An adenine nucleotide-linked succinic thiokinase of animal origin

Succinyl-CoA Synthetase Dysfunction as a Mechanism of Mitochondrial Encephalomyopathy: More than Just an Oxidative Energy Deficit

Biallelic pathogenic variants in subunits of succinyl-CoA synthetase (SCS), a tricarboxylic acid (TCA) cycle enzyme, are associated with mitochondrial encephalomyopathy in humans. SCS catalyzes the interconversion of succinyl-CoA to succinate, coupled to substrate-level phosphorylation of either ADP or GDP, within the TCA cycle. SCS-deficient encephalomyopathy typically presents in infancy and early childhood, with many patients succumbing to the disease during childhood. Common symptoms include abnormal brain MRI, basal ganglia lesions and cerebral atrophy, severe hypotonia, dystonia, progressive psychomotor regression, and growth deficits. Although subunits of SCS were first identified as causal genes for progressive metabolic encephalomyopathy in the early 2000s, recent investigations are now beginning to unravel the pathomechanisms underlying this metabolic disorder. This article reviews the current understanding of SCS function within and outside the TCA cycle as it relates to the complex and multifactorial mechanisms underlying SCS-related mitochondrial encephalomyopathy.

Reconsideration of the significance of substrate-level phosphorylation in the citric acid cycle*

For nearly 50 years, students of metabolism in animals have been taught that a substrate-level phosphorylation in the Krebs citric acid cycle produces GTP that subsequently undergoes a transphosphorylation with ADP catalyzed by nucleoside diphosphate kinase. Research in the past decade has revealed that animals also express an ADP-forming succinate-CoA ligase whose activity exceeds that of the GDP-forming enzyme in some tissues. Here I argue that the primary fate of GTP is unlikely to be transphosphorylation with ADP. Rather, two succinate-CoA ligases with different nucleotide specificities have evolved to better integrate and regulate the central metabolic pathways that involve the citric acid cycle. The products of substrate-level phosphorylation, ATP and/or GTP, may represent a pool of nucleotide that has a different phosphorylation potential than the ATP made by oxidative phosphorylation and may be channeled to meet specific needs within mitochondria and the cell. Further research is needed to determine the applicable mechanisms and how they vary in tissues.

Genetic evidence for the expression of ATP- and GTP-specific succinyl-CoA synthetases in multicellular eucaryotes

Highly ATP- and GTP-specific isoforms of succinyl-CoA synthetase in pigeon incorporate the same alpha-subunit, but different beta-subunits (Johnson, J. D., Muhonen, W. W., and Lambeth, D. O. (1998) J. Biol. Chem. 273, 27573-27579). The sequences of the mature subunits were determined by methods based on reverse transcription-polymerase chain reaction. The 306-residue mature alpha-subunit in pigeon shows >88% identity to its homologues in pig and rat. The sequences of the mature ATP- and GTP-specific beta-subunits (A-beta and G-beta, respectively) in pigeon are 54% identical. These sequences were used to identify expressed sequence tags for human and mouse that were highly homologous to G-beta and A-beta, respectively. The sequences for mature A-beta and G-beta in mouse and human were completed and verified by polymerase chain reaction. The sequence of A-beta in pig was also obtained. The mammalian A-beta sequences show >89% identity to each other; the G-beta sequences are similarly related. However, pairwise comparisons of the A-beta and G-beta sequences revealed <53% identity. Alignment with two sequences of the beta-subunit in Caenorhabditis elegans suggests that the A-beta and G-beta genes arose by duplication early in the evolution of multicellular eucaryotes. The expression of A-beta is strong in numerous mouse and human tissues, which suggests that ATP-specific succinyl-CoA synthetase also plays an important role in species throughout the animal kingdom.

Characterization of the ATP- and GTP-specific succinyl-CoA synthetases in pigeon. The enzymes incorporate the same alpha-subunit

