence of nongelling hemoglobins. In addition, the enhancement of the relaxation rates as a function of pH is consistent with the number of sickled forms observed.     

Detection of two forms of glutamine synthetase in Bacillus brevis (A. G. 4)

A laboratory isolate of $\text{Bacillus}\text{brevis}$ could grow and sporulate on an amino acid, viz., alanine or glutamate or aspartate as single source of carbon and nitrogen. It failed to sporulate if the amino acid was replaced by the corresponding keto acid and ammonium sulphate in the medium, although, normal growth was observed. One of the key enzymes in nitrogen assimilation, the glutamine synthetase, has been purified by DE-52 and affinity column chromatography from both alanine and pyruvate grown cells. The kinetic and other properties of both of these enzymes were studied. The enzyme isolated from alanine grown cells differed significantly from that isolated from pyruvate grown cells (viz.,pH optima, response to Mg⁺⁺ and other effectors). A possible role of glutamine synthetase in the initiation of bacterial sporulation is discussed.     

Preferential utilization of glutamine for amination of xanthosine 5'-phosphate to guanosine 5'-phosphate by purified enzymes from Escherichia coli

GMP synthetase was purified 180-fold from $\text{E. coli}$ B and 18-fold from the derepressed purine auxotroph, $\text{E. coli}$ B-96. The enzymes from both sources show the same preference for glutamine over ammonia as amino donor. Each is dimeric, consisting of subunits of molecular weight about 60,000. Thus the two are apparently identical. The similarities between GMP synthetase and xanthosine 5′-phosphate aminase of $\text{E. coli}$ B-96 (N. Sakamoto, G.W. Hatfield, and H.S. Moyed, J. Biol. Chem. (1972) $\text{247}$, 5880–5887) in respect to structure, state of derepression, and behavior during purification, lead us to the conclusion that the synthetase and the aminase are a single entity. We observe no loss or separation of glutamine-dependent activity upon purification of GMP synthetase and we suggest that such loss, reported by other workers, results artifactually by inactivation of an intrinsic glutamine-binding site. GMP synthetase appears not to contain a glutamine-binding subunit which is separable from the xanthosine 5′-phosphate-aminating component.     

Disappearance of calcium-induced phase separation in phosphatidylserine-phosphatidylcholine membranes caused by protonation and by electric current

Disappearance of Ca²⁺-induced phase separation in phosphatidylserine-phosphatidylcholine membranes has been studied under several conditions by monitoring electron spin resonance spectrum of spin-labeled phosphatidylcholine. The membranes were prepared in Millipore filters. Electron micrographs of the preparations showed formation of multilayered structures lined on the pore surface. The phase separation was disappeared when the membrane was soaked in non-buffered salt solution (100 ml KCl, pH 5.5). It was markedly contrasting that when the bathing salt solution was buffered no disappearance was observed. Disappearance of the phase separation was also observed when the Ca²⁺-treated membrane was transferred to acidic salt solutions ($\text{?}\text{pH 2.5}$) or to low ionic strength media ($\text{? 10}\text{mM}$) buffered at pH 5.5, and then to the buffered salt solution (100 mM KCl, pH 5.5). These are due to replacement of Ca²⁺ by proton, proton-induced separation, followed by disappearance of the phase separation inthe buffered salt solution. Biological significance of the competition between Ca²⁺ and proton for the phase separation or domain formation in the membranes was emphasized.     

The use of affinity elution from Blue Dextran Sepharose by yeast tRNA2Val in the complete purification of the cytoplasmic valyl-tRNA synthetase from Euglena gracilis

Two distinct monomers, α and β participate in the structures of diffe$\text{r}$ ent oligomers of $\text{Neurospora crassa}$ glutamine synthetase (EC 6. 3. 1. 2). In ammonium-limited cultures a tetrameric form composed mainly of α monomers was found. In excess of nitrogen an octameric form composed mainly from β monomers is the predominant oligomeric state. The presence of both monomers was observed in intermediate oligomeric forms.     

Adaptive synthesis of rat liver fatty acid synthetase: evidence for in vitro formation of active enzyme from inactive protein precursors and 4'-phosphopantetheine

Evidence is presented in support of the hypothesis that an important step in the adaptive synthesis of fatty acid synthetase is the conversion of inactive enzyme precursors to active enzyme via the incorporation of the 4′-phosphopantetheine prosthetic group. Fatty acid synthetase activity was generated $\text{in vitro}$ when CoA or $\text{E. coli}$ acyl carrier protein was incubated with enzymatically inactive extracts from livers of rats fed a fat-free diet for 0–5 hr following starvation, and a factor present in liver extracts from rats refed for more than 6 hr. When (¹⁴C)-CoA, labelled in the pantetheine moiety, was used in the above system, radioactivity was incorporated into a protein bound form, from which it could be released by mild alkaline hydrolysis.     

Biosynthesis of monoterpenes: preliminary characterization of bornyl pyrophosphate synthetase from sage (Salvia officinalis) and demonstration that Geranyl pyrophosphate is the preferred substrate for cyclization.

Previous studies with soluble enzyme preparations from sage (Salvia officinalis) demonstrated that the monoterpene ketone (+)-camphor was synthesized by the cyclization of neryl pyrophosphate to (+)-bornyl pyrophosphate followed by hydrolysis of this unusual intermediate to (+)-borneol and then oxidation of the alcohol to camphor (R. Croteau, and F. Karp, 1977, Arch. Biochem. Biophys.184, 77–86). Preliminary investigation of the (+)-bornyl pyrophosphate synthetase in crude preparations indicated that both neryl pyrophosphate and geranyl pyrophosphate could be cyclized to (+)-bornyl pyrophosphate, but the presence of high levels of phosphatases in the extract prevented an accurate assessment of substrate specificity. The competing phosphatases were removed by combination of gel filtration on Sephadex G-150, chromatography on hydroxylapatite, and chromatography on O-(diethylaminoethyl)-cellulose. In these fractionation steps, activities for the cyclization of neryl pyrophosphate and geranyl pyrophosphate to bornyl pyrophosphate were coincident, and on the removal of competing phosphatases, the synthetase was shown to prefer geranyl pyrophosphate as substrate ($\text{V}\text{K}{}_{\mathrm{m}}$ for geranyl pyrophosphate was 20-fold that of neryl pyrophosphate). No interconversion of geranyl and neryl pyrophosphates was detected. The partially purified bornyl pyrophosphate synthetase had an apparent molecular weight of 95,000, and required Mg²⁺ for catalytic activity (K_m for Mg²⁺ ~ 3.5 mm). Mn²⁺ and other divalent cations were ineffective in promoting the formation of bornyl pyrophosphate. The enzyme exhibited a pH optimum at 6.2 and was strongly inhibited by both p-hydroxymercuribenzoate and diisopropylfluorophosphate. Bornyl pyrophosphate synthetase is the first monoterpene synthetase to be isolated free from competing phosphatases, and the first to show a strong preference for geranyl pyrophosphate as substrate. A mechanism for the cyclization of geranyl pyrophosphate to bornyl pyrophosphate is proposed.