A comparison of the secondary structure of human brain mitochondrial and cytosolic 'malic' enzyme investigated by Fourier-transform infrared spectroscopy

Effect of stereotactic radiosurgery on lipids and proteins of normal and hypoperfused rat brain homogenates: A Fourier transform infrared spectroscopy study

PURPOSE

The effect of stereotactic radiosurgery on lipids and proteins of normal and hypoperfused rat brain was investigated to see if hypoxic areas are really more resistant to radiation effects or not.

MATERIALS AND METHODS

Rat brain samples from control, stereotactically irradiated and chronically hypoperfused plus stereotactically irradiated groups were homogenized separately with saline phosphate buffer, and centrifuged at 125,000 g for 15 min. Membrane rich parts (pellet) of these homogenates were used for Fourier Transform Infrared (FTIR) spectroscopy studies. Mann-Whitney U tests were performed on the groups, two by two, to test the significance of the differences between the control group and stereotactically irradiated group as well as the control group and chronically hypoperfused plus stereotactically irradiated group.

RESULTS

After a single high dose of X-rays to healthy rat brain, the lipid concentration increased slightly, protein content decreased significantly (p < 0.05) and protein-to-lipid ratio decreased slightly. The secondary structure of the proteins was altered in the irradiated brain samples such that the content of a-helical structure decreased significantly (p < 0.01) and random coil increased dramatically (p < 0.05). The effect of radiation on the content of a-helical structure was not found to be significant in the hypoperfused group, but the decrease in the content of random coil was significant (p < 0.01).

CONCLUSION

Stereotactic radiosurgery of the brain increased the lipid concentration, decreased the protein concentration and consequently resulted in a decrease in the protein to lipid ratio compared to un-irradiated brain. Radiation also altered the secondary structure of protein. The variations in lipid and protein content and the resulting lipid to protein ratio imply that chronically hypoperfused brain is more vulnerable to radiation than non-hypoperfused brain and suggests chronic hypoperfusion does not prevent cerebral damage caused by irradiation. However, irradiation of hypoperfused brain resulted in less alteration in protein structure than in non-hyperfused brain, suggesting higher resistance to irradiation using this endpoint.

Chronic hypoperfusion alters the content and structure of proteins and lipids of rat brain homogenates: a Fourier transform infrared spectroscopy study

Arteriovenous malformations (AVMs), masses of abnormal blood vessels which grow in the brain, produce high flow shunts that steal blood from surrounding brain tissue, which is chronically hypoperfused. Hypoperfusion is a condition of inadequate tissue perfusion and oxygenation, resulting in abnormal tissue metabolism. Fourier transform infrared (FTIR) spectroscopy is used in this study to investigate the effect of hypoperfusion on homogenized rat brain samples at the molecular level. The results suggest that the lipid content increases, the protein content decreases, the lipid-to-protein ratio increases, and the state of order of the lipids increases in the hypoperfused brain samples. FTIR results also revealed that, owing to hypoperfusion, not only the protein synthesis but also the protein secondary structure profile is altered in favor of beta-sheets and random coils. These findings clearly demonstrate that, FTIR spectroscopy can be used to extract valuable information at the molecular level so as to have a better understanding of the effect of hypoperfusion on rat brain.

Malo-ethanolic fermentation in Saccharomyces and Schizosaccharomyces

Yeast species are divided into the K(+) or K(-) groups, based on their ability or inability to metabolise tricarboxylic acid (TCA) cycle intermediates as sole carbon or energy source. The K(-) group of yeasts includes strains of Saccharomyces, Schizosaccharomyces pombe and Zygosaccharomyces bailii, which is capable of utilising TCA cycle intermediates only in the presence of glucose or other assimilable carbon sources. Although grouped together, these yeasts have significant differences in their abilities to degrade malic acid. Typically, strains of Saccharomyces are regarded as inefficient metabolisers of extracellular malic acid, whereas strains of Sch. pombe and Z. bailii can effectively degrade high concentrations of malic acid. The ability of a yeast strain to degrade extracellular malic acid is dependent on both the efficient transport of the dicarboxylic acid and the efficacy of the intracellular malic enzyme. The malic enzyme converts malic acid into pyruvic acid, which is further metabolised to ethanol and carbon dioxide under fermentative conditions via the so-called malo-ethanolic (ME) pathway. This review focuses on the enzymes involved in the ME pathway in Sch. pombe and Saccharomyces species, with specific emphasis on the malate transporter and the intracellular malic enzyme.