Two succinyl-CoA synthetases, one highly specific for GTP/GDP and the other for ATP/ADP, have been purified to homogeneity from pigeon liver and breast muscle. The two enzymes are differentially distributed in pigeon, with only the GTP-specific enzyme detected in liver and the ATP-specific enzyme in breast muscle. Based on assays in the direction of CoA formation, the ratios of GTP-specific to ATP-specific activities in kidney, brain, and heart are approximately 7, 1, and 0.1, respectively. Both enzymes have the characteristic alpha- and beta-subunits found in other succinyl-CoA synthetases. Studies of the alpha-subunit by electrophoresis, mass spectrometry, reversed-phase high performance liquid chromatography, and peptide mapping showed that it was the same in the two enzymes. Characterization of the beta-subunits by the same methods indicated that they were different, with the tryptic peptide maps providing evidence that the beta-subunits likely differ along their entire sequences. Because the two succinyl-CoA synthetases incorporate the same alpha-subunit, the determinants of nucleotide specificity must reside within the beta-subunit. Determination of the apparent Michaelis constants showed that the affinity of the GTP-specific enzyme for GDP is greater than that of the ATP-specific enzyme for ADP (7 versus 250 microM). Rather large differences in apparent Km values were also observed for succinate and phosphate.

Characterization and cloning of a nucleoside-diphosphate kinase targeted to matrix of mitochondria in pigeon

Nucleoside-diphosphate kinase (NDP kinase) from the matrix space of mitochondria in pigeon liver was purified to homogeneity. Degenerate oligonucleotide primers to the N-terminal sequence of the purified protein and the region containing the active site histidine were used in reverse transcriptase-polymerase chain reaction to obtain a major portion of the coding sequence for the mature protein. The sequences of the C and N termini of the mature protein, and eight residues in the signal peptide, were obtained by rapid amplification of cDNA end procedures. The entire coding sequence of a cytosolic form of NDP kinase was also determined. Both isoforms, which share 53% sequence identity, possess the characteristically conserved regions of known NDP kinases. The mature mitochondrial NDP kinase protein migrates in molecular sieving columns with an apparent molecular mass of about 66 kDa. It shows very high thermal stability even though it lacks the proline residue in the killer of prune loop, and the Tyr/Glu C termini that are important in stabilizing other NDP kinases. The affinity of the mitochondrial isoform for adenine and guanine nucleotides is much higher than for pyrimidine nucleotides, but the enzyme is especially susceptible to substrate inhibition by GDP. Semi-quantitative reverse transcriptase-polymerase chain reaction showed that the relative levels of expression of the mitochondrial isoform are liver > kidney >> heart = brain > breast muscle. The cytosolic isoform is strongly and approximately equally expressed in these same five tissues. This work is the first characterization of a NDP kinase isoform that is found in the matrix space of mitochondria.

Occurrence of two distinct succinate thiokinases in animal tissues

Although succinate thiokinase from mammalian sources has hitherto been described as showing substrate specificity for guanine nucleotide, a range of mammalian tissues has here been found to display succinate thiokinase activity with both guanine and adenine nucleotides as substrates. Evidence is presented for the existence of two distinct succinate thiokinases and this is confirmed by their separation by affinity chromatography. Each enzyme is specific for one nucleotide and is inhibited by the non-substrate nucleotide. The physiological roles of the two enzymes is yet to be established.

Succinate thiokinase in pigeon breast muscle mitochondria

Succinate thiokinase has been purified from pigeon breast muscle. It has been confirmed that the enzyme is entirely specific for ATP, and Km is very high (approximately 0.8 mM). Activity in mitochondrial sonicates is low enough for it to be doubtful whether the enzyme can support citric acid cycle flux in the tissue. The enzyme appears to have an Mr of 80 000-100 000, and to have two unequal subunits. As determined by SDS gel electrophoresis one subunit certainly has an Mr of 40 000.

Regulation of pyruvate oxidation in blowfly flight muscle mitochondria: requirement for ADP

Blowfly (Phormia regina) flight muscle mitochondria oxidized pyruvate ( + proline) in the presence of either ADP (coupled respiration) or carbonylcyanide-p-trifluoromethoxyphenylhydrazone (FCCP-uncoupled respiration). There was an absolute requirement for ADP (Km = 8.0 microM) when pyruvate oxidation was stimulated by FCCP in the presence of oligomycin. This requirement for ADP was limited to the oxidation of pyruvate; uncoupled alpha-glycerolphosphate oxidation proceeded maximally even in the absence of added ADP. Atractylate inhibited uncoupled pyruvate oxidation whether added before (greater than 99%) or after (95%) initiation of respiration with FCCP. In the presence of FCCP, oligomycin, and limiting concentrations of ADP (less than 110 microM), there was a shutoff in the uptake of oxygen. This inhibition of respiration was completely reversed by the addition of more ADP. Plots of net oxygen uptake as a function of the limiting ADP concentration were linear; the observed ADP/O ratio was 0.22 +/- 0.025. An ADP/O ratio of 0.2 was predicted if phosphorylation occurred only at the succinyl-CoA synthetase step of the tricarboxylate cycle. Experiments performed in the presence of limiting concentrations of ADP, and designed to monitor changes in the mitochondrial content of ADP and ATP, demonstrated that the shutoff in oxygen uptake was not due to the presence of a high intramitochondrial concentration of ATP. Indeed, ATP, added to the medium prior to the addition of FCCP, inhibited uncoupled pyruvate oxidation; the apparent KI was 0.8 mM. These results are consistent with the hypothesis that it is the intramitochondrial ATP/ADP ratio that is one of the controlling factors in determining the rate of flux through the tricarboxylate cycle. Changes in the mitochondrial content of citrate, isocitrate, alpha-ketoglutarate, and malate during uncoupled pyruvate oxidation in the presence of a limiting concentration of ADP were consistent with the hypothesis that the mitochondrial NAD + -linked isocitric dehydrogenase is a major site for such control through the tricarboxylate cycle.

