Biochemistry of fluoroacetate poisoning; isolation of an active tricarboxylic acid fraction from poisoned kidney homogenates

Structural and biochemical studies of a fluoroacetyl-CoA-specific thioesterase reveal a molecular basis for fluorine selectivity

We have initiated a broad-based program aimed at understanding the molecular basis of fluorine specificity in enzymatic systems, and in this context, we report crystallographic and biochemical studies on a fluoroacetyl-coenzyme A (CoA) specific thioesterase (FlK) from Streptomyces cattleya. Our data establish that FlK is competent to protect its host from fluoroacetate toxicity in vivo and demonstrate a 10(6)-fold discrimination between fluoroacetyl-CoA (k(cat)/K(M) = 5 × 10⁷ M⁻¹ s⁻¹) and acetyl-CoA (k(cat)/K(M) = 30 M⁻¹ s⁻¹) based on a single fluorine substitution that originates from differences in both substrate reactivity and binding. We show that Thr 42, Glu 50, and His 76 are key catalytic residues and identify several factors that influence substrate selectivity. We propose that FlK minimizes interaction with the thioester carbonyl, leading to selection against acetyl-CoA binding that can be recovered in part by new C═O interactions in the T42S and T42C mutants. We hypothesize that the loss of these interactions is compensated by the entropic driving force for fluorinated substrate binding in a hydrophobic binding pocket created by a lid structure, containing Val 23, Leu 26, Phe 33, and Phe 36, that is not found in other structurally characterized members of this superfamily. We further suggest that water plays a critical role in fluorine specificity based on biochemical and structural studies focused on the unique Phe 36 "gate" residue, which functions to exclude water from the active site. Taken together, the findings from these studies offer molecular insights into organofluorine recognition and design of fluorine-specific enzymes.

Drug resistance in trypanosomes: effects of metabolic inhibitiors, pH and oxidation-reduction potential on normal and resistant Trypanosoma rhodesiense

A wide variety of metabolic inhibitors tested in vitro for trypanocidal activity on normal and drug-resistant strains of Trypanosoma rhodesiense showed no relation between acquired drug resistance and changes in specific enzymatic function. Oxidation-reduction potential is an important factor in trypanocidal action but is not obviously related to the development of resistance. The dependence on pH of the trypanocidal action of ionizing drugs against both normal and resistant trypanosomes supports the postulate that the development of resistance involves physical changes in cell structures associated with the uptake of drug.

Characterization of Ethanol Production from Xylose and Xylitol by a Cell-Free Pachysolen tannophilus System

Whole cells and a cell extract of Pachysolen tannophilus converted xylose to xylitol, ethanol, and CO 2 . The whole-cell system converted xylitol slowly to CO 2 and little ethanol was produced, whereas the cell-free system converted xylitol quantitatively to ethanol (1.64 mol of ethanol per mol of xylitol) and CO 2 . The supernatant solution from high-speed centrifugation (100,000 × g ) of the extract converted xylose to ethanol, but did not metabolize xylitol unless a membrane fraction and oxygen were also present. Fractionation of the crude cell extract by gel filtration resulted in an inactive fraction in which ethanol production from xylitol was fully restored by the addition of NAD + and ADP. The continued conversion of xylose to xylitol in the presence of fluorocitrate, which inhibited aconitase, demonstrated that the tricarboxylic acid cycle was not the source of the electrons for the production of xylitol from xylose. Therefore, the source of the electrons is indirectly identified as an oxidative pentose-hexose cycle.

Effect of inhibitors on acid production by baker's yeast

Glucose-induced acid extrusion, respiration and anaerobic fermentation in baker's yeast was studied with the aid of sixteen inhibitors. Uranyl(2+) nitrate affected the acid extrusion more anaerobically than aerobically; the complexing of Mg2+ and Ca2+ by EDTA at the membrane had no effect. Inhibitors of glycolysis (iodoacetamide, N-ethylmaleimide, fluoride) suppressed acid production markedly, and so did the phosphorylation-blocking arsenate. Fluoroacetate, inhibiting the citric-acid cycle, had no effect. Inhibition by uncouplers depended on their pKa values: 2,4,6-trinitrophenol (pKa 0.4) less than 2,4-dinitrophenol (4.1) less than azide (4.7) less than 3-chlorophenylhydrazonomalononitrile (6.0). Inhibition by trinitrophenol was only slightly increased by its acetylation. Cyanide and nonpermeant oligomycin showed practically no effect; inhibition by dicyclohexylcarbodiimide was delayed but potent. The concentration profiles of inhibition of acid production differed from those of respiration and fermentation. Thus, though the acid production is a metabolically dependent process, it does not reflect the intensity of metabolism, except partly in the first half of glycolysis.

