Endogenous factor in rat liver that converts tyrosine aminotransferase form III to form I: a rapid assay of this converting factor.

After induction by cortisol, tyrosine aminotransferase (l-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) present in rat liver homogenates can be resolved into three peaks of activity by CM-Sephadex chromatography. Based on differential elution of these forms by a linear KCl gradient, a three-tube assay was developed that quantitates the amount of form III relative to total enzyme. The assay was used to determine the presence of a factor in the liver that converts tyrosine aminotransferase form III to form I. Definitive evidence for the liberation of such a factor is presented.     

Influence of glutathione on the mutagenicity of 2-chloroethylnitrosoureas. Mutagenic potential of glutathione derivatives formed from 2-chloroethylnitrosoureas and glutathione

2-Chloroethylnitrosoureas (CNU) are antineoplastic agents whose therapeutic dose is limited by toxic and carcinogenic side effect. The clinically used drugs, bis-(2-chloroethyl)nitrosourea (BCNU) and 1-(2-chloroethyl)-3-(2-hydroxyethyl)-1-nitrosourea (HECNU) and their analogue N-(2-chloroethyl)-N-nitrosocarbamoyl-glycinamide (CNC-GA) were tested for mutagenicity and toxicity in the Salmonella typhimurium tester strain TA1535 in the presence and absence of glutathione (GSH). All 3 compounds proved to be potent mutagens. The cytotoxicity of these CNUs, however, varied depending on their carbamoylating activity. These cytotoxic effects were decreased considerably by the addition of GSH. It has been shown that the isocyanate decomposition product of the 2-chloroethylnitrosoureas reacts with GSH yielding S-carbamoylated GSH derivatives. The adducts resulting from coincubation of BCNU or HECNU with GSH, 2-chloroethyl-S-carbamoyl-GSH and 2-hydroxy-S-carbamoyl-GSH, were also tested for their mutagenic activity. While the hydroxyethylated compound exhibited no effects, 2-chloroethyl-S-carbamoyl-GSH and its cysteine analogue, 2-chloroethyl-S-carbamoyl-GSH, were strong mutagens. Further experiments with 3-chloropropyl-S-carbamoyl-GSH and t-butyl-S-carbamoyl-GSH indicate that a chlorine substituent in the beta position is necessary for the induction of a potent mutagenic response.     

Selective modulation of nucleotide levels in rat liver and hepatomas by high-orotate or arginine-deficient diets and by carbamoylating agents

Transplanted Morris hepatomas in Buffalo-strain rats were found to be resistant to the changes in ribonucleotide levels in rat liver caused by a high-orotate diet or an arginine-deficient diet. The increase in UTP levels and decrease in ATP levels seen in the livers of rats on a 1%-orotate diet were less marked in the livers of BUB- and DBA-strain mice on this diet. Although the changes were less than in rat liver, there was a 2-3-fold increase in UTP concentration in the livers of mice on the high-orotate diet. However, there was a similar response in nucleotide levels in the two species when the animals were maintained on an arginine-deficient diet, and there was a greater than 10-fold increase in the UTP level in the livers of both rats and mice. These diets had much less effect on the levels of deoxyribonucleotides than of ribonucleotides. In contrast to the insensitivity of hepatomas to dietary modulation of nucleotide levels, treatment of hepatoma-bearing rats with carbamoylating agents (sodium cyanate and 2-chloroethyl isocyanate) caused decreases in the levels of nucleotides in the tumors which were generally greater than in host livers. For example, 2-chloroethyl isocyanate depressed ATP levels in the Morris hepatomas 5123C and 20 under conditions in which there was no significant effect on host liver ATP. The data revealed selective modulation of nucleotide levels in normal and neoplastic liver which may be achieved by either dietary modification or drug treatment.     

Studies on the conversion of multiple forms of tyrosine aminotransferase in rat liver.

