Plasmodium falciparum: inhibitors of lysosomal cysteine proteinases inhibit a trophozoite proteinase and block parasite development

Trophozoites of Plasmodium falciparum obtain free amino acids for protein synthesis by degrading host erythrocyte hemoglobin in an acidic food vacuole. We previously reported that leupeptin and L-trans-epoxysuccinyl-leucylamido(4-guanidino)butane (E-64), two inhibitors of the cysteine class of proteinases, blocked hemoglobin degradation in the trophozoite food vacuole, and we identified a 28-kDa trophozoite cysteine proteinase as a potential food vacuole hemoglobinase. We now report that the biochemical properties of the trophozoite cysteine proteinase closely resembled those of the lysosomal cysteine proteinases cathepsin B and cathepsin L. The trophozoite proteinase had a pH optimum of 5.5-6.0, near that of both lysosomal proteinases, and it was efficiently inhibited by highly specific diazomethylketone and fluoromethylketone inhibitors of cathepsin B and cathepsin L. The trophozoite proteinase preferred peptide substrates with arginine adjacent to hydrophobic amino acids, as does cathepsin L. Micromolar concentrations of the fluoromethylketone inhibitor Z-Phe-Ala-Ch2F blocked the degradation of hemoglobin in the trophozoite food vacuole and prevented parasite multiplication. In previous studies much higher concentrations of the inhibitor were not toxic for mice. Our results provide additional evidence that the 28-kDa trophozoite proteinase is a food vacuole hemoglobinase and suggest that specific inhibitors of the enzyme may have potential as antimalarial drugs.

Stimulation of rat liver growth by a 1,3-dithiole derivative, KZ-1026

The oral administration of 2-(1,3-dithiol-2-ylidene)-1-phenyl-1,3-butanedione (KZ-1026) to normal rats at doses of 50, 100 and 200 mg/kg/day for 3 days accelerated liver enlargement in association with a dose-dependent increase in the total amounts of protein, RNA and DNA in the liver. The liver weight at 24 hr after the third dose of 200 mg/kg reached 174% of the control. With respect to the effect on liver enlargement, KZ-1026 differed from phenobarbital, since KZ-1026, unlike phenobarbital, increased hepatic DNA content without significant effects on P-450 and aminopyrine-N-demethylase. Incorporation of [3H]thymidine into liver DNA was stimulated by a single dose of KZ-1026 (200 mg/kg), and it peaked at 24 hr post dose (18 times the control), followed by an increase in the number of liver nuclei. Liver growth was also accompanied by an increasing hepatic reserve cell mass, expressed by the capacity of eliminating exogenous galactose from the blood stream. Pretreatment with KZ-1026 (200 mg/kg/day) for 3 days significantly improved the survival rate of subtotally hepatectomized rats from 39% to 78%. These findings indicate that KZ-1026 accelerates hepatocyte proliferation, resulting in an enhancement of liver functional mass in normal rats.

New fluoroketones as human renin inhibitors

Renin inhibition has been evaluated for a new class of fluorinated ketones, true analogues of peptides that have been retroinverted at the C-terminal position. The readily formed hydrate of the ketone is proposed to mimic the tetrahedral intermediate that occurs during the enzyme-catalyzed hydrolysis of amide linkage. From this series of compounds it appears that the number of reverted amide bonds is crucial in terms of activity. Furthermore, a shortening of the C-terminal part of our peptide analogues and the replacement of the leucine residue in P1 by a cyclohexylalanine leads to the tripeptide analogue 12 a potent renin inhibitor (IC50 = 3.5 x 10(-9) M).

