Transport of pyridoxine and pyridoxal 5'-phosphate in isolated rat liver mitochondria

Maintenance of cellular vitamin B6 levels and mitochondrial oxidative function depend on pyridoxal 5'-phosphate homeostasis protein

Recently, biallelic variants in PLPBP coding for pyridoxal 5'-phosphate homeostasis protein (PLPHP) were identified as a novel cause of early-onset vitamin B6-dependent epilepsy. The molecular function and precise role of PLPHP in vitamin B6 metabolism are not well understood. To address these questions, we used PLPHP-deficient patient skin fibroblasts and HEK293 cells and YBL036C (PLPHP ortholog)-deficient yeast. We showed that independent of extracellular B6 vitamer type (pyridoxine, pyridoxamine, or pyridoxal), intracellular pyridoxal 5'-phosphate (PLP) was lower in PLPHP-deficient fibroblasts and HEK293 cells than controls. Culturing cells with pyridoxine or pyridoxamine led to the concentration-dependent accumulation of pyridoxine 5'-phosphate and pyridoxamine 5'-phosphate (PMP), respectively, suggesting insufficient pyridox(am)ine 5'-phosphate oxidase activity. Experiments utilizing 13C4-pyridoxine confirmed lower pyridox(am)ine 5'-phosphate oxidase activity and revealed increased fractional turnovers of PLP and pyridoxal, indicating increased PLP hydrolysis to pyridoxal in PLPHP-deficient cells. This effect could be partly counteracted by inactivation of pyridoxal phosphatase. PLPHP deficiency had a distinct effect on mitochondrial PLP and PMP, suggesting impaired activity of mitochondrial transaminases. Moreover, in YBL036C-deficient yeast, PLP was depleted and PMP accumulated only with carbon sources requiring mitochondrial metabolism. Lactate and pyruvate accumulation along with the decrease of tricarboxylic acid cycle intermediates downstream of α-ketoglutarate suggested impaired mitochondrial oxidative metabolism in PLPHP-deficient HEK293 cells. We hypothesize that impaired activity of mitochondrial transaminases may contribute to this depletion. Taken together, our study provides new insights into the pathomechanisms of PLPBP deficiency and reinforces the link between PLPHP function, vitamin B6 metabolism, and mitochondrial oxidative metabolism.

A Comparative Study on the Effects of the Lysine Reagent Pyridoxal 5-Phosphate and Some Thiol Reagents in Opening the Tl+-Induced Mitochondrial Permeability Transition Pore

Lysine residues are essential in regulating enzymatic activity and the spatial structure maintenance of mitochondrial proteins and functional complexes. The most important parts of the mitochondrial permeability transition pore are F1F0 ATPase, the adenine nucleotide translocase (ANT), and the inorganic phosphate cotransporter. The ANT conformation play a significant role in the Tl⁺-induced MPTP opening in the inner membrane of calcium-loaded rat liver mitochondria. The present study tests the effects of a lysine reagent, pyridoxal 5-phosphate (PLP), and thiol reagents (phenylarsine oxide, tert-butylhydroperoxide, eosin-5-maleimide, and mersalyl) to induce the MPTP opening that was accompanied by increased swelling, membrane potential decline, and decreased respiration in 3 and 3U_DNP (2,4-dinitrophenol uncoupled) states. This pore opening was more noticeable in increasing the concentration of PLP and thiol reagents. However, more significant concentrations of PLP were required to induce the above effects comparable to those of these thiol reagents. This study suggests that the Tl⁺-induced MPTP opening can be associated not only with the state of functionally active cysteines of the pore parts, but may be due to a change in the state of the corresponding lysines forming the pore structure.

