The Proteome of Human Liver Peroxisomes: Identification of Five New Peroxisomal Constituents by a Label-Free Quantitative Proteomics Survey

The peroxisome is a key organelle of low abundance that fulfils various functions essential for human cell metabolism. Severe genetic diseases in humans are caused by defects in peroxisome biogenesis or deficiencies in the function of single peroxisomal proteins. To improve our knowledge of this important cellular structure, we studied for the first time human liver peroxisomes by quantitative proteomics. Peroxisomes were isolated by differential and Nycodenz density gradient centrifugation. A label-free quantitative study of 314 proteins across the density gradient was accomplished using high resolution mass spectrometry. By pairing statistical data evaluation, cDNA cloning and in vivo colocalization studies, we report the association of five new proteins with human liver peroxisomes. Among these, isochorismatase domain containing 1 protein points to the existence of a new metabolic pathway and hydroxysteroid dehydrogenase like 2 protein is likely involved in the transport or β-oxidation of fatty acids in human peroxisomes. The detection of alcohol dehydrogenase 1A suggests the presence of an alternative alcohol-oxidizing system in hepatic peroxisomes. In addition, lactate dehydrogenase A and malate dehydrogenase 1 partially associate with human liver peroxisomes and enzyme activity profiles support the idea that NAD⁺ becomes regenerated during fatty acid β-oxidation by alternative shuttling processes in human peroxisomes involving lactate dehydrogenase and/or malate dehydrogenase. Taken together, our data represent a valuable resource for future studies of peroxisome biochemistry that will advance research of human peroxisomes in health and disease.     

Activity of liver mitochondrial Krebs cycle NAD<Superscript>+</Superscript>-dependent dehydrogenases in rats with hepatitis induced by acetaminophen under conditions of alimentary protein deficiency

Activity of isocitrate dehydrogenase, α-ketoglutarate dehydrogenase, malate dehydrogenase, and the NAD⁺/NADН ratio were studied in the liver mitochondrial fraction of rats with toxic hepatitis induced by acetaminophen under conditions of alimentary protein deficiency. Acetaminophen-induced hepatitis was characterized by a decrease of isocitrate dehydrogenase, α-ketoglutarate dehydrogenase and malate dehydrogenase activities, while the mitochondrial NAD⁺/NADН ratio remained at the control level. Modeling of acetaminophen-induced hepatitis in rats with alimentary protein deficiency caused a more pronounced decrease in the activity of studied Krebs cycle NAD⁺-dependent dehydrogenases and a 2.2-fold increase of the mitochondrial NAD⁺/NADН ratio.     

Inhibition of 15-hydroxyprostaglandin dehydrogenase by papaverine.

Papaverine was found to inhibit NAD⁺-linked 15-hydroxyprostaglandin dehydrogenase partially purified from guinea pig lung. The inhibition was noncompetitive with prostaglandin E₂, uncompetitive with NAD⁺, and reversible. The Ki was calculated to be 26 μM. Papaverine also inhibited the enzyme from swine lung, chicken and dog heart, and rat and dog kidney. The inhibitory effects of papaverine on the 15-hydroxyprostaglandin dehydrogenase were compared with those on cyclic AMP phosphodiesterases in these tissues.     

Helminthosporium maydis Race T Toxin Induces Leakage of NAD from T Cytoplasm Corn Mitochondria

The mechanism by which Helminthosporium maydis race T toxin inhibits respiration dependent on NAD⁺-linked substrates in T cytoplasm corn mitochondria was investigated. The toxin did not cause leakage of the soluble matrix enzyme malate dehydrogenase from the mitochondria or inhibit malate dehydrogenase or isocitrate dehydrogenase directly. The toxin did increase the permeability of the inner membranes of T cytoplasm, but not N cytoplasm, mitochondria to NAD⁺. Added NAD⁺ partially or fully restored toxin-inhibited electron transport in T cytoplasm mitochondria. Thiamin pyrophosphate had a similar effect when malate was the substrate. It was concluded that the inhibition of respiration of NAD⁺-linked substrates by the toxin is due to depletion of the intramitochondrial pool of NAD⁺ and other coenzymes.     