Acute exposure to formaldehyde induces hepatic metallothionein synthesis in mice

Humans risk inadvertent intraperitoneal or intravenous exposure to formaldehyde (HCHO), commonly used for disinfection of implanted or extracorporeal medical devices. Various chemical and physical stresses are known to induce hepatic metallothionein. This study examined the effect of acute parenteral administration of HCHO on induction of hepatic metallothionein synthesis. Adult male CF1 mice were administered HCHO ip and hepatic metallothionein was quantified by the cadmium-radioassay method. HCHO (50 mg/kg) increased hepatic metallothionein as early as 8 hr after dosing with maximal levels (27-fold increase) occurring at 72 hr. Metallothionein concentrations were elevated (15-fold) 24 hr after 50 or 100 mg HCHO/kg but not at lower dosages. Concomitant elevations in hepatic zinc and copper content were observed. No increases in metallothionein were observed in kidney, pancreas, or intestine 24 hr after HCHO administration (100 mg/kg, ip). Induction of metallothionein by HCHO may reflect direct de novo synthesis since the response was abolished by pretreatment with the RNA synthesis inhibitor, actinomycin D. HCHO induction of metallothionein also does not appear to be mediated by stress-induced release of corticosteroids or catecholamines from the adrenal since the response was unaltered in adrenalectomized mice. Interference with the glutathione (GSH)-dependent oxidation of HCHO by reducing hepatic GSH concentrations to 40% of control after a 2-hr pretreatment with phorone decreased the metallothionein induction response to HCHO by 33%. This result suggests that the induction may be partially due to a HCHO metabolite, e.g., formate. Confirmation of metallothionein synthesis was obtained following spectral and chromatographic analysis. Thus, HCHO and/or a metabolite produces a marked increase in hepatic metallothionein and alters hepatic zinc and copper homeostasis, all of which are transient responses. Although HCHO was only mildly hepatotoxic at the highest dose (as evidenced by an increase in plasma alanine aminotransferase activity), such changes in metallothionein synthesis and essential metal homeostasis may be part of a cellular repair mechanism operant after acute toxic cell injury.

The properties of peptidyl diazoethanes and chloroethanes as protease inactivators

Earlier work has demonstrated the irreversible inactivation of serine and cysteine proteinases by peptides with a C-terminal chloromethyl ketone group. With a C-terminal diazomethyl ketone, on the other hand, peptides become reagents specific for cysteine proteinases. We have now synthesized and examined the properties of reagents with an additional methyl side chain near the reactive grouping with the goal of diminishing side reactions in a cellular environment. Derivatives of neutral amino acids as well as of lysine and arginine have been prepared. The chloroethyl ketones are about 60% less reactive to chemical nucleophiles than the chloromethyl ketones. However, the susceptibilities of the proteases examined varied remarkably. Cathepsins B and L of the papain family of cysteine proteinases were much less susceptible (about 2 orders of magnitude less) to both peptidyl diazoethyl and chloroethyl ketones. In marked contrast, clostripain, a cysteine proteinase of a separate family was decisively more susceptible to chloroethyl ketones. The serine proteinases showed a drop in susceptibility to the chloroethyl ketones generally, and this was similar to the drop in chemical reactivity in proceeding from the chloromethyl to the chloroethyl ketone.

The ketone cinnamoyl-(1-13C-Phe)-CGly-Pro-Pro is a tetrahedral transition state analog inhibitor of C. histolyticum collagenase

The ketone cinnamoyl-(1-13C-Phe)-CGly-Pro-Pro [(4-13C-5-cinnamido-4-oxo-6-phenylhexanoyl)-Pro-Pro 2] competitively inhibits a mixture of collagenases from Clostridium histolyticum with a Ki of 40 +/- 6 nM. 13C-nmr spectroscopy of the ketone in the presence of this collagenase shows a bound 13C resonance at 102.6 ppm and the resonance of the free ketone at 212 ppm. Ketone alone shows no trace (less than 0.5%) of a resonance in the region around 100 ppm. The bound resonance is displaceable by another competitive inhibitor. This ketone is thus a transition state analog which is rehybridized from trigonal planar to tetrahedral upon binding to collagenase.

[Detection of the presence of formaldehyde in textiles by using color tests with acetylacetone and chromotropic acid]

In 61 textile materials of cotton, wool and synthetic fibre formaldehyde was determined using colour tests with acetylacetone and chromotropic acid. Full agreement of the results of both tests was demonstrated. In 61 studied materials formaldehyde was demonstrated in 44 samples and in 17 samples it was absent. Formaldehyde was present in materials submitted to end-use finish. The possibility was showed also of formaldehyde passage from the finished materials into materials not containing it following their storage together. Frequent washing and ironing of materials after finish may lead to formaldehyde loss from the materials. The usefulness is stressed of colour tests for the diagnosis and prevention of formaldehyde-induced clothing-related eczema.

