Crystalline L-amino-acid oxidase from the soluble fraction of rat-kidney cells

From Glucose to Lactate and Transiting Intermediates Through Mitochondria, Bypassing Pyruvate Kinase: Considerations for Cells Exhibiting Dimeric PKM2 or Otherwise Inhibited Kinase Activity

A metabolic hallmark of many cancers is the increase in glucose consumption coupled to excessive lactate production. Mindful that L-lactate originates only from pyruvate, the question arises as to how can this be sustained in those tissues where pyruvate kinase activity is reduced due to dimerization of PKM2 isoform or inhibited by oxidative/nitrosative stress, posttranslational modifications or mutations, all widely reported findings in the very same cells. Hereby 17 pathways connecting glucose to lactate bypassing pyruvate kinase are reviewed, some of which transit through the mitochondrial matrix. An additional 69 converging pathways leading to pyruvate and lactate, but not commencing from glucose, are also examined. The minor production of pyruvate and lactate by glutaminolysis is scrutinized separately. The present review aims to highlight the ways through which L-lactate can still be produced from pyruvate using carbon atoms originating from glucose or other substrates in cells with kinetically impaired pyruvate kinase and underscore the importance of mitochondria in cancer metabolism irrespective of oxidative phosphorylation.

Purification and characterization of 3-mercaptolactic acid S-conjugate oxidases

Two enzymes catalysing the oxidative formation of 3-mercaptopyruvic acid S-conjugates from L-3-mercaptolactic acid S-conjugates were purified to apparent homogeneity from rat liver cytosol. The two enzymes, tentatively designated MLO-I and MLO-II, showed a molecular mass of 160 and 250 kDa and were composed of four and six subunits of 41 and 39 kDa, respectively. Both enzymes possessed flavin mononucleotide as prosthetic group and oxidized several aromatic and aliphatic S-substituted L-3-mercaptolactic acids as well as alpha-hydroxy acids such as L-3-phenyllactic acid and L-2-hydroxyisocaproic acid. Glycolic acid and 3-(4-hydroxyphenyl)-lactic acid were the specific substrates for MLO-I and MLO-II, respectively. Neither of the enzymes oxidized beta- and gamma-hydroxy acids such as 3- and 4-hydroxybutyric acid. 2-Hydroxyisobutyric acid, ethyl-2-hydroxybutyrate, malic acid, 1-butanol, benzyl alcohol and L-leucine did not act as substrates for the enzymes. MLO-I and MLO-II exerted their maximum activities around pH 5.5 with Km of 0.5 and 0.25 mM and Vmax of 0.9 and 0.2 mumol/min/mg, respectively, when S-(4-bromophenyl)-3-thiolactic acid was used as substrate. MLO-I was inhibited by sulphydryl-modifying agents, while MLO-II was not. Both enzymes were strongly inhibited by divalent metal ions. These results indicate that MLO-I and MLO-II are different from L-amino acid oxidase (EC 1.4.3.2), malate oxidase (EC 1.1.3.3), L-alpha-hydroxy acid oxidase (EC 1.1.3.15) and glycolate oxidase (EC 1.1.3.1). The present enzymes are likely to be involved in the formation of cysteine conjugates from L-3-mercaptolactic acid S-conjugates in conjunction with cysteine conjugate aminotransferases.

Enzymatic synthesis of S-aminoethyl-L-cysteine from pantetheine

The recently characterized compound S-aminoethylcysteine ketimine can be synthesized from purified S-aminoethylcysteine by enzymatic systems (transaminases or L-amino acid oxidase) present in mammalian tissues. S-Aminoethylcysteine, which could be considered as the natural precursor of the ketimine, is produced from L-serine and cysteamine by the action of the enzyme cystathionine-beta-synthase. We demonstrate in this paper that pantetheine, a normal cellular component, is an efficient cysteamine donor for the synthesis of S-aminoethylcysteine and of S-aminoethylcysteine ketimine in the place of free cysteamine, and we describe the enzymatic system, composed of partially purified enzymes, for the in vitro synthesis of S-aminoethylcysteine ketimine from pantetheine. This seems to indicate a new biological role for pantetheine.