Loss of a pyridoxal-phosphate phosphatase rescues Arabidopsis lacking an endoplasmic reticulum ATP carrier

The endoplasmic reticulum (ER)-located ATP/ADP-antiporter (ER-ANT1) occurs specifically in vascular plants. Structurally different transporters mediate energy provision to the ER, but the cellular function of ER-ANT1 is still unknown. Arabidopsis (Arabidopsis thaliana) mutants lacking ER-ANT1 (er-ant1 plants) exhibit a photorespiratory phenotype accompanied by high glycine levels and stunted growth, pointing to an inhibition of glycine decarboxylase (GDC). To reveal whether it is possible to suppress this marked phenotype, we exploited the power of a forward genetic screen. Absence of a so far uncharacterized member of the HaloAcid Dehalogenase (HAD)-like hydrolase family strongly suppressed the dwarf phenotype of er-ant1 plants. Localization studies suggested that the corresponding protein locates to chloroplasts, and activity assays showed that the enzyme dephosphorylates, with high substrate affinity, the B6 vitamer pyridoxal 5'-phosphate (PLP). Additional physiological experiments identified imbalances in vitamin B6 homeostasis in er-ant1 mutants. Our data suggest that impaired chloroplast metabolism, but not decreased GDC activity, causes the er-ant1 mutant dwarf phenotype. We present a hypothesis, setting transport of PLP by ER-ANT1 and chloroplastic PLP dephosphorylation in the cellular context. With the identification of this HAD-type PLP phosphatase, we also provide insight into B6 vitamer homeostasis.

Intracellular trafficking of the pyridoxal cofactor. Implications for health and metabolic disease

The importance of the vitamin B6-derived pyridoxal cofactor for human health has been established through more than 70 years of intensive biochemical research, revealing its fundamental roles in metabolism. B6 deficiency, resulting from nutritional limitation or impaired uptake from dietary sources, is associated with epilepsy, neuromuscular disease and neurodegeneration. Hereditary disorders of B6 processing are also known, and genetic defects in pathways involved in transport of B6 into the cell and its transformation to the pyridoxal-5'-phosphate enzyme cofactor can contribute to cardiovascular disease by interfering with homocysteine metabolism and the biosynthesis of vasomodulatory polyamines. Compared to the processes involved in cellular uptake and processing of the B6 vitamers, trafficking of the PLP cofactor across intracellular membranes is very poorly understood, even though the availability of PLP within subcellular compartments (particularly the mitochondrion) may have important health implications. The aim of this review is to concisely summarize the state of current knowledge of intracellular trafficking of PLP and to identify key directions for future research.

Towards closing the remaining gaps in photorespiration--the essential but unexplored role of transport proteins

Photorespiration is an essential prerequisite for all autotrophic organisms performing oxygenic photosynthesis. In contrast to the well-characterised enzymes accomplishing photorespiratory metabolism, current knowledge on the involved transport processes and the respective proteins is still quite limited. In this review, we focus on the status quo of translocators involved in photorespiratory metabolism. Although the transport of some of the photorespiratory intermediates could be characterised biochemically, using isolated organelles, the genes encoding these transporters have to date not been identified in most cases. Here, we describe the postulated transport processes, present information on established or hypothetical photorespiratory transporters, depict strategies on how to identify the transport proteins on the molecular level and, finally, discuss strategies for how to find the remaining candidates.

The riboflavin/FAD cycle in rat liver mitochondria

Here we provide evidence that mitochondria isolated from rat liver can synthesize FAD from riboflavin that has been taken up and from endogenous ATP. Riboflavin uptake takes place via a carrier-mediated process, as shown by the inverse relationship between fold accumulation and riboflavin concentration, the saturation kinetics [riboflavin Km and Vmax values were 4.4+/-1.3 microM and 35+/-5 pmol x min(-1) (mg protein)(-1), respectively] and the inhibition shown by the thiol reagent mersalyl, which cannot enter the mitochondria. FAD synthesis is due to the existence of FAD synthetase (EC 2.7.7.2), localized in the matrix, which has as a substrate pair mitochondrial ATP and FMN synthesized from taken up riboflavin via the putative mitochondrial riboflavin kinase. In the light of certain features, including the protein thermal stability and molecular mass, mitochondrial FAD synthetase differs from the cytosolic isoenzyme. Apparent Km and apparent Vmax values for FMN were 5.4+/-0.9 microM and 22.9+/-1.4 pmol x min(-1) x (mg matrix protein)(-1), respectively. Newly synthesized FAD inside the mitochondria can be exported from the mitochondria in a manner sensitive to atractyloside but insensitive to mersalyl. The occurrence of the riboflavin/FAD cycle is proposed to account for riboflavin uptake in mitochondria biogenesis and riboflavin recovery in mitochondrial flavoprotein degradation; both are prerequisites for the synthesis of mitochondrial flavin cofactors.