Evidence that formation of an intermediate filament-protein complex plays a primary role in aggregation of neurofilaments, glial fibrillary acidic protein (GFAP)-filaments and vimentin-filaments by 2,5-hexanedione

Using a variety of cell types in culture, we have investigated the relevance of intermolecular crosslinking involving intermediate filament proteins (IF-proteins) to the changes in distribution of intermediate filaments induced by the neurotoxicant, 2,5-hexanedione (2,5HD). Aggregation of vimentin-filaments (vimentin-IF), glial fibrillary acidic protein (GFAP)-IF and neurofilaments was preceded by appearance on immunoblots of a high molecular weight complex (IF-complex) labeled by antibodies against the IF-protein of the cell type: pV-170 from human skin fibroblasts and pV-130 from 3T3 mouse fibroblasts, which were labeled by anti-vimentin and, from cultures of dissociated mouse spinal cord and dorsal root ganglia, pNFH-300 labeled by antibody against the 200 kDa neurofilament protein (NF-200 or NF-H) and a doublet labeled by antiserum to GFAP (pGFAP-145).pV-170 was detected in human skin fibroblasts within one hour of exposure to 2,5HD and the amount of both pV-170 and pNFH-300 was related to the concentration of 2,5HD and the duration of exposure. Intermediate filament-complexes were not detected in PtK1 epithelial cells by labeling of transblots with anti-cytokeratin, nor are keratin-IF aggregated by 2,5HD. Intermediate filament-complexes were not detected in fibroblasts with IF-aggregates secondary to disruption of microtubules by colchicine or in fibroblasts from patients with giant axonal neuropathy.

Human plasma lecithin-cholesterol acyltransferase. Inhibition of the phospholipase A2-like activity by sn-2-difluoroketone phosphatidylcholine analogues

Lecithin-cholesterol acyltransferase (LCAT) is a plasma enzyme which catalyzes the transacylation of the sn-2-fatty acid of lecithin to cholesterol, forming lysolecithin and cholesteryl ester. We have recently proposed a covalent catalytic mechanism for LCAT in which lecithin cleavage proceeds via the formation of a transition state tetrahedral adduct between the oxygen atom of the catalytic serine residue and the sn-2-carbonyl carbon atom of the substrate (Jauhiainen, M., Ridgway, N.D., and Dolphin, P.J. (1987) Biochim. Biophys. Acta 918, 175-188). This proposal is evaluated here by use of nonhydrolyzable sn-2-difluoroketone phosphatidylcholine analogues, known to inhibit calcium-dependent phospholipase A2. These compounds inhibited the calcium-independent phospholipase A2 activity of LCAT in a time and concentration dependent manner. The most potent analogues had a 100-fold higher affinity for the enzyme than the substrate, lecithin, when present within lecithin/apoA-I proteoliposomes. The inhibition was dependent upon the presence of a difluoromethylene group alpha to the sn-2-carbonyl carbon of the analogues. The inhibition is attributed to the formation of a tetrahedral adduct between the catalytic serine residue of LCAT and the sn-2-carbonyl carbon atom of the analogues which is stabilized by the electronegative fluorine atoms present upon the carbon atom alpha to the carbonyl carbon. This adduct mimics that proposed by us to occur during lecithin cleavage by LCAT, and the data substantiate the existence of this transition state adduct prior to the release of lysolecithin and formation of a fatty acylserine oxyester of the enzyme.

Cytoskeletal changes induced in vitro by 2,5-hexanedione: an immunocytochemical study

The effects of 2,5-hexanedione, the main metabolite of the solvents hexane and methyl butyl ketone, have been explored in different in vitro epithelial (CG5 and HEp-2) and melanoma (JR8) cells by means of immunochemistry and electron microscopy. The administration of the toxicant to the cell monolayers at noncytolytic concentrations for 24 and 48 hr exerted several effects on the cell lines studied. Most epithelial and melanoma cells detached from the substrate were in the mitotic phase, whereas cells adhering to the substrate showed time-dependent organelle changes. In fact, after treatment with 2,5-hexanedione, mitochondria appeared swollen, with distorted cristae and rarefied matrix; changes in intracytoplasmic vesicles were also detected. Cytoskeletal components were also investigated. A remarkable rearrangement of microfilaments and intermediate filaments (keratin and vimentin) was detected in a time-dependent manner. In particular, actin ruffles and intermediate filament aggregates were observed. Furthermore, the microtubular apparatus seemed to be less affected. The results here reported seem to indicate cytoskeletal components as probable targets of 2,5-hexanedione cytotoxicity in cultured cells.

