Suicide substrates for mitochondrial enzymes

Reduced clearance of proteins labeled with diisopropylfluorophosphate in portacaval-shunted rats

Portacaval shunting is a model for hepatic encephalopathy that causes chronic hyperammonemia, disruption of metabolic, signaling, and neurotransmitter systems, and progressive morphological changes. Exposure of cultured cells to ammonia raises intralysosomal pH and inhibits proteolysis, and the present study tested the hypothesis that proteolytic capacity is diminished in portacaval-shunted rats. Proteins were labeled in vivo with tracer doses of diisopropylfluorophosphate (DFP) and clearance of label was assayed. This approach labeled proteins independent of protein synthesis, which is reported to be altered in shunted rats, and avoided complications arising from re-utilization of labeled amino acids that causes underestimation of degradation rate. Characterization of DFP labeling showed that protein labeling was fast, about 50% of the label was released during a 24 h interval, labeling by DFP metabolites was negligible, inhibition of brain acetylcholinesterase was not detectable, and labeling by [(3)H]- and [(14)C]DFP was equivalent. To assay degradative capacity, proteins were first labeled with [(3)H]DFP, followed by labeling with [(14)C]DFP that was given 24 or 72 h later. The (3)H/(14)C ratio in each animal was used as a relative measure of removal of (3)H-labeled proteins. (3)H/(14)C ratios were generally significantly higher in portacaval-shunted rats than in controls, consistent with reduced proteolytic capacity. Assays of amino acid incorporation into brain protein generally replicated literature reports, supporting the conclusion that protein synthesis unlikely to be markedly inhibited and amino acid recycling influences calculated protein synthesis rates in shunted rats. Therapeutic strategies to reduce ammonia level would help normalize lysosomal functions and protein and lipid turnover.

Reversible metabolic depression in hepatocytes of lamprey (Lampetra fluviatilis) during pre-spawning: regulation by substrate availability

The regulation of oxidative metabolism in hepatocytes of lampreys (Lampetra fluviatilis) during the freshwater pre-spawning period of their life cycle was studied. The energy metabolism in these cells is characterized by a simplified scheme, where glycolytic ATP production is insignificant and fatty acids are the major respiratory substrates. Seasonal changes in aerobic cell metabolism include a considerable reversible depression of metabolic rate in lamprey hepatocytes during the winter months of the pre-spawning period. The depression is characterized by a more than twofold decrease in hepatocyte endogenous respiration rate, a reduction of oxidative phosphorylation and drop in cellular ATP content. The addition of fatty acids to the hepatocyte incubation medium prevents the decrease in the metabolic rate. In spring, before spawning, a marked activation of energy metabolism in lamprey hepatocytes is found. These observations support the conclusion that the regulation of lamprey hepatocyte energy metabolism is realized through the availability of fatty acids for oxidation.

Active site generation of a protonically unstable suicide substrate from a stable precursor: glucose oxidase and dibromonitromethane

Bromonitromethane is an inefficient suicide substrate for glucose oxidase (in contrast to the case of CH(3)CCl=NO(2)(-) and D-amino acid oxidase) because, in the enzyme-substrate encounter step, the required ionization states of enzyme (EH(0)(+), pK(a) approximately 3.5) and substrate (CHBr=NO(2)(-), pK(a) approximately 8.3) cannot be highly populated simultaneously. Because reprotonation of CHBr=NO(2)(-) is rapid at the pH value used for the assay of glucose oxidase, presentation of the enzyme with the preformed anion could not be exploited in this case. We circumvent this difficulty by allowing the enzyme to reductively dehalogenate CHBr(2)NO(2), thereby generating the desired protonically unstable suicide substrate in situ (E(r) + CHBr(2)NO(2) --> E(o) + CHBr=NO(2)(-) + HBr + H(+)). Irreversible inactivation of the enzyme, because of the formation of a dead-end N-5 formylflavin adduct, is more than 100-fold faster when CHBr=NO(2)(-) is generated in situ than when it is externally applied. The remaining competitive fates of CHBr=NO(2)(-) at the active site are protonation and release or oxidation to HCOBr (or HCONO(2)). Strong support for these conclusions comes from (1) the brisk evolution of CH(3)CBr=NO(2)(-) (which is too bulky to act further as an efficient suicide substrate) from the enzyme-catalyzed reductive debromination of CH(3)CBr(2)NO(2), (2) the 1:1 stoichiometry of enzyme inactivation, and (3) the identification of the modified flavin as 5-formyl-1, 5-dihydro-FAD.

Poisoning by Coprinus atramentarius

The ink cap--Coprinus atramentarius (Bulliard ex Fries) Fries--is responsible for poisoning when ingested with alcohol. The investigation of the "Coprinus syndrome," although a minor poisoning incident, stimulated numerous research programs because the results were expected to yield a novel drug useful during the treatment of alcoholism. This work led to the identification of the active principle--coprine--and to an explanation of its mode of action; nevertheless, detailed toxicology investigations have shown that the mutagenic and gonadotoxic properties of this compound made it unsuitable for therapeutic use. Our current knowledge of the poisoning, the chemistry of the toxin, and its mode of action are here reviewed.