Studies from several laboratories have demonstrated the existence of at least three separable forms of the hepatic enzyme, tyrosine aminotransferase. The significance of these separable forms of the enzyme isolated in vitro for the nature and regulation of the enzyme in vivo has been the subject of some controversy. The studies reported in this paper demonstrate the existence of a heat-labile, pH- and temperature-dependent, nondialyzable component associated predominantly with the lysosomal and mitochondrial fraction of rat liver which catalyzes the conversion of form II to forms III and IV of the enzyme. The activity of this conversion factor is not significantly affected by F^?, molybdate ions, or two inhibitors of proteases. On the other hand, the cyanate ion completely inhibits the conversion of form II to forms III and IV of tyrosine aminotransferase, as do iodoacetate and oxidized glutathione. p-Chloromercuribenzoate also markedly inhibits the conversion. Kinetic studies suggest that the shift from one form to another follows the pathway: II to III to IV. Titration of the available sulfhydryl groups of the three forms of the enzyme demonstrates that form II possesses between 16 and 17 titratable SH groups per mole, while forms III and IV possess 15 and 13 or 14, respectively. The possible catalytic mechanism by which the conversion of the multiple forms of tyrosine aminotransferase is accomplished is discussed.     

Stabilization of rat liver tyrosine aminotransferase by tetracycline.

Rat liver tyrosine aminotransferase was purified 200-fold and an antiserum raised against it in rabbits. 2. Hepatic tyrosine aminotransferase activity was increased fourfold by tyrosine, twofold by tetracycline, 2.5-fold by cortisone 21-acetate and ninefold by a combination of tyrosine and cortisol administered intraperitoneally to rats. 3. Radioimmunoassay with 14C-labelled tyrosine aminotransferase, in conjunction with rabbit antiserum against the enzyme, revealed that cortisol stimulates the synthesis of the enzyme de novo, but that tetracycline has no such effect. 4. Incubation of rat liver homogenates with purified tyrosine aminotransferase in vitro leads to a rapid inactivation of the enzyme, which tetracycline partially inhibits. 5. The inactivation is brought about by intact lysosomes, and the addition of 10mM-cysteine increases the rate of enzyme inactivation, which is further markedly increased by 10mM-Mg2+ and 10mM-ATP. Here again tetracycline partially inhibits the decay rate, leading to the inference that the increase of tyrosine aminotransferase activity in vivo by tetracycline is brought about by the latter inhibiting the lysosomal catheptic action.     

Functional significance of aromatic amino acid: aromatic keto acid aminotransferases in rat brain and liver: competition for tryptophan between aminotransferases and tryptophan hydroxylase in vitro.

Using low concentrations of substrates and cofactors, a comparison was made of the relative rates by which aminotransferases catalysed transaminations between aromatic amino acids and aromatic or aliphatic keto acids. Tryptophan aminotransferase in homogenates of rat midbrain and liver transaminated phenylpyruvate at a rate 70 to 150-fold greater than the rate with α-ketoglutarate at low concentrations of substrates. Phenylalanine aminotransferase in liver and midbrain also was more active with aromatic keto acids than with aliphatic keto acids. However, tyrosine aminotransferase in dialysed homogenates of midbrain transaminated α-ketoglutarate and phenylpyruvate at approximately equal rates. Fresh homogenates of midbrain contained an inhibitor which markedly decreased tyrosine aminotransferase activity with α-ketoglutarate but not with phenylpyruvate. Tyrosine aminotransferase in homogenates of rat liver transaminated α-ketoglutarate and phenylpyruvate at equal rates below 10 μM keto acid, but above 10 μM, transamination of α-ketoglutarate was favoured. With homogenates of liver, transamination of α-ketoglutarate, but not phenylpyruvate, by tyrosine was increased 650% by exogenous pyridoxal phosphate. Since tryptophan aminotransferase in the brain may compete with tryptophan hydroxylase for available tryptophan, a comparison was made of the relative activities of tryptophan hydroxylase and tryptophan aminotransferase. At concentrations above 7.5 μM phenylpyruvate, transamination was 8 to 17-fold greater than the rate of hydroxylation of 50 μM tryptophan.     

Interconversion of multiple forms of tyrosine aminotransferase in vitro and in vivo in cultured hepatoma cells.