The metabolism of fluoroacetate in lettuce

1. Whole lettuce plants were incubated with (1) [1-(14)C]acetate, (2) fluoroacetate followed by [1-(14)C]acetate, (3) fluoro[1-(14)C]acetate, (4) fluoro[2-(14)C]acetate or (5) S-carboxy[(14)C]methylglutathione. 2. Fluoroacetate did not affect the expiration of (14)CO(2) from [1-(14)C]acetate and only a small amount of (14)CO(2) was produced from either fluoro[1-(14)C]-acetate or fluoro[2-(14)C]acetate in 43h. 3. Fluoroacetate at 50mg/kg wet wt. doubled the plant citrate concentration after 43h incubation, and depending on the age and size of the plant 50-100% of the compound was metabolized. 4. With both fluoro[1-(14)C]acetate and fluoro[2-(14)C]acetate all the radioactivity except that in the CO(2) was found in the water-soluble acid fraction. About 2% was in fluorocitrate and the remainder, apart from unchanged fluoroacetate, was in a number of compounds devoid of fluorine but containing nitrogen and sulphur. These were peptide-like and could be separated by chromatography on an amino acid analyser. 5. Identical compounds were obtained from the spontaneous reaction between iodo[2-(14)C]acetate and glutathione, the major product being S-carboxymethylglutathione. 6. S-Carboxymethylcysteine was also isolated and its mass spectrum compared with a commercial sample. 7. Reaction rates of all the monohaloacetates with glutathione were studied at pH7 at 25 degrees C. No reaction was observed with fluoroacetate. 8. The metabolism of fluoroacetate by lettuce is discussed in relation to that of aliphatic and aromatic halogen compounds, including fluoroacetate, by mammalian liver and to the metabolism of fluoroacetate by different plants reported by other workers.

Light and electron microscopic studies of the rat kidney after administration of inhibitors of the citric acid cycle in vivo. I. Effects of sodium fluoroacetate on the proximal convoluted tubule

Light and electron microscopic studies of morphologic changes in the rat proximal convoluted tubule after intraperitoneal injection of sodium fluoroacetate (FAc), 60, 20 and 3.5 mg/kg body weight, have been made. Particular attention was directed toward appreciating different changes in the first (S(1)) and second (S(2)) segments of the proximal tubule. The earliest change was loss of mitochondrial granules and pallor of the mitochondrial matrix, not necessarily associated with matrix swelling. Matrix swelling was greatest at 3 hours after 3.5 mg/kg and was reversible. However, the mitochondria retained their elongate shape and cristae persisted. At 48 hours, some mitochondria appeared normal; in others, abnormal matrix densities of unknown nature were present. Mitochondrial changes were similar in S(1) and S(2) at all times. Enlarged apical vacuoles, most pronounced in S(1), occurred in all rats after 20 mg/kg. The change was uncommon after 3.5 mg/kg. The hypothesis proposed is that vacuoles arise during an FAc-induced hyperglycemic phase, when pinocytotic activity is maintained but the normal pathway of glucose catabolism is inhibited. Moderate dilatation of the rough-surfaced endoplasmic reticulum occurred during the first 2-hour period in S(1) and S(2) tubules after high and low doses, but between 6 and 24 hours, dilatation was extensive in S(1) tubules after 3.5 mg/kg. This change was reversible. Two types of abnormal vacuolar bodies, large and small, have been described, and were unique to S(1) tubules. Acid phosphatase activity was demonstrated in a proportion of the small ones, indiciating that they were a type of lysosome. The larger ones shared features in common with cytosomes of control cells, but acid phosphatase activity was not demonstrated in them and their origins and functions remain obscure. The biochemical lesions induced by fluoroacetate have been discussed and a tentative interpretation of some of the morphologic changes has ben made.

Respiration of the Mycelia and Mitochondria of the Filamentous Watermold, Allomyces Macrogynus

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