The in vitro effects of alkanes, alcohols, and ketones on rat lung cytochrome P450-dependent alkoxyphenoxazone dealkylase activities

The sensitivities of cytochrome P450 (EC 1.14.14.1)-dependent benzyloxy- and ethoxyphenoxazone dealkylase (BzOPh'ase and EtOPh'ase, respectively) activities towards a series of aliphatic hydrocarbons were measured in the microsomal fraction of lung obtained from beta-naphthoflavone-treated rats. The unsubstituted hydrocarbons were straight-chain (n-hexane through n-undecane) and branch-chain (eight carbons). The substituted compounds were alcohols and ketones of hexane and octane. The data are expressed as I50 values, i.e. the hydrocarbon concentration required to cause 50% decrease in the rate of enzyme-catalyzed product (resorufin) formation. The unsubstituted aliphatic hydrocarbons exhibited I50 values towards BzOPh'ase from 0.76 microM (2,5-dimethylhexane) to 8.8 microM (n-hexane). The lung EtOPh'ase activity was insensitive towards the tested unsubstituted aliphatic hydrocarbons. When the alcohols and ketones of hexane and octane were tested against lung BzOPh'ase activity, I50 values ranged from 16 microM (1-octanol) to 4.8 mM (2,5-hexanedione). Lung EtOPh'ase activity exhibited some sensitivity towards the alcohols and ketones, and I50 values ranged from 0.52 mM (4-octanol) to 40.5 mM (2-hexanol). The data show rat lung BzOPh'ase and EtOPh'ase activities are differentially sensitive towards the selected unsubstituted aliphatic hydrocarbons and corresponding alcohols and ketones. The difference in sensitivities may reflect different requirements for an adventitious interaction between a hydrocarbon and enzyme active site.

Anti-hypoxic effect of glutathione depletors

The effect of various reduced glutathione (GSH) depletors on the survival time under normobaric and hypobaric hypoxia was examined in mice. The survival time was markedly prolonged in mice treated with glutathione S-transferase substrate, 2-cyclohexene-1-one (50-100 mg/kg, ip) and phorone (100-250 mg/kg, ip). The anti-hypoxic effect lasted for at least 3 hr and the maximum effect was found 0.5 hr after injection. Further, both compounds significantly elevated blood glucose levels 0.5-1 hr after treatment. The extent of the elevated blood glucose was nearly comparable to that of the mice treated with glucose (1-2 g/kg, ip), which was found to possess an anti-hypoxic effect. However, a GSH synthesis inhibitor, buthionine sulfoximine, could cause neither a prolongation of survival time of hypoxic mice nor an elevation of blood glucose. Moreover, unlike the depletion of hepatic GSH, brain GSH was markedly decreased by 2-cyclohexene-1-one and phorone, but not by buthionine sulfoximine. These findings suggest that the elevated blood glucose may involve in one of the mechanisms of the anti-hypoxic effect of 2-cyclohexene-1-one and phorone. A relationship between the anti-hypoxic effect and the depletion of brain GSH was also discussed.

Tris acetylacetonate aluminium(III) induces osmotic fragility and acanthocyte formation in suspended erythrocytes

Tris acetylacetonate aluminium(III) (Al(acac)3), dissolved in water, is effective in producing osmotic fragility in suspended erythrocytes in the concentration range of 0.034-0.34 mmol/l. Water solutions of Tris maltolate aluminium(III) (Al(malt)3) and aluminium lactate (Al(lac)3) are also effective but the dose-response behavior is less pronounced. Moreover, only Al(acac)3 induces a prominent generation of acanthocytes. The stronger effects of Al(acac)3 on membrane stability are attributed to the greater solubility of this complex in the cell membrane.