Corticosteroid-induced tyrosine aminotransferase (EC 2.6.1.5) from cultured hepatoma cells was separated by carboxymethyl-Sephadex chromatography into three molecular forms resembling those described previously in the rat liver. Enzyme forms were isolated and used as purified substrates to examine their in vitro interconversion by various subcellular fractions. Isolated form III was converted to forms II and I, and isolated form II was converted to form I by the coarse particulate fraction sedimenting at 1000 X g. This activity was inhibited by the serine enzyme inhibitor phenylmethane sulfonyl fluoride or by raising the pH to 8.7. Conversion of enzyme forms in vitro in the opposite direction (I leads to II leads to III) could not be detected. The distribution of enzyme forms in vivo was examined by the use of experimental conditions that prevent their in vitro interconversion during cell extraction. Tyrosine aminotransferase extracted from cell subjected to various treatments that affect the rates of enzyme synthesis or degradation existed always predominantly as form III. It appears, therefore, that multiple forms of tyrosine aminotransferase are not related to the turnover of this enzyme in vivo.     

Semichronic Inhibition of Glutathione Reductase Promotes Oxidative Damage to Proteins and Induces both Transcription and Translation of Tyrosine Hydroxylase in the Nigrostriatal System

We have evaluated the effect of N,N-bis (2-chloroethyl)-N-nitrosourea (BCNU), an inhibitor of glutathione reductase (GR), on the oxidative status along with the integrity of the nigrostriatal dopaminergic system of the rat. The oxidative status was studied by the quantification of carbonyl groups coupled to protein homogenates. Moreover, the specific oxidations in glial fibrillary acidic protein (GFAP) and neurofilament-200 (NF-200) were also measured. The results show that oxidative damage in proteins in the nigrostriatal system is confined to the striatum. Specific carbonyl groups coupled to native NF-200 and GFAP were also increased. These changes were accompanied by reactive astrocytosis in striatum but not in substantia nigra. In substantia nigra, decreased levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) were observed following BCNU treatment. In contrast, DA levels were increased in the striatum along with an overall decrease in the ratios of DA metabolites to DA. We also studied the mRNA levels for tyrosine hydroxylase (TH) and the dopamine transporter (DAT) by in situ hybridization. TH mRNA but not DAT mRNA was significantly induced in substantia nigra following BCNU treatment, which was consistent with significant elevations in TH enzyme amount and activity and unchanged DA uptake in striatum. All these results support the DA free radical hypothesis and the key role of the striatal glutathione system in protecting the striatal system against oxidative stress.     

Semichronic inhibition of glutathione reductase promotes oxidative damage to proteins and induces both transcription and translation of tyrosine hydroxylase in the nigrostriatal system

We have evaluated the effect of N,N-bis (2-chloroethyl)-N-nitrosourea (BCNU), an inhibitor of glutathione reductase (GR), on the oxidative status along with the integrity of the nigrostriatal dopaminergic system of the rat. The oxidative status was studied by the quantification of carbonyl groups coupled to protein homogenates. Moreover, the specific oxidations in glial fibrillary acidic protein (GFAP) and neurofilament-200 (NF-200) were also measured. The results show that oxidative damage in proteins in the nigrostriatal system is confined to the striatum. Specific carbonyl groups coupled to native NF-200 and GFAP were also increased. These changes were accompanied by reactive astrocytosis in striatum but not in substantia nigra. In substantia nigra, decreased levels of dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) were observed following BCNU treatment. In contrast, DA levels were increased in the striatum along with an overall decrease in the ratios of DA metabolites to DA. We also studied the mRNA levels for tyrosine hydroxylase (TH) and the dopamine transporter (DAT) by in situ hybridization. TH mRNA but not DAT mRNA was significantly induced in substantia nigra following BCNU treatment, which was consistent with significant elevations in TH enzyme amount and activity and unchanged DA uptake in striatum. All these results support the DA free radical hypothesis and the key role of the striatal glutathione system in protecting the striatal system against oxidative stress.     

Subcellular distribution of a factor inactivating tyrosine aminotransferase. Study of its mechanism and relationship to different forms of the enzyme.

The subcellular distribution of a tyrosine aminotransferase inactivating factor in rat liver has been investigated. Most of its activity is associated with plasma membranes, with minor amounts in mitochondria and endoplasmatic reticulum. The factor is also found in kidney and inactivates the enzyme reversibly in presence of cysteine, most likely by modification of -SH groups. ATP counteracts this inactivation only, when crude enzyme extracts are inactivated by purified subcellular fractions or when the purified enzyme is inactivated in presence of liver or kidney cortex homogenates. The relationship of this inactivation to reported different forms of the enzyme has been investigated. Form I of three different forms, that can be obtained by hydroxyl-apatite chromatography, is readily inactivated, form III can be partly converted to form I by incubation in presence of purified plasma membranes. The relationship of these findings to a possible multistep mechanism in the turnover of the enzyme discussed.     