Effect of oxidative stress on stability and structure of neurofilament proteins

Neurofilament proteins are highly phosphorylated molecules in the axonal compartment of the adult nervous system. We report the structural analysis of neurofilament proteins after oxidative damage. SDS-PAGE, immunoblotting, circular dichroism, and Fourier transform infrared spectroscopy were used to investigate the relative sensitivity of neurofilaments to oxidative stress and to identify changes in their molecular organization. An ascorbate-Fe+3-O2 buffer system as well as catechols were used to generate free radicals on a substrate of phosphorylated and dephosphorylated neurofilaments. By Fourier Transform Infrared spectroscopy and circular dichroism, we established that the neurofilament secondary structure is mainly composed of α-helices and that after free radical damage of the peptide backbone of neurofilaments, those helices are partly modified into β-sheet and random coil structures. These characteristic reorganizations of the neurofilament structure after oxidative exposure suggest that free radical activity might play an important role in the biogenesis of the cytoplasmic inclusions found in several neurodegenerative diseases.Key words: neurofilaments, oxidative stress, neurodegeneration, phosphorylation, infrared spectroscopy, circular dichroism.

Mitochondrial malic enzyme activity is much higher in mitochondria from cortical synaptic terminals compared with mitochondria from primary cultures of cortical neurons or cerebellar granule cells

Most of the malic enzyme activity in the brain is found in the mitochondria. This isozyme may have a key role in the pyruvate recycling pathway which utilizes dicarboxylic acids and substrates such as glutamine to provide pyruvate to maintain TCA cycle activity when glucose and lactate are low. In the present study we determined the activity and kinetics of malic enzyme in two subfractions of mitochondria isolated from cortical synaptic terminals, as well as the activity and kinetics in mitochondria isolated from primary cultures of cortical neurons and cerebellar granule cells. The synaptic mitochondrial fractions had very high mitochondrial malic enzyme (mME) activity with a Km and a Vmax of 0.37 mM and 32.6 nmol/min/mg protein and 0.29 mM and 22.4 nmol/min mg protein, for the SM2 and SM1 fractions, respectively. The Km and Vmax for malic enzyme activity in mitochondria isolated from cortical neurons was 0.10 mM and 1.4 nmol/min/mg protein and from cerebellar granule cells was 0.16 mM and 5.2 nmol/min/mg protein. These data show that mME activity is highly enriched in cortical synaptic mitochondria compared to mitochondria from cultured cortical neurons. The activity of mME in cerebellar granule cells is of the same magnitude as astrocyte mitochondria. The extremely high activity of mME in synaptic mitochondria is consistent with a role for mME in the pyruvate recycling pathway, and a function in maintaining the intramitochondrial reduced glutathione in synaptic terminals.

Different regulatory properties of the cytosolic and mitochondrial forms of malic enzyme isolated from human brain

The human brain contains a cytosolic and mitochondrial form of NADP(+)-dependent malic enzyme. To investigate their possible metabolic roles we compared the regulatory properties of these two iso-enzymes. The mitochondrial malic enzyme exhibited a sigmoid substrate saturation curve at low malate concentration which was shifted to the right at both higher pH values and in the presence of low concentration of Mn2+ or Mg2+. Succinate or fumarate increased the activity of the mitochondrial malic enzyme at low malate concentration. Both activators shifted the plot of reaction velocity versus malate concentration to the left, and removed sigmoidicity, but the maximum velocity was unaffected. The activation was associated with a decrease in Hill coefficient from 2.3 to 1.1. The human brain cytosolic malic enzyme displayed a hyperbolic substrate saturation kinetics and no sigmoidicity was detected even at high pH and low malate concentrations. Succinate or fumarate exerted no effect on the enzyme activity. Excess of malate inhibited the oxidative decarboxylation catalysed by cytosolic enzyme at pH 7.0 and below. In contrast, decarboxylation catalysed by mitochondrial malic enzyme, was unaffected by the substrate. These results suggest that under in vivo conditions, cytosolic malic enzyme catalyses both oxidative decarboxylation of malate and reductive carboxylation of pyruvate, whereas the role of mitochondrial enzyme is limited to decarboxylation of malate. One may speculate that in vivo the reaction catalysed by cytosolic malic enzyme supplies dicarboxylic acids (anaplerotic function) for the formation of neurotransmitters, while the mitochondrial enzyme regulates the flux rate via Krebs cycle by disposition of the tricarboxylic acid cycle intermediates (cataplerotic function).