Cytoskeletal effects of acrylamide and 2,5-hexanedione: selective aggregation of vimentin filaments

Several neurotoxic compounds cause aggregation of neurofilaments in peripheral axons. This may represent a primary action of these chemicals or a secondary response to other cellular damage. To distinguish between these possibilities, the effects of acrylamide and 2,5-hexanedione (2,5-HD) on vimentin were examined in PtK2 cultured cells. Vimentin intermediate filaments were chosen because they are closely related, in structure, to neurofilaments. Effects on other components of the cytoskeleton (cytokeratin filaments, microtubules, and microfilaments) were also determined. Both acrylamide and 2,5-HD caused aggregation of vimentin filaments in a concentration-dependent fashion; these effects occurred at a lower concentration than alterations in other cytoskeletal filaments. The effects of both acrylamide and 2,5-HD were reversible, except at high concentrations of 2,5-HD. Crosslinking of cytoskeletal proteins was also examined. High-molecular-weight proteins with vimentin-like immunoreactivity were detected on blots from cells exposed to high concentrations of 2,5-HD. No crosslinked protein was detected after acrylamide treatment. These results suggest that both acrylamide and 2,5-HD cause a primary collapse of vimentin intermediate filaments in cultured cells. The initial redistribution of vimentin filaments occurred without apparent crosslinking of cytoskeletal proteins. The aggregation of vimentin filaments in cultured cells and of neurofilaments in vivo may share a common molecular mechanism.

Peptide methyl ketones as reversible inhibitors of cysteine proteinases

Peptide methyl ketones represent a new class of reversible, competitive cysteine proteinase inhibitor with little or no effect on serine proteinases. The affinity of the inhibitors to papain (EC 3.4.22.3), cathepsin B (EC 3.4.22.1) and cathepsin L (EC 3.4.22.15) depends on the peptide chain length and on side-chain effects. Variations in the P1 and P4 positions (terminology of Schechter and Berger) and their influence on the efficiency of the inhibitors have been investigated. The most effective inhibitors display inhibition constants in the micromolar range. In contrast to the endopeptidases papain and the cathepsins B and L, the aminoendopeptidase cathepsin H (EC 3.4.22.16) is not inhibited by N-acylated peptide methyl ketones but only by amino methyl ketones containing a free alpha-amino group. The endopeptidases are not affected by amino methyl ketones.

Methods for depleting brain glutathione

To search for a technique to deplete reduced glutathione (GSH) in brain, the influence of various types of compounds on brain GSH levels was investigated in mice. Of the compounds tested, cyclohexene-1-one, cycloheptene-1-one and diethyl maleate were shown to be potent GSH depletors in brain as well as in liver. The depletion of cerebral GSH ranged about 40-60% of control levels at 1 and 3 hr after intraperitoneal injection. Cyclohexene, cycloheptene, phorone, acetaminophen, and benzyl chloride caused mild depletion of cerebral GSH, but buthionine sulfoximine did not alter cerebral GSH levels. Further, intracerebroventricular injection of cyclohexene-1-one and cycloheptene-1-one caused depletion of brain GSH to about 60-80% of control levels at 1 hr after injection, and the effects persisted for at least 6 hr. Under these conditions, hepatic GSH was not altered. These results demonstrated that cyclohexene-1-one and cycloheptene-1-one can cause not only a marked depletion of brain GSH by systemic administration, but also depletion of cerebral GSH by intracerebroventricular injection by virtue of being water-soluble compounds. Thus, methods for depleting brain GSH employing both compounds are available for exploring possible functions of cerebral GSH in in vivo systems.

Effect of glutathione depletion on sulfate activation and sulfate ester formation in rats

Sulfation of organic compounds requires activation of inorganic sulfate via formation of adenosine 3'-phosphate 5'-phosphosulfate (PAPS). Inorganic sulfate can be formed by sulfoxidation of cysteine, which can be derived from GSH. Thus, a decrease in hepatic GSH may impair formation of inorganic sulfate, the synthesis of PAPS, and the sulfation of chemicals. This hypothesis was tested by investigating the effect of GSH depletion on the levels of inorganic sulfate in serum and of PAPS in liver, and on the capacity to form the sulfate conjugate of harmol in rats. Phorone (2 mmol/kg, i.p.) decreased hepatic GSH (97%), serum inorganic sulfate (63%), and hepatic PAPS (48%). Diethyl maleate and vinylidene chloride (6 mmol/kg, each, i.p.) were less effective than phorone in decreasing GSH in liver and inorganic sulfate in serum, and they did not alter hepatic PAPS levels. Three hours after phorone treatment, the nadir of hepatic PAPS concentration, harmol was injected in order to assess sulfation in vivo. After administration of harmol (100 and 300 mumol/kg, i.v.), less harmol sulfate and more harmol glucuronide were found in the serum of phorone-treated rats as compared to control rats. At the higher dosage of harmol, phorone reduced the biliary excretion of harmol sulfate while increasing the biliary excretion of harmol glucuronide. These results indicate that severe GSH depletion decreases PAPS formation and sulfation of chemicals. However, an increase in glucuronidation may compensate for the impaired sulfation.