Effects of inhibitors of gluconeogenesis on weak organic acid uptake in rat renal tubules

Using inhibitors of gluconeogenesis (phenylpyruvate, alpha-cyano-4-hydroxycinnamate, quinolinate, D-malate, aminooxyacetate), we analysed mechanisms by which the gluconeogenic substrates, lactate and pyruvate, as well as a short-chain fatty acid, acetate, stimulate the uptake of a weak organic acid, fluorescein, in the rat kidney. We have shown that these inhibitors modified both the rate of glucose production from lactate and pyruvate in the renal cortex fragment suspension and the stimulatory effects of the metabolic substrates on fluorescein uptake in superficial proximal tubules in the renal cortex slices. The peculiarities of the effects of lactate and pyruvate on the uptake were correlated with the partial divergence of the pathways of gluconeogenesis from these precursors. The linkage of the weak organic acid uptake with gluconeogenesis is interpreted in terms of the hypothesis that the uptake is controlled by the cytoplasmic pyridine nucleotide redox potential, which is maintained with the participation of certain processes involved in glucose synthesis.

Nature and distribution of brain lesions in rats intoxicated with 3-nitropropionic acid: a type of hypoxic (energy deficient) brain damage

The clinical signs and morphological brain lesions associated with histotoxic hypoxia induced by subcutaneous injection of 3-nitropropionic acid (NPA) in rats are described, and compared to hypoxic brain damage from other causes including ischemia and hypoglycemia. The brains were perfusion-fixed with paraformaldehyde/glutaraldehyde fixative, and examined by light and electron microscopy. Intoxicated rats developed severe neurological disease characterized by somnolence, uncoordinated gait with stereotypical paddling movements, and ventral or lateral recumbency. Recumbent rats had a selective, bilaterally symmetrical pattern of severe morphological injury in the caudate-putamen, hippocampus, and thalamus. Recumbency was a consistent indicator of the development of morphological brain lesions. In contrast to reports describing rat models of ischemia and hypoglycemia, morphological injury was not seen in the cerebral and cerebellar cortices of NPA-intoxicated rats. Ultrastructurally, neuronal alterations ranged from chromatin clumping with increased cytoplasmic lucency to severe cellular shrinkage or swelling with marked mitochondrial swelling (high amplitude swelling). White matter alterations included axonal swelling and adaxonal splitting of myelin lamellae. Vascular changes included perivascular deposits of proteinaceous material presumably from leakage of serum proteins, variable electron lucency of endothelial cell cytoplasm, an apparent increase in pinocytotic vesicles, rare platelet thrombosis of capillaries, and rare intravascular blebs of luminal plasma membrane. As a model of brain damage following energy deficiency, NPA intoxication has the advantages of producing morphological brain injury in a highly predictable anatomical pattern, and at a time paralleling the onset of clinical recumbency.

Inactivation of GABA aminotransferase by 3-nitro-1-propanamine

3-Nitro-1-propanamine is a close structural analog of the neuro-transmitter GABA. The nitro compound is a good substrate for the GABA aminotransferase from porcine brain. However, it inactivates the GABA aminotransferase from GABA-grown Pseudomonas fluorescens in a slowly reversible reaction. Both enzymes are inactivated by the homolog 4-nitro-1-butanamine.

Overview of the present state of MAO inhibitors

In this paper an overview of the present state of monoamine oxidase inhibitors (MAOIs) is presented. The irreversible inhibitors are firstly considered. They have been divided into four chemical types: substituted hydrazine, cyclopropylamine, propargylamine and allylamine derivatives. Moreover, a tetrahydropyridine derivative (MPTP), recently described as an irreversible inhibitor of MAO-B, has been included among the irreversible MAOIs. The reversible inhibitors such as tetrahydro-beta-carbolines and salsolinol, phenylalkylamines: amphetamine, amiflamine and 2,3-dichloro-alpha-methyl-benzylamine. Among the short acting or reversible inhibitors the 4-(2-benzofuranyl) piperidine series and the morpholinoethylamino derivatives are discussed. Finally the oxazolidinone series is presented separately, as in this series reversible or irreversible inhibitors of the A or B form of MAO have been obtained.

Brain enzyme and clinical alterations induced in rats and mice by nitroaliphatic toxicants

The effects of a single subcutaneous (s.c.) injection of the nitroaliphatic toxicants 3-nitropropanol (NPOH) and 3-nitropropionic acid (NPA) dissolved in physiological saline solution were studied in mice and rats, respectively. Clinical signs observed in both NPOH-treated mice and NPA-treated rats included depression, abnormal motor activity, and recumbency. Succinate dehydrogenase (SDH) activity, demonstrated histochemically in frozen brain sections, was markedly reduced in intoxicated mice and rats. The SDH activity of mitochondrial preparations from brains of intoxicated mice and rats was diminished to 18-24% of control values, although the activity of another mitochondrial flavoprotein enzyme, alpha-glycerophosphate dehydrogenase (alpha-GPDH), was not altered.

