Thiamine Mono- and Diphosphate Phosphatases in Bovine Brain Synaptosomes.

Neurodegenerative diseases are accompanied by changes in the activity of thiamine mono- and diphosphate phosphatases, but molecular identification of these mammalian enzymes is incomplete. In this work, the protein fraction of bovine brain synaptosomes displaying phosphatase activity toward thiamine derivatives was subjected to affinity chromatography on thiamine-Sepharose. Protein fractions eluted with thiamine (pH 7.4 or 5.6), NaCl, and urea were assayed for the phosphatase activity against thiamine monophosphate (ThMP), thiamine diphosphate (ThDP), and structurally similar purine nucleotides. Proteins in each fraction were identified by mass spectrometry using the SwissProt database for all organisms because of insufficient annotation of the bovine genome. Peptides of two annotated bacterial phosphatases, alkaline phosphatase L from the DING protein family and exopolyphosphatase, were identified in the acidic thiamine eluate. The abundance of peptides of alkaline phosphatase L and exopolyphosphatase in the eluted fractions correlated with ThMPase and ThDPase activities, respectively. The elution profiles of the ThMPase and ThDPase activities differed from the elution profiles of nucleotide phosphatases, thus indicating the specificity of these enzymes toward thiamine derivatives. The search for mammalian DING phosphatases in the eluates from thiamine-Sepharose revealed X-DING-CD4, mostly eluted by the acidic thiamine solution (pH 5.6). The identified exopolyphosphatase demonstrated structural similarity with apyrases possessing the ThDPase activity. The obtained results demonstrate that mammalian DING proteins and apyrases exhibit ThMPase and ThDPase activity, respectively.

Regulation of Malate Dehydrogenases and Glutamate Dehydrogenase of Mammalian Brain by Thiamine in vitro and in vivo.

To study the mechanisms of the non-coenzyme action of thiamine and its diphosphate (ThDP) on brain proteins, proteins of acetone extract of bovine brain synaptosomes or the homogenate of rat brain cortex were subjected to affinity chromatography on thiamine-modified Sepharose. In the step-wise eluates by thiamine (at pH 7.4 or 5.6), NaCl, and urea, the occurrence of glutamate dehydrogenase (GDH) and isoenzymes of malate dehydrogenase (MDH) along with the influence of thiamine and/or ThDP on the enzymatic activities were characterized using mass spectrometry and kinetic experiments. Maximal activation of the malate dehydrogenase reaction by thiamine is observed after the protein elution with the acidic thiamine solution, which does not elute the MDH1 isoenzyme. Effects of exogenous thiamine or ThDP on the GDH activity may depend on endogenous enzyme regulators. For example, thiamine and/or ThDP activate the brain GDH in eluates from thiamine-Sepharose but inhibit the enzyme in the crude preparations applied to the sorbent. Inhibition of GDH by ThDP is observed using the ADP-activated enzyme. Compared to the affinity chromatography employing the elution by thiamine at pH 7.4, the procedure at pH 5.6 decreases the activation of GDH by thiamine (but not ThDP) in the eluates with NaCl and urea. Simultaneously, the MDH2 content and total GDH activity are higher after the affinity elution at pH 5.6 than at pH 7.4, suggesting the role of the known interaction of GDH with MDH2 in stabilizing the activity of GDH and in the regulation of GDH by thiamine. The biological potential of thiamine-dependent regulation of the brain GDH is confirmed in vivo by demonstration of changes in regulatory properties of GDH after administration of a high dose of thiamine to rats. Bioinformatics analysis of the thiamine-eluted brain proteins shows a specific enrichment of their annotation terms with "phosphoprotein", "acetylation", and "methylation". The relationship between thiamine and the posttranslational modifications in brain may contribute to the neuroprotective effects of high doses of thiamine, including the regulation of oxidation of the major excitatory neurotransmitter in brain - glutamate.