Bioactivation mechanism of cytotoxic homocysteine S-conjugates

S-(1,2-Dichlorovinyl)-L-homocysteine is a much more potent nephrotoxin than the corresponding cysteine S-conjugate S-(1,2-dichlorovinyl)-L-cysteine (A. A. Elfarra, L. H. Lash, and M. W. Anders (1986) Proc. Natl. Acad. Sci. USA 83, 2667-2671). The objective of the present experiments was to test the hypothesis that the increased toxicity of homocysteine S-conjugates may be associated with the formation of the reactive metabolite 2-oxo-3-butenoic acid, which may arise via a nonenzymatic retro-Michael elimination reaction from the 2-oxo acid metabolites of homocysteine S-conjugates. S-(2-Benzothiazolyl)-L-homocysteine, which was a substrate for purified bovine kidney cysteine conjugate beta-lyase (glutamine transaminase K) and whose metabolism was dependent on the presence of a 2-oxo acid, was cytotoxic in isolated rat kidney cells and was toxic to rat renal mitochondria, whereas the cysteine S-conjugate S-(2-benzothiazolyl)-L-cysteine had little effect. L-Methionine sulfoximine, L-canavanine, and the Michael acceptor methyl vinyl ketone were cytotoxic. The 2-hydroxy acid analogs of S-(1,2-dichlorovinyl)-L-homocysteine and 2-oxo-3-butenoic acid, S-(1,2-dichlorovinyl)-2-hydroxy-4-mercaptobutanoic acid and 2-hydroxy-3-butenoic acid, respectively, which are expected to be metabolized by rat renal L-2-hydroxy (L-amino) acid oxidase to yield 2-oxo-3-butenoic acid, were also cytotoxic. To obtain evidence for the formation of 2-oxo-3-butenoic acid as a product of the metabolism of L-homocysteine S-conjugates and analogs, trapping experiments were conducted. S-(2-Benzothiazolyl)-L-homocysteine, S-(1,2-dichlorovinyl)-L-homocysteine, L-methionine sulfoximine, and L-canavanine were converted by snake venom L-amino acid oxidase to 2-oxo-3-butenoic acid, which was trapped by the nucleophile methanethiol to yield 4-methylthio-2-oxobutanoic acid; the trapped product was derivatized with 2,4-dinitrophenylhydrazine and was identified by its electronic absorption spectrum and by high-performance liquid chromatography. Similar trapping experiments conducted with kidney homogenates and purified beta-lyase were not successful. The data indicate that the bioactivation of homocysteine S-conjugates and analogs involves the enzymatic formation of the corresponding 2-oxo acids followed by a nonenzymatic retro-Michael elimination reaction to yield the Michael acceptor 2-oxo-3-butenoic acid, which may contribute to the observed cytotoxicity of homocysteine S-conjugates.

Bioactivation mechanism of L-thiomorpholine-3-carboxylic acid

L-Thiomorpholine-3-carboxylic acid (L-TMC) is a cyclized analog of S-(2-chloroethyl)-L-cysteine, which is cytotoxic in vitro and nephrotoxic in vivo. To determine whether L-TMC may play a role in S-(2-chloroethyl)-L-cysteine-induced toxicity, the cytotoxicity of L-TMC was studied in isolated rat kidney cells. L-TMC produced time- and concentration-dependent cytotoxicity. Probenecid, an inhibitor of the renal anion transport system, and L-alpha-hydroxyisocaproic acid, a substrate for L-amino acid oxidase, inhibited L-TMC-induced cytotoxicity. Rat kidney cytosol catalyzed the metabolism of L-TMC to a product absorbing at 300 nm. The increase in absorbance at 300 nm was accompanied by an increase in oxygen consumption and was inhibited by L-alpha-hydroxyisocaproic acid; moreover, the absorbance of the metabolite was quenched by addition of potassium cyanide or sodium borohydride, which indicated the formation of an imine. When L-TMC was incubated with rat kidney cytosol and sodium borodeuteride was added at the end of the incubation period, analysis by gas chromatography/mass spectrometry of the tert-butyldimethylsilyl ester of L-TMC showed the formation of [2H]TMC, indicating the intermediate formation of the imine 5,6-dihydro-2H-1,4-thiazine-3-carboxylic acid; chemically synthesized TMC imine showed similar behavior. The enzyme responsible for the metabolism of L-TMC was purified from rat kidney and was identified as L-amino acid oxidase. These observations indicate a role for L-amino acid oxidase in the bioactivation and cytotoxicity of L-TMC.

Biosynthesis and biotransformation of glutathione S-conjugates to toxic metabolites

