Inactivation of glucose-6-phosphate dehydrogenase isozymes from human and pig brain by aluminum

Influence of the green tea leaf extract on neurotoxicity of aluminium chloride in rats

Aluminium may have an important role in the aetiology/pathogenesis/precipitation of Alzheimer's disease. Because green tea (Camellia sinensis L.) reportedly has health-promoting effects in the central nervous system, we evaluated the effects of green tea leaf extract (GTLE) on aluminium chloride (AlCl3 ) neurotoxicity in rats. All solutions were injected into the cornu ammonis region 1 hippocampal region. We measured the performance of active avoidance (AA) tasks, various enzyme activities and total glutathione content (TGC) in the forebrain cortex (FbC), striatum, basal forebrain (BFb), hippocampus, brain stem and cerebellum. AlCl3 markedly reduced AA performance and activities of cytochrome c oxidase (COX) and acetylcholinesterase (AChE) in all regions. It decreased TGC in the FbC, striatum, BFb, hippocampus, brain stem and cerebellum, and increased superoxide dismutase activity in the FbC, cerebellum and BFb. GTLE pretreatment completely reversed the damaging effects of AlCl3 on AA and superoxide dismutase activity, markedly corrected COX and AChE activities, and moderately improved TGC. GTLE alone increased COX and AChE activities in almost all regions. GTLE reduces AlCl3 neurotoxicity probably via antioxidative effects and improves mitochondrial and cholinergic synaptic functions through the actions of (-)-epigallocatechin gallate and (-)-epicatechin, compounds most abundantly found in GTLE. Our results suggest that green tea might be beneficial in Alzheimer's disease.

Effect of cadmium acetate on the conformation of lysozyme: functional implications

Structural variations of lysozyme as a consequence of its interaction with CdAc2, as well as the implications on the protein functionality have been studied. Variations in the conformation of the macromolecule are seen, however these changes are not reflected on the secondary structure. The interaction of the salt with the polypeptide chain is weak and thermodynamically unfavourable. Molecular aggregates (dimer forms) are observed at the highest salt concentrations. This interaction causes an inhibitory effect on lysozyme, the activity loss being 50% at the highest salt concentration studied. The inhibition is of mixed type with an uncompetitive component. Thus cadmium does not bind to the active site of the enzyme which is in accordance with the not very large activity loss observed. The substrate inhibition of lysozyme is favoured in the presence of the salt, so interaction with the macromolecule is at low affinity sites.

Aluminum inhibition of hexokinase activity in vitro: a study in biological availability

We have used the HK/G6PDH coupled enzyme assay to determine the biological availability of aluminum in mixed-ligand media of biological interest. The biological availability of aluminum was measured as the inhibition of the activity rate of the assay and was shown to be dependent upon the equilibration state of the aluminum stock solutions (prior to their addition to the assay) and the comparative reaction kinetics of competitive aluminum equilibria in the assays. Aluminum was found to inhibit the assay, however, the inhibition by aluminum was abolished when silicic acid was present in both the aluminum stock solution and the assay medium. The assay is proposed as a model system for investigating the biological availability of aluminum in heterogenous media of biochemical significance.

Ferritin: the role of aluminum in ferritin function

We previously showed that human brain ferritin (HBF) binds aluminum (Al) in vivo and in vitro and HBF isolated from Alzheimer's brain had more Al bound compared to aged matched controls (7). To further understand the role ferritin may play in Al neurotoxicity, we have studied in vitro the effect of Al on the function of human ferritin isolated from Alzheimer's (AD) and normal brain tissue, and compared the results with other mammalian ferritins. Al causes a concentration-dependent decrease in the initial rate of iron loading into apo-horse spleen and human brain ferritin and the rates were similar for ferritin isolated from both AD and normal brains. The rates of iron release of mammalian ferritins from different tissues were determined: horse spleen much greater than human liver greater than rat brain greater than human brain = rat liver ferritin. The rates of iron release of AD and normal human brain ferritin were similar and were unaffected by preloading with Al. Several mammalian ferritins were compared for their total iron uptake: horse spleen = human liver greater than human brain (normal) = human brain (AD) ferritin. In 20 mM HEPES (pH 6.0) buffer holoferritin is more resistant to precipitation by Al than apoferritin suggesting that holoferritin is a better chelator for nonferrous metal ions.

Neurochemical hypothesis: participation by aluminum in producing critical mass of colocalized errors in brain leads to neurological disease

1. Aluminum is an established neurotoxin. Prolonged exposure to even low levels of aluminum permit its chelation and subsequent transport to brain where it is non-uniformly distributed. 2. Available evidence suggests that (i) aluminum interferes with glucose metabolism by inhibiting hexokinase and glucose-6-phosphate dehydrogenase; (ii) it binds to calmodulin and affects numerous phosphorylation-dephosphorylation reactions; (iii) it binds to transferrin and ferritin, affects the function of these proteins which in turn affect iron metabolism. 3. Thus accumulation of aluminum-induced metabolic errors colocalized in specific areas of the brain may lead to neurological disorders.

Characterization of glucose-6-phosphate dehydrogenase isozymes from human and pig brain

Homogenates of human and pig brain in 10 mM Tris-HCl, pH 8.0 were centrifuged at 25,400 x g for 1 h. The supernatants were electrophoresed in polyacrylamide gels were stained for glucose-6-phosphate dehydrogenase (EC 1.1.1.49) activity. Five distinct bands were visible. Isozymes corresponding to two of those bands were purified from human and pig brain. The isozymes were electrophoretically homogeneous. The native proteins, Mr, 220,000, dissociated in sodium dodecyl sulphate-polyacrylamide gels into a 57,000 Mr subunit. Therefore, the native isozymes are tetramers. None of the isozymes required additional metal ions for activity. At 1 mM concentration Mg2+ and Ca2+, independently or together, activated the isozymes 1.5-fold. The isozymes were NADP(+)-specific. Kmapp values of the G6PD isozymes were similar for NADP+ (6-8 microM), but different for G6P (56-180 microM). The specific activities of the isozymes varied from 50 to 210 units per mg of protein. All isozymes were inhibited by NADPH. The inhibition was competitive with respect to NADP+ and non-competitive with respect to G6P. NADH did not affect any of the isozymes. ATP inhibited the isozymes competitively with respect to G6P and non-competitively with respect to NADP+. Palmitoyl-CoA dissociated the active tetramers into enzymatically inactive dimeric forms. This treatment also abolished the 6-phosphogluconate activity of the isozyme II from both sources. High performance liquid chromatography peptide maps of the tryptic digest and amino acid analyses of the isozymes showed extensive homologies between the corresponding isozymes from the two species. Interestingly, only the isozyme II in human and pig brain was active with 6-phosphogluconate as a substrate (Kmapp = 864 and 279 microM). The specific activities of the isozyme II with 6-phosphogluconate (14 and 48 unit per mg of protein for human and pig brain isozyme II, respectively) was four times less than those with G6P. It is therefore suggested that isozyme II is a bifunctional enzyme.