Irreversible inhibition of delta 5-3-oxosteroid isomerase by 2-substituted progesterones

2 alpha-Cyanoprogesterone (I) and 2-hydroxymethyleneprogesterone (II) were synthesized and screened as irreversible active-site-directed inhibitors of the delta 5-3-oxosteroid isomerase (EC 5.3.3.1) from Pseudomonas testosteroni. Both compounds were found to inhibit the purified bacterial enzyme in a time-dependent manner. In either case the inactivated enzyme could be dialysed without return of activity, indicating that a stable covalent bond had formed between the inhibitor and the enzyme. Inactivation mediated by compounds (I) and (II) followed pseudo-first-order kinetics, and at higher inhibitor concentrations saturation was observed. The competitive inhibitor 17 beta-oestradiol offered protection against the inactivation mediated by both compounds, and initial-rate studies indicated that compounds (I) and (II) can also act as competitive inhibitors yielding Ki values identical with those generated during inactivation experiments. 2 alpha-Cyanoprogesterone (I) and 2-hydroxymethyleneprogesterone (II) thus appear to be active-site-directed. To compare the reactivity of these 2-substituted progesterones with other irreversible inhibitors of the isomerase, 3 beta-spiro-oxiranyl-5 alpha-pregnan-20 beta-ol (III) was synthesized as the C21 analogue of 3 beta-spiro-oxiranyl-5 alpha-androstan-17 beta-ol, which is a potent inactivator of the isomerase [Pollack, Kayser & Bevins (1979) Biochem. Biophys. Res. Commun. 91, 783-790]. Comparison of the bimolecular rate constants for inactivation (k+3/Ki) mediated by compounds (I)-(III) indicated the following order of reactivity: (III) greater than (II) greater than (I). 2-Mercaptoethanol offers complete protection against the inactivation of the isomerase mediated by 2 alpha-cyanoprogesterone (I). Under the conditions of inactivation compound (I) appears to be completely stable, and no evidence could be obtained for enolate ion formation in the presence or absence of enzyme. It is suggested that cyanoprogesterone inactivates the isomerase after direct nucleophilic attack at the electropositive 2-position, and that tautomerization plays no role in the inactivation event. By contrast, 2-mercaptoethanol offers no protection against the inactivation mediated by 2-hydroxymethyleneprogesterone, and under the conditions of inactivation this compound appears to exist in the semi-enolized form.

Pyruvate kinase catalyzed phosphorylation of 3-nitro-2-hydroxypropionate by ATP and concomitant inactivation of the enzyme

Pyruvate kinase catalyzes the phosphorylation of 3-nitro-2-hydroxypropionate by ATP. The product (3-nitro-2-phosphopropionate) decomposes nonenzymatically and inactivates the enzyme. The inactivation process is irreversible and the enzyme is completely protected against inactivation by mercaptoethanol or the competitive inhibitor oxalate. The Km for 3-nitro-2-hydroxypropionate (DL) is 40 mM and kcat = 5 s-1 at pH 8.6. In terms of kcat/km, 3-nitro-2-hydroxypropionate is a fourfold better substrate than glycolate.

Chromatographic resolution of amino acid adducts of aliphatic halides

Numerous xenobiotics are known to be bioactivated and to covalently and to proteins, but the resulting amino acid adducts (AAAs) are unknown. In this study the AAAs of twelve 14C-labeled aliphatic halides were examined after formation in an in vitro microsomal system. After exhaustive solvent extraction of the precipitated microsomal protein, the AAAs were isolated by Pronase digestion, followed by filtration through a 500 mol. wt. exclusion membrane. The liberated AAAs were applied to a constant flow DC-4A cation exchange column, resolved by stepwise buffer elution, collected and counted for radioactivity. Column recovery for applied radioactivity was 100 +/- 4%. Generally, 1-4 different AAAs (defined by eluting radioactivity) were resolved, with each organohalogen displaying a characteristic elution profile. Methyl iodide, trichloroethylene and 1,2-dichloroethylene had a single major AAA while bromotrichloromethane, 1,2-dibromoethane, 1,1,1-trichloroethane, 1,2-dichloroethane, 1,1,2-trichloroethane, 2-bromo-2-chloro-1,1,1-trifluoroethane, chloroform and carbon tetrachloride had up to 4 AAAs or more, indicating combinations of binding site(s) and reactive intermediate(s). The single AAA formed following incubation of methyl iodide with the microsomes was identified as S-methylcysteine. Thus, this method appears capable of resolving binding sites and is the initial isolation step for identifying specific adducts to proteins.