Binding of inactivated tyrosine aminotransferase to microsomal membranes

Tyrosine aminotransferase from guinea pig liver spontaneously inactivates both in crude homogenates and in tissue slices. In the course of inactivation the cytosolic enzyme progressively translocates only into the microsomal fraction under an inactive form. Translocated enzyme activity can be restored by dithiothreitol addition which also produces the release of the enzyme from the microsomal particles. The specific binding of tyrosine aminotransferase to microsomal particles as a critical event for subsequent proteolytic degradation of the enzyme is postulated.     

Carbamoylating Activity Associated with the Activation of the Antitumor Agent Laromustine Inhibits Angiogenesis by Inducing ASK1-Dependent Endothelial Cell Death

The anticancer agent 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)-2-[(methylamino)carbonyl]hydrazine (laromustine), upon decomposition in situ, yields methyl isocyanate and the chloroethylating species 1,2-bis(methylsulfonyl)-1-(2-chloroethyl)hydrazine (90CE). 90CE has been shown to kill tumor cells via a proposed mechanism that involves interstrand DNA cross-linking. However, the role of methyl isocyanate in the antineoplastic function of laromustine has not been delineated. Herein, we show that 1,2-bis(methylsulfonyl)-1-[(methylamino)carbonyl]hydrazine (101MDCE), an analog of laromustine that generates only methyl isocyanate, activates ASK1-JNK/p38 signaling in endothelial cells (EC). We have previously shown that ASK1 forms a complex with reduced thioredoxin (Trx1) in resting EC, and that the Cys residues in ASK1 and Trx1 are critical for their interaction. 101MDCE dissociated ASK1 from Trx1, but not from the phosphoserine-binding inhibitor 14-3-3, in whole cells and in cell lysates, consistent with the known ability of methyl isocyanate to carbamoylate free thiol groups of proteins. 101MDCE had no effect on the kinase activity of purified ASK1, JNK, or the catalytic activity of Trx1. However, 101MDCE, but not 90CE, significantly decreased the activity of Trx reductase-1 (TrxR1). We conclude that methyl isocyanate induces dissociation of ASK1 from Trx1 either directly by carbamoylating the critical Cys groups in the ASK1-Trx1 complex or indirectly by inhibiting TrxR1. Furthermore, 101MDCE (but not 90CE) induced EC death through a non-apoptotic (necroptotic) pathway leading to inhibition of angiogenesis in vitro. Our study has identified methyl isocyanates may contribute to the anticancer activity in part by interfering with tumor angiogenesis.     

Altered protein synthesis rate in ovaries of D. melanogaster caused by new antitumour alkylating agents.

1. The effect of two homo-aza-steroidal esters with antineoplastic activity, namely 3 beta-hydroxy-13 alpha-amino-13,17-seco-5 alpha-androstan-17-oic-13,17-lactam-p-bis(2-chloroethyl)aminoph enoxyacetate (NSC 294859) and 3 beta-hydroxy-13 alpha-amino-13,17-seco-5 alpha-androstan-17-oic-13,17-lactam-p-bis(2-chloroethyl)aminoph enylacetate (ASE) on protein synthesis rate was studied in ovaries of Drosophila melanogaster females. 2. Two different concentrations for each compound were examined. 3. Both esters containing the same alkylating agent have been shown to decrease protein synthesis in relation to control.     