Endogenous protein phosphorylation in rat brain mitochondria: occurrence of a novel ATP-dependent form of the autophosphorylated enzyme succinyl-CoA synthetase

When rat brain mitochondria are incubated with [gamma-32P]ATP, there is a rapid (10 s) phosphorylation of proteins designated E1 and F of M.W. 42,000 and 32,000, respectively. Although [gamma-32P]ATP was the preferred substrate for protein F, a small amount of labeling did occur with [gamma-32P]GTP. Phosphorylation of E1 was absolutely ATP-dependent. On the other hand, a 32,000 M.W. protein from rat liver mitoplasts (mitochondria devoid of an outer membrane) was highly phosphorylated when [gamma-32P]GTP was used but not at all phosphorylated within short time periods with [gamma-32P]ATP. Both the ATP-labeled brain phosphoprotein F and GTP-labeled liver protein migrated to identical positions on high-resolution two-dimensional polyacrylamide gels, and both contained acid-labile phosphoryl groups. Furthermore, both phosphoproteins were identified as the autophosphorylated subunit of succinyl-CoA synthetase (SCS, EC 6.2.1.4) by using antibody directed against purified GTP-dependent porcine SCS. However, immunotitration experiments with anti-porcine SCS revealed that ATP- and GTP-labeled protein F in brain differed in their interactions with antibody, suggesting that in rat brain mitochondria two different forms of the enzyme exist that are immunologically distinct and differ in substrate specificity. When mitochondrial preparations enriched in particular brain cell or subcellular types were examined, an unequal distribution of E1 and the two forms of protein F were observed. A brain subfraction containing neuronal cell body and glial mitochondria (CM) was found to contain E1 and approximately equal amounts of the ATP- and GTP-dependent forms of protein F. Light synaptic mitochondria (SM1) contained ATP-dependent protein F almost exclusively and were depleted in E1. Dense synaptic mitochondria (SM2) are rich in the ATP form of SCS but also contain low amounts of the GTP enzyme.

The control of tricarboxylate-cycle of oxidations in blowfly flight muscle. The steady-state concentrations of coenzyme A, acetyl-coenzyme A and succinyl-coenzyme A in flight muscle and isolated mitochondria

(1) A ;cycling' method involving citrate synthase (EC 4.1.3.7) and malate dehydrogenase (EC 1.1.1.37) was modified by the inclusion of succinyl-CoA synthetase (EC 6.2.1.5) and hexokinase (EC 2.7.1.1) to permit the determination of very small amounts of succinyl-CoA in addition to CoA and acetyl-CoA. (2) Application of this technique to blowfly (Phormia regina) flight-muscle extracts reveals no change in acetyl-CoA concentration, a slight fall in CoA concentration and a rise in succinyl-CoA concentration during flight. (3) Extraction of isolated mitochondria during controlled (state 4) pyruvate oxidation reveals essentially only acetyl-CoA. Activation of respiration by ADP (state 3) or uncoupling agents leads to a fall in acetyl-CoA and a rise in CoA and succinyl-CoA content. (4) The presence of glycerol phosphate in addition to pyruvate results in a lower acetyl-CoA content in state 4. (5) It is contended that these results are consistent with a primary control of one of the reactions of the tricarboxylate cycle, rather than of pyruvate dehydrogenase, during the state 4 oxidation of pyruvate by isolated mitochondria, and that the modulation of citrate synthase activity by the ratio of acetyl-CoA/succinyl-CoA is unimportant under these conditions.