Glyceraldehyde metabolism in human erythrocytes in comparison with that of glucose and dihydroxyacetone

Solution Chemistry of Dihydroxyacetone and Synthesis of Monomeric Dihydroxyacetone

Dihydroxyacetone (DHA) is a major byproduct of e-cigarette combustion and is the active ingredient in sunless tanning products. Mounting evidence points to its damaging effects on cellular functions. While developing a simple synthetic route to monomeric [13C3]DHA for flux metabolic studies that compared DHA and glyceraldehyde (GA) metabolism, we uncovered that solid DHA ages upon storage and differences in the relative abundance of each of its isomer occur when reconstituted in an aqueous solution. While all three of the dimeric forms of DHA ultimately resolve to the ketone and hydrated forms of monomeric DHA once in water at room temperature, these species require hours rather than minutes to reach an equilibrium favoring the monomeric species. Consequently, when used in bolus or flux experiments, the relative abundance of each isomer and its effects at the time of application is dependent on the initial DHA isomeric composition and concentration, and time of equilibration in solution before use. Here, we make recommendations for the more consistent handling of DHA as we report conditions that ensure that DHA is present in its monomeric form while in solutions, conditions used in an isotopic tracing study that specifically compared monomeric DHA and GA metabolism in cells.

Toxicity of dihydroxyacetone is exerted through the formation of methylglyoxal in Saccharomyces cerevisiae: effects on actin polarity and nuclear division

Dihydroxyacetone (DHA) is the smallest ketotriose, and it is utilized by many organisms as an energy source. However, at higher concentrations, DHA becomes toxic towards several organisms including the budding yeast Saccharomyces cerevisiae. In the present study, we show that DHA toxicity is due to its spontaneous conversion to methylglyoxal (MG) within yeast cells. A mutant defective in MG-metabolizing enzymes (glo1Δgre2Δgre3Δ) exhibited higher susceptibility to DHA. Intracellular MG levels increased following the treatment of glo1Δgre2Δgre3Δ cells with DHA. We previously reported that MG depolarized the actin cytoskeleton and changed vacuolar morphology. We herein demonstrated the depolarization of actin and morphological changes in vacuoles following a treatment with DHA. Furthermore, we found that both MG and DHA caused the morphological change in nucleus, and inhibited the nuclear division. Our results suggest that the conversion of DHA to MG is a dominant contributor to its cytotoxicity.

Bifunctional homodimeric triokinase/FMN cyclase: contribution of protein domains to the activities of the human enzyme and molecular dynamics simulation of domain movements

Mammalian triokinase, which phosphorylates exogenous dihydroxyacetone and fructose-derived glyceraldehyde, is neither molecularly identified nor firmly associated to an encoding gene. Human FMN cyclase, which splits FAD and other ribonucleoside diphosphate-X compounds to ribonucleoside monophosphate and cyclic X-phosphodiester, is identical to a DAK-encoded dihydroxyacetone kinase. This bifunctional protein was identified as triokinase. It was modeled as a homodimer of two-domain (K and L) subunits. Active centers lie between K1 and L2 or K2 and L1: dihydroxyacetone binds K and ATP binds L in different subunits too distant (≈ 14 Å) for phosphoryl transfer. FAD docked to the ATP site with ribityl 4'-OH in a possible near-attack conformation for cyclase activity. Reciprocal inhibition between kinase and cyclase reactants confirmed substrate site locations. The differential roles of protein domains were supported by their individual expression: K was inactive, and L displayed cyclase but not kinase activity. The importance of domain mobility for the kinase activity of dimeric triokinase was highlighted by molecular dynamics simulations: ATP approached dihydroxyacetone at distances below 5 Å in near-attack conformation. Based upon structure, docking, and molecular dynamics simulations, relevant residues were mutated to alanine, and kcat and Km were assayed whenever kinase and/or cyclase activity was conserved. The results supported the roles of Thr(112) (hydrogen bonding of ATP adenine to K in the closed active center), His(221) (covalent anchoring of dihydroxyacetone to K), Asp(401) and Asp(403) (metal coordination to L), and Asp(556) (hydrogen bonding of ATP or FAD ribose to L domain). Interestingly, the His(221) point mutant acted specifically as a cyclase without kinase activity.