Flavin adenine dinucleotide and flavin mononucleotide metabolism in rat liver--the occurrence of FAD pyrophosphatase and FMN phosphohydrolase in isolated mitochondria

In order to gain some insight into mitochondrial flavin biochemistry, rat liver mitochondria essentially free of lysosomal and microsomal contamination were prepared and their capability to metabolise externally added and endogenous FAD and FMN tested both spectroscopically and via HPLC. The existence of two novel mitochondrial enzymes, namely FAD pyrophosphatase (EC 3.6.1.18) and FMN phosphohydrolase (EC 3.1.3.2), which catalyse FAD-->FMN and FMN-->riboflavin conversion, respectively, is shown. They differ from each other and from extramitochondrial enzymes, as judged by their pH profile and inhibitor sensitivity, and can be separated in a partial FAD pyrophosphatase purification. Digitonin titration and subfractionation experiments show that FAD pyrophosphatase is located in the outer mitochondrial membrane and FMN phosphohydrolase in the intermembrane space. Since these enzymes can metabolise endogenous FAD and FMN, which are made available by using both Triton X-100 and the effector oxaloacetate, a proposal is made that FAD pyrophosphatase and FMN phosphohydrolase play a major role in mitochondrial flavoprotein turnover.

3'-Azido-3'-deoxythmidine uptake into isolated rat liver mitochondria and impairment of ADP/ATP translocator

To gain some insight into the mechanism by which 3'-azido-3'-deoxythymidine (AZT) impairs mitochondrial metabolism, [14C]AZT uptake by rat liver mitochondria (RLM) in vitro was investigated. AZT accumulated in mitochondria in a time-dependent manner and entered the mitochondrial matrix. The rate of AZT uptake into mitochondria showed a hyperbolic dependence on the drug concentration and was inhibited by mersalyl, a thiol reagent that cannot enter mitochondria, thus showing that a membrane protein is involved in AZT transport. Investigation into the capability of AZT to affect certain mitochondrial carriers demonstrated that AZT was able to impair the ADP/ATP translocator, but had no effect on Pi, dicarboxylate, tricarboxylate, or oxodicarboxylate carriers. AZT inhibited ADP/ATP antiport in either mitochondria or mitoplasts in a competitive manner with different sensitivity (Ki values were 18.3 +/- 2.9 and 70.2 +/- 5.8 microM, respectively). Consistent with this were isotopic measurements showing that AZT accumulates in the intermembrane space. AZT does not use ADP/ATP carrier to enter mitochondria, as shown by the failure of both carboxyatractyloside (CAT) to inhibit AZT transport into mitochondria and AZT to induce ATP efflux from ATP-loaded mitochondria. ADP/ATP translocator impairment by AZT as one of the biochemical processes responsible for the ATP deficiency syndrome is discussed.

Thiamine pyrophosphate uptake into isolated rat liver mitochondria

The fact that thiamine pyrophosphate is synthesized in cytosol necessitates its uptake into mitochondria. The ability of mitochondria to take up externally added thiamine pyrophosphate was investigated by measuring the intramitochondrial thiamine pyrophosphate content using an enzymatic method. Thiamine pyrophosphate uptake by isolated rat liver mitochondria was found to occur in a time- and temperature-dependent manner. Uptake shows saturation characteristics with Km and Vmax values equal to about 20 microM and 700 pmol/min x mg protein, respectively, and is inhibited by certain nonpenetrating compounds. The inhibition of thiamine uptake by thiamine pyrophosphate and the efflux of endogenous thiamine pyrophosphate, caused by externally added thiamine, suggest the existence of a thiamine pyrophosphate/thiamine antiporter which could play an active role in the turnover of intramitochondrial thiamine pyrophosphate linked enzymes.