Morphology of noncardiogenic pulmonary edema induced by Perilla ketone in sheep

A single infusion of Perilla ketone (PK) into sheep causes marked increases in lung fluid and solute exchange in the absence of any alteration in either pulmonary arterial or left atrial pressures. These alterations are most compatible with increased pulmonary microvascular permeability. The present paper describes the morphologic changes that accompany the previously described alterations in lung function. In five anesthetized open-chest sheep, lung biopsy tissue was taken at baseline and at 15, 30, 60, 120, and 180 minutes after the start of a single infusion of PK (15-20 mg/kg given over a 20 minute period). Biopsy tissue was taken from different lobes of the lung in random sequence, fixed, and processed for light and electron microscopic examination. Three control sheep received the vehicle, dimethyl sulfoxide, alone. Just 15 minutes after the start of PK infusion, alveolar capillary congestion, accumulation of peripheral lung neutrophils, and intraalveolar and interstitial edema were apparent. Electron microscopy revealed early evidence of damage to both the microvascular endothelial cells and Type I pneumonocytes. The damage became more severe with time. From 30 minutes, occasional nonciliated cells in the airway epithelium exhibited dilated rough and agranular endoplasmic reticulum. Thus, PK causes rapid onset of pulmonary edema accompanied by structural evidence of damage to the microvascular endothelium and Type I pneumonocytes. Pulmonary inflammation was also evident. These structural changes occur before the described alterations in either pulmonary microvascular permeability or reduction in pulmonary compliance.

Favorable effect of catechol-O-methyltransferase inhibition by OR-462 in experimental models of Parkinson's disease

A selective catechol-O-methyltransferase inhibitor, OR-462, was studied for its ability to affect pharmacokinetic properties of L-dopa after the p.o. administration of the inhibitor to rats and mice. When OR-462 was given to rats at the dose range of 0.3 to 30 mg/kg in conjunction with L-dopa and carbidopa, a dose-related and long-lasting (greater than 5 hr) increase in striatal L-dopa and dopamine levels as well as a reduction in 3-O-methyldopa levels were shown. For a 50% reduction of the 3-O-methyldopa levels a dose of 6 mg/kg of OR-462 was needed. The increase in striatal homovanillic acid, an O-methylated metabolite of dopamine which poorly penetrates the blood brain barrier, indicates that O-methylation was not inhibited in the brain. In order to get the same dopamine levels in striatum the L-dopa dose could be lowered to one-fourth when OR-462 was added. The L-dopa-sparing effect of OR-462 given p.o. was also demonstrated in two behavioral parkinsonian models. OR-462 given at doses of 3 to 30 mg/kg in conjunction with L-dopa and carbidopa, dose-dependently potentiated the L-dopa-induced reversal of hypoactivity in reserpinized mice. Likewise, the same doses of OR-462 caused a marked potentiation of L-dopa-induced contralateral turning behavior in rats with unilateral nigrostriatal lesions produced by 6-hydroxydopamine. The data suggest a possible beneficial effect of OR-462 in the therapy of Parkinson's disease.

Mutagenicity studies on ketone solvents: methyl ethyl ketone, methyl isobutyl ketone, and isophorone

3 ketone solvents (methyl ethyl ketone (MEK), methyl isobutyl ketone (MiBK), and isophorone) were tested for potential genotoxicity. The assays of MEK and MiBK included the Salmonella/microsome (Ames) assay, L5178Y/TK+/- mouse lymphoma (ML) assay, BALB/3T3 cell transformation (CT) assay, unscheduled DNA synthesis (UDS) assay, and micronucleus (MN) assay. Only the ML, UDS, and MN assays were conducted on samples of isophorone. No genotoxicity was found for MEK or isophorone. The presence of a marginal response only at the highest, cytotoxic concentration tested in the ML assay, the lack of reproducibility in the CT assay, and clearly negative results in the Ames assay, UDS and MN assays, suggest that MiBK is unlikely to be genotoxic in mammalian systems.