The role of GLUT1 in the sugar-induced dielectric response of human erythrocytes

We propose a key role for the glucose transporter 1 (GLUT1) in mediating the observed changes in the dielectric properties of human erythrocyte membranes as determined by dielectric spectroscopy. Cytochalasin B, a GLUT1 transport inhibitor, abolished the membrane capacitance changes in glucose-exposed red cells. Surprisingly, D-fructose, known to be transported primarily by GLUT5, exerted similar membrane capacitance changes at increasing D-fructose concentrations. In order to evaluate whether the glucose-mediated membrane capacitance changes originated directly from intracellularly bound adenosine triphosphate (ATP) or other components of the glycolysis process, we studied the dielectric responses of swollen erythrocytes with a decreased ATP content and of nucleotide-filled ghosts. Resealed ghosts containing physiological concentrations of ATP yielded the same glucose-dependent capacitance changes as biconcave intact red blood cells, further supporting the finding that ATP is the effector of the glucose-mediated dielectric response where the ATP concentration is also the mediating factor in swollen red blood cells. The results suggest that ATP binding to GLUT1 elicits a membrane capacitance change that increases with the applied concentration gradient of D-glucose. A simplified model of the membrane capacitance alteration with glucose uptake is proposed.

Antiproliferative effect of dihydroxyacetone on Trypanosoma brucei bloodstream forms: cell cycle progression, subcellular alterations, and cell death

We evaluated the effects of dihydroxyacetone (DHA) on Trypanosoma brucei bloodstream forms. DHA is considered an energy source for many different cell types. T. brucei takes up DHA readily due to the presence of aquaglyceroporins. However, the parasite is unable to use it as a carbon source because of the absence of DHA kinase (DHAK). We could not find a homolog of the relevant gene in the genomic database of T. brucei and have been unable to detect DHAK activity in cell lysates of the parasite, and the parasite died quickly if DHA was the sole energy source in the medium. In addition, during trypanosome cultivation, DHA induced growth inhibition with a 50% inhibitory concentration of about 1 mM, a concentration that is completely innocuous to mammals. DHA caused cell cycle arrest in the G(2)/M phase of up to 70% at a concentration of 2 mM. Also, DHA-treated parasites showed profound ultrastructural alterations, including an increase of vesicular structures within the cytosol and the presence of multivesicular bodies, myelin-like structures, and autophagy-like vacuoles, as well as a marked disorder of the characteristic mitochondrion structure. Based on the toxicity of DHA for trypanosomes compared with mammals, we consider DHA a starting point for a rational design of new trypanocidal drugs.

Crystal structure of the nucleotide-binding subunit DhaL of the Escherichia coli dihydroxyacetone kinase

Dihydroxyacetone (Dha) kinases are a family of sequence-related enzymes that utilize either ATP or phosphoenolpyruvate (PEP) as source of high energy phosphate. The PEP-dependent Dha kinase of Escherichia coli consists of three subunits. DhaK and DhaL are homologous to the Dha and nucleotide-binding domains of the ATP-dependent kinase of Citrobacter freundii. The DhaM subunit is a multiphosphorylprotein of the PEP:sugar phosphotransferase system (PTS). DhaL contains a tightly bound ADP as coenzyme that gets transiently phosphorylated in the double displacement of phosphate between DhaM and Dha. Here we report the 2.6A crystal structure of the E.coli DhaL subunit. DhaL folds into an eight-helix barrel of regular up-down topology with a hydrophobic core made up of eight interlocked aromatic residues and a molecule of ADP bound at the narrower end of the barrel. The alpha and beta phosphates of ADP are complexed by two Mg2+ and by a hydrogen bond to the imidazole ring of an invariant histidine. The Mg ions in turn are coordinated by three gamma-carboxyl groups of invariant aspartate residues. Water molecules complete the octahedral coordination sphere. The nucleotide is capped by an alpha-helical segment connecting helices 7 and 8 of the barrel. DhaL and the nucleotide-binding domain of the C.freundii kinase assume the same fold but display strongly different surface potentials. The latter observation and biochemical data indicate that the domains of the C.freundii Dha kinase constitute one cooperative unit and are not randomly interacting and independent like the subunits of the E.coli enzyme.

Dihydroxyacetone and methylglyoxal as permeants of the Plasmodium aquaglyceroporin inhibit parasite proliferation

The aquaglyceroporin of Plasmodium falciparum (PfAQP) is a bi-functional channel with permeability for water and solutes. Its functions supposedly are in osmotic protection of parasites and in facilitation of glycerol permeation for glycerolipid biosynthesis. Here, we show PfAQP permeability for the glycolysis-related metabolites methylglyoxal, a cytotoxic byproduct, and dihydroxyacetone, a ketotriose. AQP3, the red cell aquaglyceroporin, also passed dihydroxacetone but excluded methylglyoxal. Proliferation of malaria parasites was inhibited by methylglyoxal with an IC50 around 200 microM. Surprisingly, also dihydroxyacetone, which is an energy source in human cells, was antiproliferative in chloroquine-sensitive and resistant strains with an IC50 around 3 mM. We expressed P. falciparum glyceraldehyde 3-phosphate dehydrogenase (PfGAPDH) to examine whether it is inhibited by either carbonyl compound. Methylglyoxal did not affect PfGAPDH on incubation with 2.5 mM for 20 h. Treatment with 2.5 mM dihydroxyacetone, however, abolished PfGAPDH activity within 6 h. Aquaglyceroporin permeability for glycolytic metabolites may thus be of physiological significance.