Conversion of 1,3-Butanediol or Acetaldol to β-Hydroxybutyric Acid in Chicks

Enzymic oxidizing system of 1,3-butanediol in chicks was studied in comparison with that in rats. 1,3-Butanediol was oxidized with NAD⁺ as a cofactor in the cytosol fraction of chick liver, kidney, and rat liver. NADP⁺ was about 40% as effective as NAD⁺ in chick liver, kidney, whereas the former was ineffective in rat liver. Inhibitory effect of pyrazole on 1,3-butanediol dehydrogenase activity was recognized distinctly both in chicks and rats, but the effect was much higher in rat liver than that in chick liver and kidney. Both in chicks and rats, the conversion of 1,3-butanediol to β-hydroxybutyric acid was detected clearly, and the rate of β-hydroxybutyrate production increased remarkably by employing NAD⁺-regenerating system, i.e., lactic dehydrogenase and pyruvic acid. β-Hydroxybutyric acid was also formed from acetaldol, the most probable intermediate of 1,3-butanediol, with NAD⁺ both in chicks and rats. From the observations for coenzyme specificity, inhibitory effect of pyrazole, and so on, it is concluded that appreciable difference may be present in 1,3-butanediol oxidizing system between chicks and rats.     

Studies on the metabolism of galactose-6-phosphate in rat heart and brain.

The subcellular distribution of NADP⁺ and NAD⁺-dependent glucose-6-phosphate and galactose-6-phosphate dehydrogenases were studied in rat liver, heart, brain, and chick brain. Only liver particulate fractions oxidized glucose-6-phosphate and galactose-6-phosphate with either NADP⁺ or NAD⁺ as cofactor. While all of the tissues examined had NADP⁺-dependent glucose-6-phosphate dehydrogenase activity, only rat liver and rat brain soluble fractions had NADP⁺-dependent galactose-6-phosphate dehydrogenase activity. Rat liver microsomal and rat brain soluble galactose-6-phosphate dehydrogenase activities were kinetically different (K_m's 0.5 mm and 10 mm, respectively, for galactose-6-phosphate), although their reaction products were both 6-phosphogalactonate. Rat brain subcellular fractions did not oxidize 6-phosphogalactonate with either NADP⁺ or NAD⁺ cofactors but phosphatase activities hydrolyzing 6-phosphogalactonate, galactose-6-phosphate and galactose-1-phosphate were found in crude brain homogenates. In addition, galactose-6-phosphate and 6-phosphogalactonate were tested as inhibitors of various enzymes, with largely negative results, except that 6-phosphogalactonate was a competitive inhibitor (K_i = 0.5 mM) of rat brain 6-phosphogluconate dehydrogenase.     

NADP-Utilizing Enzymes in the Matrix of Plant Mitochondria

Purified potato tuber (Solanum tuberosum L. cv Bintie) mitochondria contain soluble, highly latent NAD⁺- and NADP⁺-isocitrate dehydrogenases, NAD⁺- and NADP⁺-malate dehydrogenases, as well as an NADPH-specific glutathione reductase (160, 25, 7200, 160, and 16 nanomoles NAD(P)H per minute and milligram protein, respectively). The two isocitrate dehydrogenase activities, but not the two malate dehydrogenase activities, could be separated by ammonium sulfate precipitation. Thus, the NADP⁺-isocitrate dehydrogenase activity is due to a separate matrix enzyme, whereas the NADP⁺-malate dehydrogenase activity is probably due to unspecificity of the NAD⁺-malate dehydrogenase. NADP⁺-specific isocitrate dehydrogenase had much lower K_ms for NADP⁺ and isocitrate (5.1 and 10.7 micromolar, respectively) than the NAD⁺-specific enzyme (101 micromolar for NAD⁺ and 184 micromolar for isocitrate). A broad activity optimum at pH 7.4 to 9.0 was found for the NADP⁺-specific isocitrate dehydrogenase whereas the NAD⁺-specific enzyme had a sharp optimum at pH 7.8. Externally added NADP⁺ stimulated both isocitrate and malate oxidation by intact mitochondria under conditions where external NADPH oxidation was inhibited. This shows that (a) NADP⁺ is taken up by the mitochondria across the inner membrane and into the matrix, and (b) NADP⁺-reducing activities of malate dehydrogenase and the NADP⁺-specific isocitrate dehydrogenase in the matrix can contribute to electron transport in intact plant mitochondria. The physiological relevance of mitochondrial NADP(H) and soluble NADP(H)-consuming enzymes is discussed in relation to other known mitochondrial NADP(H)-utilizing enzymes.     

