Specific and reversible inactivation of Phycomyces blakesleeanus isocitrate lyase by ascorbate-iron: role of two redox-active cysteines

Regulation by glutathionylation of isocitrate lyase from Chlamydomonas reinhardtii

Post-translational modification of protein cysteine residues is emerging as an important regulatory and signaling mechanism. We have identified numerous putative targets of redox regulation in the unicellular green alga Chlamydomonas reinhardtii. One enzyme, isocitrate lyase (ICL), was identified both as a putative thioredoxin target and as an S-thiolated protein in vivo. ICL is a key enzyme of the glyoxylate cycle that allows growth on acetate as a sole source of carbon. The aim of the present study was to clarify the molecular mechanism of the redox regulation of Chlamydomonas ICL using a combination of biochemical and biophysical methods. The results clearly show that purified C. reinhardtii ICL can be inactivated by glutathionylation and reactivated by glutaredoxin, whereas thioredoxin does not appear to regulate ICL activity, and no inter- or intramolecular disulfide bond could be formed under any of the conditions tested. Glutathionylation of the protein was investigated by mass spectrometry analysis, Western blotting, and site-directed mutagenesis. The enzyme was found to be protected from irreversible oxidative inactivation by glutathionylation of its catalytic Cys(178), whereas a second residue, Cys(247), becomes artifactually glutathionylated after prolonged incubation with GSSG. The possible functional significance of this post-translational modification of ICL in Chlamydomonas and other organisms is discussed.

A protective association between catalase and isocitrate lyase in peroxisomes

Glyoxysomes are specialized peroxisomes in germinating seeds, which catalyze many reactions that convert fatty acids into carbohydrates thus generating H(2)O(2). They are characterized by the presence of catalase (CAT, E.C. 1.11.1.6) in their matrix which protects cells from oxidative stress. Here, we investigated the possibility that a protein can be protected from oxidative damage by its association with CAT. We purified peroxisomal CAT from germinating castor beans by ion exchange, gel filtration, and hydroxylapatite chromatography. Gel filtration of the matrix proteins, cross-linking, and co-immunoprecipitation studies indicate that CAT associates with a glyoxysomal matrix protein, isocitrate lyase (ICL, E.C. 4.1.3.1). In addition, we found that H(2)O(2) inactivates ICL and degrades its product, glyoxylate, when CAT is inactive. ICL and its product appear to be sensitive to oxidative damage; thus, association of CAT with ICL would afford protection from H(2)O(2).

New thioredoxin targets in the unicellular photosynthetic eukaryote Chlamydomonas reinhardtii

Proteomics were used to identify the proteins from the eukaryotic unicellular green alga Chlamydomonas reinhardtii that can be reduced by thioredoxin. These proteins were retained specifically on a thioredoxin affinity column made of a monocysteinic thioredoxin mutant able to form mixed disulfides with its targets. Of a total of 55 identified targets, 29 had been found previously in higher plants or Synechocystis, but 26 were new targets. Biochemical tests were performed on three of them, showing a thioredoxin-dependent activation of isocitrate lyase and isopropylmalate dehydrogenase and a thioredoxin-dependent deactivation of catalase that is redox insensitive in Arabidopsis. In addition, we identified a Ran protein, a previously uncharacterized nuclear target in a photosynthetic organism. The metabolic and evolutionary implications of these findings are discussed.

Antioxidant-independent ascorbate enhancement of catecholamine-induced contractions of vascular smooth muscle

Ascorbate reduces the oxidation rate of catecholamines and, by an independent mechanism, enhances rabbit aortic ring contractions initiated by catecholamines. The largest significantly different fractional increases in force produced by ascorbate enhancement of norepinephrine (NE), epinephrine, phenylpropanolamine (PPA), and ephedrine (Eph) are 5.5, 1.8, 1.6, and 1.3 times, respectively. In physiological salt solutions bubbled with 95% O(2) at 37 degrees C, NE, PPA, and Eph have oxidation rate constants of 1.24, 247, and 643 h, respectively. Ascorbate significantly enhances 100 nM NE contractions by at least twofold at all ascorbate concentrations >15 microM, including the entire physiological range of 40-100 microM. Ascorbate preloading and washout followed by NE exposure produces significantly greater contractions than NE without ascorbate preloading but significantly lower than NE simultaneously with ascorbate. Ascorbate does not enhance K(+)- or angiotensin II-induced contractions. Ascorbate enhancement of catecholamine contractions occurs in addition to the reduction in oxidation rate, because the increases in force occur faster than oxidation can occur, the increases occur with compounds that have negligible oxidation rates, and the increases occur when ascorbate and NE are not physically present together. These results are consistent with ascorbate acting on the adrenergic receptor. Ascorbate may play a role in shock and asthma treatments and potentiate the cardiovascular health consequences of PPA and Eph (Ephedra).