Interconversion of multiple forms of tyrosine aminotransferase in vitro and in vivo in cultured hepatoma cells

Corticosteroi-induced tyrosine aminotransferase (EC 2.6.1.5) from cultured hepatoma cells was separated by carboxymethyl-Sephadex chromatography into three molecular forms resembling those described previously in the rat liver. Enzyme forms were isolated and used as purified substrates to examine their in vitro interconversion by various subcellular fractions. Isolated form III was converted to forms II and I, and isolated form II was converted to form I by the coarse particulate fraction sedimenting at 1000 × g. This activity was inhibited by the serine enzyme inhibitor phenylmethane sulfonyl fluoride or by raising the pH to 8.7. Conversion of enzyme forms in vitro in the opposite direction (I → II → III) could not be detected. The distribution of enzyme forms in vivo was examined by the use of experimental conditions that prevent their in vitro interconversion during cell extraction. Tyrosine aminotransferase extracted from cells subjected to various treatments that affect the rates of enzyme synthesis or degradation existed always predominantly as form III. It appears, therefore, that multiple forms of tyrosine aminotransferase are not related to the turnover of this enzyme in vivo.     

Tyrosine aminotransferase converting factor: kinetic properties, cellular localization, and tissue distribution.

Utilizing a more rapid procedure for determining tyrosine aminotransferase-converting factor activity, the kinetic properties of this factor were characterized further. Tyrosine aminotransferase-converting factor is a heat-labile substance present in the particulate fraction of rat liver that converts tyrosine aminotransferase form III to I at 4 degrees C. Analysis of the distribution of marker enzymes for mitochondria and lysosomes, and of converting factor, following differential and discontinuous sucrose gradient centrifugation indicated that this factor was associated with light lysosomes. The activity of converting factor was not altered following administration of cortisol. Converting factor activity, equivalent to that in liver, was also observed in particulate fractions from kidney and spleen, and to a lesser extent, in pancreas and salivary gland. No detectable activity was observed in brain, heart, small intestine, skeletal muscle, red blood cells, serum, or plasma. The presence of converting factor activity in kidney and spleen suggests that other proteins are substrates for this factor since tyrosine aminotransferase is virtually absent from these tissues. Alternatively, the absence of converting factor from other tissues need not indicate they are devoid of converting factor-like activity merely, that such activity in these tissues has different specificities and does not utilize tyrosine aminotransferase as a substrate.     

Tyrosine aminotransferase converting factor kinetic properties,cellular localization,and tissue distribution

Utilizing a more rapid procedure for determining tyrosine aminotransferase-converting factor activity, the kinetic properties of this factor were characterized further. Tyrosine aminotransferase-converting factor is a heat-labile substance present in the particulate fraction of rat liver converts tyrosine aminotransferase form III to 4°C. Analysis of the distribution of marker enzymes for mitochondria and lysosomes, and of converting factor, following differential and discontinuous sucrose gradient centrifugation indicated that this factor was associated with light lysosomes. The activity of converting factor was not altered following administration of cortisol.Converting factor activity, equivalent to that in liver, was also observed in particulate fractions from kidney and spleen, and to a lesser extent, in pancreas and salivary gland. No detectable activity was observed in brain, heart, small intestine, skeletal muscle, red blood cells, serum, or plasma. The presence of converting factor activity in kidney and spleen suggests that other proteins are substrates for this factor since tyrosine aminotransferase is virtually absent from these tissues. Alternatively, the absence of converting factor from other tissues need not indicate they are devoid of converting factor-like activity merely, that such activity in these tissues has different specificities and does not utilize tyrosine aminotransferase as a substrate.     

The effect of cytosine arabinoside and bromodeoxyuridine on the appearance of tyrosine aminotransferase in cultured foetal hepatocytes of the rat.

1. The acquisition of dexamethasone-inducibility of tyrosine aminotransferase activity by hepatocytes cultured from 15-day-foetal rat liver is blocked in the presence of cytosine arabinoside. 2. Similar results are obtained in the presence of bormodeoxyuridine. 3. No effects on steroid-inducibility of tyrosine aminotransferase are obtained with either of the above compounds in hepatocytes cultured from 19-day-foetal liver. 4. the inhibitory effects of the agents are substantially reversed after their removal from the culture medium. 5. The effects of bromodeoxyuridine suggest that cell differentiation, with respect to tyrosine aminotransferase-inducibility, occurs in cultures of 15-day-doetal hepatocytes. 6. The effects of cytosine arabinoside suggest that such an event is dependent on mitosis.     

Tertiary structure of tRNAPhe. A possible correlation between the structural functional unit of this tRNA and its exonic sequence.