Changes in NAD(P)+-dependent malic enzyme and malate dehydrogenase activities during fibroblast proliferation

A sensitive isotope exchange method was developed to assess the requirements for and compartmentation of pyruvate and oxalacetate production from malate in proliferating and nonproliferating human fibroblasts. Malatedependent pyruvate production (malic enzyme activity) in the particulate fraction containing the mitochondria was dependent on either NAD⁺ or NADP⁺. The production of pyruvate from malate in the soluble, cytosolic fraction was strictly dependent on NADP⁺. Oxalacetate production from malate (malate dehydrogenase, EC 1.1.1.37) in both the particulate and soluble fraction was strictly dependent on NAD⁺. Relative to nonproliferating cells, NAD⁺-linked malic enzyme activity was slightly reduced and the NADP⁺-linked activity was unchanged in the particulate fraction of serum-stimulated, exponentially proliferating cells. However, a reduced activity of particulate malate dehydrogenase resulted in a two-fold increase in the ratio of NAD(P)⁺-linked malic enzyme to NAD⁺-linked malate dehydrogenase activity in the particulate fraction of proliferating fibroblasts. An increase in soluble NADP⁺-dependent malic enzyme activity and a decrease in NAD⁺-linked malate dehydrogenase indictated an increase in the ratio of pyruvate-producing to oxalacetate-producing malate oxidase activity in the cytosol of proliterating cells. These coordinate changes may affect the relative amount of malate that is oxidized to oxalacetate and pyruvate in proliferating cells and, therefore, the efficient utilization of glutamine as a respiratory fuel during cell proliferation.     

Variations in the pathways of malate oxidation and phosphorylation in different species of Mycobacteria

Mycobacterium tuberculosis H₃₇ Rv, the slow-growing human pathogenic strain of tubercle bacilli and Mycobacterium smegmatis and Mycobacterium phlei, the fast-growing saprophytes, have shown variations regarding the type of dehydrogenase that initiates malate oxidation in the respiratory chain.M. tuberculosis H₃₇Rv is characterized by having a malate oxidase system (designated MAL_NAD pathway) in which malate oxidation is mediated by the NAD⁺_? dependent malate dehydrogenase (EC 1.1.1.37) but not by FAD-dependent malatevitamin K reductase. M. smegmatis possesses a different malate oxidase system (designated MAL_FAD pathway) in which malate oxidation is exclusively carried out by the FAD-dependent malate-vitamin K reductase because NAD⁺-dependent malate dehydrogenase is absent in this organism. M. phlei has a mixed system of malate oxidase (designated MAL_NAD+FAD pathways) in which both the NAD⁺_? and FAD-dependent dehydrogenases take part. In all the three systems, the rest of the electron transport chain is common.     

Regulation of succinate oxidation by NAD+ in mitochondria purified from potato tubers

NAD⁺ supplied to purified Solanum tuberosum mitochondria caused progressive inhibition of succinate oxidation in State 3. This inhibition was especially pronounced at alkaline pH and at low succinate concentrations. Glutamate counteracted the inhibition. NAD⁺ promoted oxaloacetate accumulation in State 3; supplied oxaloacetate inhibited O₂ uptake in the presence of succinate much more severely in State 3 than in State 4. NAD reduction linked to succinate oxidation by ATP-dependent reverse electron transport was likewise inhibited by oxaloacetate. We conclude that NAD⁺-induced inhibition of succinate oxidation is due to an inhibition of succinate dehydrogenase resulting from increased accumulation of oxaloacetate generated from malate oxidation via malate dehydrogenase. The results are discussed in the context of the known regulatory characteristics of plant succinate dehydrogenase.     

The alpha beta epimerization of reduced nicotinamide adenine dinucleotide.

The proton magnetic resonance spectra of the dihydronicotinamide ring of αNADH³ and the nicotinamide ring of αNAD⁺ are reported and the proton absorptions assigned. The absolute assignment of the C4 methylene protons of αNADH is based on the generation of specifically deuterium-labeled (pro-S) B-deuterio-αNADH from enzymatically prepared B-deuterio-βNADH. The C4 proton absorption of αNAD⁺ is assigned by oxidation of B-deuterio-αNADH by the A specific, yeast alcohol dehydrogenase to yield 4-deuterio-αNAD⁺.The epimerization of either αNADH or βNADH yields an equilibrium ratio of approximately 9:1 βNADH to αNADH. The rate of epimerization of αNADH to βNADH at 38 °C in 0.05, pH 7.5, phosphate buffer is 3.1 × 10^?3 min^?1, corresponding to a half-life of 4 hr. Four related dehydrogenases, yeast and horse liver alcohol dehydrogenase and chicken M₄ and H₄ lactate dehydrogenase, are shown to oxidize αNADH to αNAD⁺ at rates three to four orders of magnitude slower than for βNADH. By using specifically labeled B-deuterio-αNADH the enzymatic oxidation by yeast alcohol dehydrogenase has been shown to occur with the identical stereospecificity as the oxidation of βNADH. The nonenzymatic epimerization of αNADH to βNADH and the enzymatic oxidation αNADH are discussed as a possible source of αNAD⁺in vivo.     