Identification of human and rat FAD-AMP lyase (cyclic FMN forming) as ATP-dependent dihydroxyacetone kinases

Rat liver FAD-AMP lyase or FMN cyclase is the only known enzymatic source of the unusual flavin nucleotide riboflavin 4',5'-cyclic phosphate. To determine its molecular identity, a peptide-mass fingerprint of the purified rat enzyme was obtained. It pointed to highly related, mammalian hypothetical proteins putatively classified as dihydroxyacetone (Dha) kinases due to weaker homologies to biochemically proven Dha kinases of plants, yeasts, and bacteria. The human protein LOC26007 cDNA was used to design PCR primers. The product amplified from human brain cDNA was cloned, sequenced (GenBank Accession No. ), and found to differ from protein LOC26007 cDNA by three SNPs. Its heterologous expression yielded a protein active both as FMN cyclase and ATP-dependent Dha kinase, each activity being inhibited by the substrate(s) of the other. Cyclase and kinase activities copurified from rat liver extracts. Evidence supports that a single protein sustains both activities, probably in a single active center. Putative Dha kinases from other mammals are likely to be FMN cyclases too. Future work will profit from the availability of the structure of Citrobacter freundii Dha kinase, which contains substrate-interacting residues conserved in human Dha kinase/FMN cyclase.

Crystal structure of the Citrobacter freundii dihydroxyacetone kinase reveals an eight-stranded alpha-helical barrel ATP-binding domain

Dihydroxyacetone kinases are a sequence-conserved family of enzymes, which utilize two different phosphoryldonors, ATP in animals, plants and some bacteria, and a multiphosphoprotein of the phosphoenolpyruvate carbohydrate phosphotransferase system in bacteria. Here we report the 2.5-A crystal structure of the homodimeric Citrobacter freundii dihydroxyacetone kinase complex with an ATP analogue and dihydroxyacetone. The N-terminal domain consists of two alpha/beta-folds with a molecule of dihydroxyacetone covalently bound in hemiaminal linkage to the N epsilon 2 of His-220. The C-terminal domain consists of a regular eight-helix alpha-barrel. The eight helices form a deep pocket, which includes a tightly bound phospholipid. Only the lipid headgroup protrudes from the surface. The nucleotide is bound on the top of the barrel across from the entrance to the lipid pocket. The phosphate groups are coordinated by two Mg2+ ions to gamma-carboxyl groups of aspartyl residues. The ATP binding site does not contain positively charged or aromatic groups. Paralogues of dihydroxyacetone kinase also occur in association with transcription regulators and proteins of unknown function pointing to biological roles beyond triose metabolism.

A mechanism of covalent substrate binding in the x-ray structure of subunit K of the Escherichia coli dihydroxyacetone kinase

Dihydroxyacetone (Dha) kinases are homologous proteins that use different phosphoryl donors, a multiphosphoryl protein of the phosphoenolpyruvate-dependent carbohydrate:phosphotransferase system in bacteria, ATP in animals, plants, and some bacteria. The Dha kinase of Escherichia coli consists of three subunits, DhaK and DhaL, which are colinear to the ATP-dependent Dha kinases of eukaryotes, and the multiphosphoryl protein DhaM. Here we show the crystal structure of the DhaK subunit in complex with Dha at 1.75 A resolution. DhaK is a homodimer with a fold consisting of two six-stranded mixed beta-sheets surrounded by nine alpha-helices and a beta-ribbon covering the exposed edge strand of one sheet. The core of the N-terminal domain has an alpha/beta fold common to subunits of carbohydrate transporters and transcription regulators of the phosphoenolpyruvate-dependent carbohydrate:phosphotransferase system. The core of the C-terminal domain has a fold similar to the C-terminal domain of the cell-division protein FtsZ. A molecule of Dha is covalently bound in hemiaminal linkage to the N epsilon 2 of His-230. The hemiaminal does not participate in covalent catalysis but is the chemical basis for discrimination between short-chain carbonyl compounds and polyols. Paralogs of Dha kinases occur in association with transcription regulators of the TetR/QacR and the SorC families, pointing to their biological role as sensors in signaling.