By using an antiserum raised against rat liver tyrosine aminotransferase, it was shown that about 60% of tryptophan aminotransferase activity in rat liver extracts is catalysed by this enzyme. Induction of tryptophan aminotransferase activity by intraperitoneal injections of tryptophan or triamcinolone can be entirely attributed to the effects of these agents on tyrosine aminotransferase. The origin of the other 40% of tryptophan aminotransferase activity remains to be established. This activity increases after starvation for 48 h. It is unlikely that tryptophan transamination plays a quantitatively important role in the metabolism of tryptophan by the liver.     

Reduction (dethiolation) of protein mixed-disulfides; distribution and specificity of dethiolating enzymes and N,N'-bis(2-chlorethyl)-N-nitrosourea inhibition of an NADPH-dependent cardiac dethiolase

The S-thiolated proteins phosphorylase b (Phb) and carbonic anhydrase III (CAIII) were prepared with [3H]glutathione in a reaction initiated with diamide. These substrates were used to measure the rate of reduction (dethiolation) of protein mixed-disulfides by enzymes with properties similar to those of thioredoxin and glutaredoxin. This enzyme activity is termed a dethiolase since the identities of the enzymes are still unknown. The dethiolation of either S-[3H]glutathiolated Phb or S-[3H]glutathiolated CAIII was employed in tissue assays and for study of two partially purified dethiolases from cardiac tissue. NADPH-dependent dethiolase activity was most abundant except in rat liver and muscle. Total dethiolase activity was approximately 10-fold higher in neutrophils, 3T3-L1 cells, and Escherichia coli than in other sources. Rat skeletal muscle had 3- to 4-fold higher dethiolase activity than rat heart or liver. These data indicate that protein dethiolase activity is ubiquitous and that normal expression of the two dethiolase activities varies considerably. A partially purified cardiac NADPH-dependent dethiolase acted on Phb approximately 1.5 times faster than CAIII, and a glutathione (GSH)-dependent dethiolase acted on Phb 3 times faster than CAIII. The Km for glutathione for the GSH-dependent dethiolase was 15 microM with Phb as substrate and 10 microM with CAIII. Thus, the GSH-dependent dethiolase is probably not affected by normal changes in the cardiac glutathione content (normally approximately 3 mM). Partially purified cardiac NADPH-dependent dethiolase was inactivated by BCNU (N,N'-bis(2-chloroethyl)-N-nitrosourea) and the GSH-dependent dethiolase was unaffected under similar conditions. In a soluble extract from bovine heart, 200 microM BCNU inhibited NADPH-dependent dethiolase by more than 60% but did not affect GSH-dependent activity. These results demonstrate that BCNU is a selective inhibitor of the NADPH-dependent dethiolase.     

A rapid bioassay to determine stabilities of anticancer agents under conditions of the clonogenic assay

Summary We developed a rapid bioassay to determine in vitro drug stabilities in the clonogenic assay. The in vitro half-lives of 11 standard antitumor agents, actinomycin D, Adriamycin, bleomycin, cis-Platinum, dacarbazine, 5-fluorouracil, melphalan, mitomycin C, 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU), vinblastine, and vincristine, and four investigational drugs, Aziridinylbenzoquinone (AZQ) (NSC 182986), diamminecyclobutane-dicarboxylatoplatinum (CBDCA) (NSC 241240), Dihydroxyanthracenedione dihydrochloride (DHAD) (NSC 301739), and 1-(2-chloroethyl)-3-(2,6-dioxo-3-piperidyl-1-nitrosourea) (PCNU) (NSC 95466), were determined under conditions of the clonogenic assay as well as under storage conditions at −40° and−196°C. BCNU and AZQ at−40°C had t1/2 of 4.7 d and 2.5 d, respectively. All other drugs were stable at −40° and −196°C with t1/2>6 wk. Under assay conditions at 37°C, actinomycin D, bleomycin, dacarbazine, 5-fluorouracil, mitomycin C, vinblastine, vincristine, and DHAD were stable, with t1/2>14 d. CBDCA, AZQ, Adriamycin, cis-Platinum, melphalan, BCNU, and PCNU had t1/2 of 94,72,29,18.5,1.8,1, and 0.5 h, respectively. This work was supported by Veterans Administration Medical Research Service and by Contract CM57710 of the National Cancer Institute.