Ethanol exposure modulates hepatic S-adenosylmethionine and S-adenosylhomocysteine levels in the isolated perfused rat liver through changes in the redox state of the NADH/NAD system

Methionine metabolism is disrupted in patients with alcoholic liver disease, resulting in altered hepatic concentrations of S-adenosylmethionine (SAM), S-adenosylhomocysteine (SAH), and other metabolites. The present study tested the hypothesis that reductive stress mediates the effects of ethanol on liver methionine metabolism. Isolated rat livers were perfused with ethanol or propanol to induce a reductive stress by increasing the NADH/NAD⁺ ratio, and the concentrations of SAM and SAH in the liver tissue were determined by high-performance liquid chromatography. The increase in the NADH/NAD⁺ ratio induced by ethanol or propanol was associated with a marked decrease in SAM and an increase in SAH liver content. 4-Methylpyrazole, an inhibitor the NAD⁺-dependent enzyme alcohol dehydrogenase, blocked the increase in the NADH/NAD⁺ ratio and prevented the alterations in SAM and SAH. Similarly, co-infusion of pyruvate, which is metabolized by the NADH-dependent enzyme lactate dehydrogenase, restored the NADH/NAD⁺ ratio and normalized SAM and SAH levels. The data establish an initial link between the effects of ethanol on the NADH/NAD⁺ redox couple and the effects of ethanol on methionine metabolism in the liver.     

Soluble and chloroplast malate dehydrogenase isoenzymes of Triticum aestivum

Three isoenzymes of malate dehydrogenase have been isolated from 9-day-old wheat shoots. The microbody (peroxisome) and chloroplast MDH are similar in their electrophoretic behaviour. The mitochondrial MDH, soluble MDH and chloroplast MDH differ in K_m values for malate and NAD. The activity of MDH isoenzymes with NAD⁺-analogues as substrate was in the order 3-AP-NAD⁺ > 3-AP-deam NAD⁺ > NAD⁺ > TN-NAD⁺ and deam NAD⁺. The thermal stabilities of the isoenzymes were significantly different: C-MDH > m-MDH > S-MDH.     

NAD(P)+-dependent isocitrate dehydrogenases in mitochondria purified from Picea abies seedlings

Isocitrate dehydrogenase (IDH) activities were measured in mitochondria isolated from aerial parts of 21-day-old spruce (Picea abies L. Karst.) seedlings. Mitochondria were purified by two methods, involving continuous and discontinuous Percoll gradients. Whatever the method of purification, the mitochondrial outer membrane was about 69% intact, and the mitochondria contained very low cytosolic, chloroplastic and peroxisomal contaminations. Nevertheless, as judged by the recovery of fumarase activity, purification on a continuous 28% Percoll gradient gave the best yield in mitochondria, which exhibited a high degree of inner membrane intactness (91%). The purified mitochondria oxidized succinate and malate with good respiratory control and ADP/O ratios. The highest oxidation rate was obtained with succinate as substrate, and malate oxidation was improved (+ 60%) by addition of exogenous NAD⁺. Experiments using standard respiratory chain inhibitors indicated that, in spruce mitochondria, the alternative pathway was present. Both NAD⁺-isocitrate dehydrogenase (EC 1.1.1.41) and NADP⁺-isocitrate dehydrogenase (EC 1.1.1.42) were present in the mitochondrial matrix fraction, and NAD⁺-IDH activity was about 2-fold higher than NADP⁺-IDH activity. The NAD⁺-IDH showed sigmoidal kinetics in response to isocitrate and standard Michaelis-Menten kinetics for NAD⁺ and Mg²⁺. The NADP⁺-IDH, in contrast, displayed lower K_m values. For NAD⁺-IDH the pH optimum was at 7.4, whereas NADP⁺-IDH exhibited a broad pH optimum between 8.3 and 9. In addition, NAD⁺-IDH was more thermolabile. Adenine nucleotides and 2-oxoglutarate were found to inhibit NAD(P)⁺-IDH activities only at high concentrations.     