Uptake of thiamin by isolated rat liver mitochondria

Investigation is made here of 14C-thiamin uptake by rat liver mitochondria in vitro. Following incubation with mitochondria, 14C-thiamin remained in the mitochondrial pellet in spite of several washings of the organelles. Accordingly, externally added thiamin produced intra/extra-mitochondrial concentration ratios up to 5.4 and 14C-thiamin space/3H2O space ratios higher than one. These ratios decreased with increasing vitamin concentrations, thus suggesting the occurrence of saturation characteristics for vitamin uptake into mitochondria. Thiamin was proven to enter both intermembrane and matrix spaces, where neither binding to intramitochondrial protein nor phosphorylation were found to occur. Moreover thiamin uptake inhibition by both metal ions and certain thiamin analogues was also found.

Biosynthesis of 4-aminobutyrate aminotransferase

Mitochondrial 4-aminobutyrate aminotransferase was synthesized in a cell-free reticulocyte lysate using polysomal RNA isolated from pig brain. Its primary translation product has a higher molecular mass than the mature enzyme. The difference in relative molecular mass is approximately 2000 as revealed by SDS/polyacrylamide gel electrophoresis. The precursor of 4-aminobutyrate aminotransferase recognizes polyvalent antibodies raised against the mature enzyme. The precursor of 4-aminobutyrate aminotransferase binds pyridoxal-5-P and displays catalytic activity. Enzymatic activity was detected using a sensitive fluorimetric method, which is based on the formation of condensation products between succinic semialdehyde and cyclohexane-1,3-dione. It is concluded that removal of an extra peptide from the precursor is not an obligatory first step in the production of biological active species.

4-Aminobutyrate aminotransferase. Conformational changes induced by reduction of pyridoxal 5-phosphate

Conformational changes induced in 4-aminobutyrate aminotransferase (4-aminobutyrate:2-oxoglutarate aminotransferase, EC 2.6.1.19) by conversion of pyridoxal-5-P to pyridoxyl-5-P were examined by two independent methods. The reactivity of the SH groups of the reduced enzyme is increased by chemical modification of the cofactor. 1.8 SH per dimer of modified enzyme react with DTNB, whereas 1.2 SH per dimer of the native enzyme react with the attacking reagent under identical experimental conditions. The modified and native forms of the enzyme bind the fluorescent probe ANS, but the number of binding sites for ANS is increased as result of conversion of P-pyridoxal to P-pyridoxyl. After the conformational changes onset by reduction of the cofactor, the modified enzyme binds one molecule of pyridoxal-5-P with a Kd of 0.1 microM to become catalytically competent. The catalytic site of the reduce enzyme was probed with P-pyridoxal analogs. Like resolved 4-aminobutyrate aminotransferase, the reduced species recognize the phosphorothioate analog and regain 40% of the total enzymatic activity. Since the catalytic parameters of reduced and native 4-aminobutyrate aminotransferase are indistinguishable, it is concluded that the additional catalytic site of the reduced enzyme is functionally identical to that of the native enzyme.

Compartmentation of labeled fixation products in intact mesophyll protoplasts from Avena sativa L. after in-situ inhibition of the chloroplast phosphate translocator

Leaf mesophyll protoplasts of oat (Avena sativa L.) were allowed to fix (14)C-labeled bicarbonate in the absence or presence of pyridoxal phosphate (PLP), a specific inhibitor of the phosphate translocator of the inner envelope membrane of chloroplasts. The incubation was terminated by a method of rapid integrated protoplast homogenization and fractionation, and compartmented levels of label contained in sugars, phosphate esters, amino acids and organic acids were determined. The results show that the addition of PLP to a suspension of intact protoplasts causes an accumulation of phosphate esters in the chloroplasts stroma for up to 2.5 min of incubation, with a corresponding decrease in the cytosol. Prolonged treatment of protoplasts with PLP in the light resulted in a decrease of starch-associated label, combined with higher levels of labeled sugars in the cytosol, indicating a switch from phosphorolytic to hydrolytic starch degradation. Together with the determination of pool sizes of triose phosphates and of inorganic phosphate, the results demonstrate that the method employed is an important tool in investigating processes of intracellular regulation. They are discussed with respect to the permeability and possible side reactions of PLP, as well as in the light of reports on PLP action on isolated chloroplasts.