The material presented in this review deals with the hypothesis that the nephrotoxicity of certain halogenated alkanes and alkenes is associated with hepatic biosynthesis of glutathione S-conjugates, which are further metabolized to the corresponding cysteine S-conjugates. Some glutathione or cysteine S-conjugates may be direct-acting nephrotoxins, but most cysteine S-conjugates require bioactivation by renal, pyridoxal phosphate-dependent enzymes, such as cysteine conjugate beta-lyase (beta-lyase). The biosynthesis of glutathione S-conjugates is catalyzed by both the cytosolic and the microsomal glutathione S-transferases, although the latter enzyme is a better catalyst for the reaction of haloalkenes with glutathione. When glutathione S-conjugate formation yields sulfur mustards, as occurs with vicinal-dihaloethanes, the S-conjugates are direct-acting toxins. In contrast, the S-conjugates formed from fluoro- and chloroalkenes yield S-alkyl- or S-vinyl glutathione conjugates, respectively, which are metabolized to the corresponding cysteine S-conjugates by gamma-glutamyltransferase and dipeptidases; inhibition of these enzymes blocks the toxicity of the glutathione S-conjugates. The cysteine S-conjugates must be metabolized by beta-lyase for the expression of toxicity; the beta-lyase inhibitor aminooxyacetic acid blocks the toxicity of cysteine S-conjugates, and the corresponding alpha-methyl cysteine S-conjugates, which cannot be metabolized by beta-lyase, are not toxic. Moreover, probenecid, an inhibitor of renal anion transport system, blocks the toxicity of cysteine S-conjugates, which cannot be metabolized by beta-lyase, are not toxic. Moreover, probenecid, an inhibitor of renal anion transport system, blocks the toxicity of cysteine S-conjugates. Homocysteine S-conjugates are also potent cyto- and nephrotoxins. The high renal content of gamma-glutamyltransferase and the renal anion transport system are probably determinants of kidney tissue as a target site. Biochemical studies indicate that renal mitochondrial dysfunction is produced by the cysteine S-conjugates. Finally, some of the glutathione and cysteine conjugates are mutagenic in the Ames test, and reactive intermediates formed by the action of beta-lyase may contribute to the nephrocarcinogenicity of certain chloroalkenes.

Light and electron microscopic localization of L-alpha-hydroxyacid oxidase in rat kidney revealed by immunocytochemical techniques

Light and electron microscopic localization of L-alpha-hydroxyacid oxidase (L-HOX) in rat kidney was studied by means of immunocytochemical techniques. Isozymes A and B of L-HOX were purified from rat liver and kidney, respectively. The apparent molecular weights of the subunits of the isozymes A and B were 35,800 and 33,500 daltons, respectively, by a slab gel electrophoresis. Antibodies to the isozymes were raised in rabbits. Anti(isozyme A) is not cross-reactive with the isozyme B and vice versa anti(isozyme B) not with the isozyme A. Using anti-isozyme B, semithin sections of Epon-embedded material and ultrathin sections of Lowicryl K4M-embedded material were stained by immunoenzyme and protein A-gold techniques, respectively. By light microscopy, fine discrete granular staining was noted in proximal tubules, but not in distal tubules including thick and thin limbs of Henle and collecting tubules. By electron microscopy, gold particles representing the antigen sites for L-HOX B were confined exclusively to peroxisomes, in which most of the gold particles were localized in electron dense peripheral matrix, but little in central matrix with low electron density. The results indicate that L-HOX B does not homogeneously distribute in peroxisomes of rat kidney but might be associated with some substructure within peroxisome matrix.

L-alpha-Hydroxyacid oxidase isozymes. Purification and molecular properties

L-alpha-Hydroxyacid oxidase isozymes from rat liver (A isozyme) and kidney (B isozyme) have been isolated in a high state of purity with specific activities of 61 and 14.7 microkatals per gram protein respectively. The subunit molecular weights determined by sodium dodecylsulphate polyacrylamide gel electrophoresis were 40000 +/- 3000; the mouse A and B isozymes were also partially purified and their subunit molecular weights shown to be 37000.

Amino acid oxidase of leukocytes in relation to H 2 O 2 -mediated bacterial killing

D-Amino acid oxidase and L-amino acid oxidase have been measured in sucrose homogenates of polymorphonuclear leukocytes (PMN) obtained from guinea pigs and humans. Subcellular distribution patterns and studies on latency indicate that these oxidases are soluble enzymes. Their hydrogen peroxide-generating capacity was verified. Chronic granulomatous disease PMN, which lack a respiratory burst and fail to generate H(2)O(2) during phagocytosis and do not kill catalase positive bacteria, had peroxide-generating amino acid oxidase activity equal to that found in PMN homogenates from patients with bacterial infections. The precise metabolic and bactericidal role of amino acid oxidases in PMN remains uncertain.

The synthesis and turnover of rat liver peroxisomes. I. Fractionation of peroxisome proteins

Rat liver peroxisomes isolated by density gradient centrifugation were disrupted at pH 9, and subdivided into a soluble fraction containing 90% of their total proteins and virtually all of their catalase, D-amino acid oxidase, L-alpha-hydroxy acid oxidase and isocitrate dehydrogenase activities, and a core fraction containing urate oxidase and 10% of the total proteins. The soluble proteins were chromatographed on Sephadex G-200, diethylaminoethyl (DEAE)-cellulose, hydroxylapatite, and sulfoethyl (SE)-Sephadex. None of these methods provided complete separation of the protein components, but these could be distributed into peaks in which the specific activities of different enzymes were substantially increased. Catalase, D-amino acid oxidase, and L-alpha-hydroxy acid oxidase contribute a maximum of 16, 2, and 4%, respectively, of the protein of the peroxisome. The contribution of isocitrate dehydrogenase could be as much as 25%, but is probably much less. After dissolution of the cores at pH 11 , no separation between their urate oxidase activity and their protein was achieved by Sephadex G-200 chromatography.