Kinetic properties of N-(2-carboxyethyl)-NAD(H) and poly(ethylene glycol)-bound NAD(H) for alcohol, lactate, malate and glyceraldehyde-3-phosphate dehydrogenase from different organisms

The steady-state kinetics of alcohol dehydrogenases (alcohol:NAD⁺ oxidoreductase, EC 1.1.1.1 and alcohol:NADP⁺ oxidoreductase, EC 1.1.1.2), lactate dehydrogenases (l-lactate:NAD⁺ oxidoreductase, EC 1.1.1.27 and d-lactate:NAD⁺ oxidoreductase, EC 1.1.1.28), malate dehydrogenase (l-malate:NAD⁺ oxidoreductase, EC 1.1.1.37), and glyceraldehyde-3-phosphate dehydrogenases [d-glyceraldehyde-3-phosphate:NAD⁺ oxidoreductase (phosphorylating), EC 1.2.1.12] from different sources (prokaryote and eukaryote, mesophilic and thermophilic organisms) have been studied using NAD(H), N⁶-(2-carboxyethyl)-NAD(H), and poly(ethylene glycol)-bound NAD(H) as coenzymes. The kinetic constants for NAD(H) were changed by carboxyethylation of the 6-amino group of the adenine ring and by conversion to macromolecular form. Enzymes from thermophilic bacteria showed especially high activities for the derivatives. The relative values of the maximum velocity (NAD = 1) of Thermus thermophilus malate dehydrogenase for N⁶-(2-carboxyethyl)-NAD and poly(ethylene glycol)-bound NAD were 5.7 and 1.9, respectively, and that of Bacillus stearothermophilus glyceraldehyde-3-phosphate dehydrogenase for poly(ethylene glycol)-bound NAD was 1.9.     

Effect of bicarbonate and oxaloacetate on malate oxidation by spinach leaf mitochondria

Mitochondria isolated from spinach leaves oxidized malate by both a NAD⁺-linked malic enzyme and malate dehydrogenase. In the presence of sodium arsenite the accumulation of oxaloacetate and pyruvate during malate oxidation was strongly dependent on the malate concentration, the pH in the reaction medium and the metabolic state condition.Bicarbonate, especially at alkaline pH, inhibited the decarboxylation of malate by the NAD⁺-linked malic enzyme in vitro and in vivo. Analysis of the reaction products showed that with 15 mM bicarbonate, spinach leaf mitochondria excreted almost exclusively oxaloacetate.The inhibition by oxaloacetate of malate oxidation by spinach leaf mitochondria was strongly dependent on malate concentration, the pH in the reaction medium and on the metabolic state condition.The data were interpreted as indicating that: (a) the concentration of oxaloacetate on both sides of the inner mitochondrial membrane governed the efflux and influx of oxaloacetate; (b) the NAD⁺/NADH ratio played an important role in regulating malate oxidation in plant mitochondria; (c) both enzymes (malate dehydrogenase and NAD⁺-linked malic enzyme) were competing at the level of the pyridine nucleotide pool, and (d) the NAD⁺-linked malic enzyme provided NADH for the reversal of the reaction catalyzed by the malate dehydrogenase.     

Two malate dehydrogenases in Methanobacterium thermoautotrophicum

Methanobacterium thermoautotrophicum (strain Marburg) was found to contain two malate dehydrogenases, which were partially purified and characterized. One was specific for NAD⁺ and catalyzed the dehydrogenation of malate at approximately one-third of the rate of oxalacetate reduction, and the other could equally well use NAD⁺ and NADP⁺ as coenzyme and catalyzed essentially only the reduction of oxalacetate. Via the N-terminal amino acid sequences, the encoding genes were identified in the genome of M. thermoautotrophicum (strain ΔH). Comparison of the deduced amino acid sequences revealed that the two malate dehydrogenases are phylogenetically only distantly related. The NAD⁺-specific malate dehydrogenase showed high sequence similarity to l-malate dehydrogenase from Methanothermus fervidus, and the NAD(P)⁺-using malate dehyrogenase showed high sequence similarity to l-lactate dehydrogenase from Thermotoga maritima and l-malate dehydrogenase from Bacillus subtilis. A function of the two malate dehydrogenases in NADPH:NAD⁺ transhydrogenation is discussed. Received: 29 December 1997 / Accepted: 4 March 1998     

Malate Oxidation in Plant Mitochondria via Malic Enzyme and the Cyanide-insensitive Electron Transport Pathway