Glutathione concentrations in rat lung bronchoalveolar lavage fluid: effects of hyperoxia

Glutathione concentrations were measured in rat bronchoalveolar lavage fluid (BALF) obtained from normal rats and rats exposed to a fraction of inspired oxygen (FiO2) of 0.8 for up to 5 days. We also perturbed rat lung glutathione concentrations by administering the compound diisopropylidene acetone (phorone) to a separate group of animals and correlated changes in BALF glutathione with changes in lung tissue glutathione. We found that reduced glutathione is present in normal rat BALF but glutathione disulfide is extremely low. Increases in lung tissue glutathione concentration and in BALF glutathione concentration occurred after 5 days of exposure to hyperoxia. Animals treated with phorone exhibited decreases in lung glutathione concentration two hours after dosing and increases in lung glutathione concentration 24 hours after dosing. Rat BALF obtained from phorone-treated animals at 2 or 24 hours after administration revealed that changes in BALF glutathione concentrations reflected changes in lung tissue glutathione concentration. The presence of glutathione in lung lavage fluid suggests that the compound could be playing an extracellular role in the lung, either as an antioxidant or as a coenzyme for other glutathione-related enzymatic reactions.

Inhibition of catechol-O-methyltransferase activity by two novel disubstituted catechols in the rat

Catechol-O-methyltransferase (COMT) has an important role in the extraneuronal inactivation of catecholamine neurotransmitters and drugs with a catechol structure. Two novel COMT inhibitors, OR-462 and OR-486, were highly effective (IC50 = 18 and 12 nM, respectively) and selective in inhibiting COMT activity in vitro. Tyrosine hydroxylase was not inhibited until micromolar concentrations of these compounds were used: the IC50 values for OR-462 and OR-486 were 10 and 14 microM, respectively. The IC50 values for dopamine-beta-hydroxylase, dopa-decarboxylase and monoamine oxidase forms A and B were greater than 50 microM. In studies ex vivo oral OR-462 inhibited mainly the COMT activity in the duodenum while OR-486 inhibited COMT activity in the liver and red blood cells as well. Oral OR-462 did not penetrate into the brain in doses up to 30 mg/kg while the same dose of OR-486 had some effect on striatal COMT activity. When tested in combination with levodopa-carbidopa, orally administered OR-462 and OR-486 were more effective in reducing the formation of 3-O-methyldopa from levodopa than was the levodopa-carbidopa treatment alone. These results indicate that OR-462 and OR-486 are effective and long-lasting inhibitors of COMT activity.

Inhibition of cathepsin B by peptidyl aldehydes and ketones: slow-binding behavior of a trifluoromethyl ketone

Inhibition of the cysteine proteinase cathepsin B by a series of N-benzyloxycarbonyl-L-phenylalanyl-L-alanine ketones and the analogous aldehyde has been investigated. Surprisingly, whereas the aldehyde was found to be almost as potent a competitive reversible inhibitor as the natural peptidyl aldehyde, leupeptin, the corresponding trifluoromethyl ketone showed comparatively weak (and slow-binding) reversible inhibition. Evaluation of competitive hydration and hemithioketal formation in a model system led to a structure-activity correlation spanning several orders of magnitude in both cathepsin B inhibition constants (Ki) and model system equilibrium data (KRSH,apparent).

Role of thiols in the in-vitro methylation of inorganic arsenic by rat liver cytosol

Rat liver cytosol inactivates inorganic arsenic (Asi) through methylation; S-adenosylmethionine is the methyl group donor and reduced glutathione (GSH) is required for full activity. The study of the combined effects of Asi, GSH and other thiols in vitro and the results of our previous in-vivo studies in humans and rats are consistent with a pathway involving the formation of a monomethylated metabolite which is either rapidly further methylated into a dimethylated derivative or is spontaneously oxidized into monomethylarsonic acid (MMA). The dimethylated metabolite gives rise to dimethylarsinic acid. The first methylation reaction is rate limiting, can be stimulated by GSH and is catalyzed by an enzyme different from that which transfers the second methyl group. The latter is sensitive to inhibition by inorganic arsenic. The stimulation of the first methylation reaction by GSH can only be evidenced at high Asi concentration because under these conditions, the second methylating enzyme can be sufficiently inhibited by Asi to allow some accumulation of MMA. The latter may also slow down the first methylation reaction. A large excess of thiol groups may prevent the methylation reactions probably by decreasing the amount of free trivalent arsenic.