Malate oxidation in plant mitochondria proceeds through the activities of two enzymes: a malate dehydrogenase and a NAD⁺-dependent malic enzyme. In cauliflower, mitochondria malate oxidation via malate dehydrogenase is rotenone- and cyanide-sensitive. Addition of exogenous NAD⁺ stimulates the oxidation of malate via malic enzyme and generates an electron flux that is both rotenone- and cyanide-insensitive. The same effects of exogenous NAD⁺ are also observed with highly cyanide-sensitive mitochondria from white potato tubers or with mitochondria from spinach leaves. Both enzymes are located in the matrix, but some experimental data also suggest that part of malate dehydrogenase activity is also present outside the matrix compartment (adsorbed cytosolic malate dehydrogenase?). It is concluded that malic enzyme and a specific pool of NAD⁺/NADH are connected to the cyanide-insensitive alternative pathway by a specific rotenone-insensitive NADH dehydrogenase located on the inner face of the inner membrane. Similarly, malate dehydrogenase and another specific pool of NAD⁺/NADH are connected to the cyanide- (and antimycin-) sensitive pathway by a rotenone-sensitive NADH dehydrogenase located on the inner face of the inner membrane. A general scheme of electron transport in plant mitochondria for the oxidation of malate and NADH can be given, assuming that different pools of ubiquinone act as a branch point between various dehydrogenases, the cyanide-sensitive cytochrome pathway and the cyanide-insensitive alternative pathway.     

The kinetics of enzyme changes in yeast under conditions that cause the loss of mitochondria

1. Aerobically grown yeast having a high activity of glyoxylate-cycle, citric acid-cycle and electron-transport enzymes was transferred to a medium containing 10% glucose. After a lag phase of 30min. the yeast grew exponentially with a mean generation time of 94min. 2. The enzymes malate dehydrogenase, isocitrate lyase, succinate–cytochrome c oxidoreductase and NADH–cytochrome c oxidoreductase lost 45%, 17%, 27% and 46% of their activity respectively during the lag phase. 3. When growth commenced pyruvate kinase, pyruvate decarboxylase, alcohol dehydrogenase, glutamate dehydrogenase (NADP⁺-linked) and NADPH–cytochrome c oxidoreductase increased in activity, whereas aconitase, isocitrate dehydrogenase (NAD⁺- and NADP⁺-linked), α-oxoglutarate dehydrogenase, fumarase, malate dehydrogenase, succinate–cytochrome c oxidoreductase, NADH–cytochrome c oxidoreductase, NADH oxidase, NADPH oxidase, cytochrome c oxidase, glutamate dehydrogenase (NAD⁺-linked), glutamate–oxaloacetate transaminase, isocitrate lyase and glucose 6-phosphate dehydrogenase decreased. 4. During the early stages of growth the loss of activity of aconitase, α-oxoglutarate dehydrogenase, fumarase and glucose 6-phosphate dehydrogenase could be accounted for by dilution by cell division. The lower rate of loss of activity of isocitrate dehydrogenase (NAD⁺- and NADP⁺-linked), glutamate dehydrogenase (NAD⁺-linked), glutamate–oxaloacetate transaminase, NADPH oxidase and cytochrome c oxidase implies their continued synthesis, whereas the higher rate of loss of activity of malate dehydrogenase, isocitrate lyase, succinate–cytochrome c oxidoreductase, NADH–cytochrome c oxidoreductase and NADH oxidase means that these enzymes were actively removed. 5. The mechanisms of selective removal of enzyme activity and the control of the residual metabolic pathways are discussed.     

The effect of 4-hydroxy-2, 3-trans-pent-en-1-al and maleylaldehyde on some mitochondrial functions

Low concentrations of HPE and MLA inhibited state 3 respiration of rat liver mitochondria in the presence of different NAD⁺-dependent substrates. MLA appeared to be more active than HPE. High aldehyde concentrations inhibited the state 3 respiration with succinate. The restraint of succinate oxidation by HPE and MLA and of glutamate plus malate oxidation by MLA correlated with the inhibition of succinate and glutamate dehydrogenase activites, respectively. HPE inhibited glutamate dehydrogenase at concentrations higher than those affecting glutamate oxidation. Malate dehydrogenase activity was slightly sensitive to HPE and MLA. Both aldehydes inhibited NADH oxidation by freeze-thawed mitochondria. These results suggest the existence of a site particularly sensitive to aldehydes in the electron transport chain between the specific NAD⁺-linked dehydrogenases and ubiquinone.