Bioenergetic consequences of accumulating the common 4977-bp mitochondrial DNA deletion.

Mutations and deletions in mitochondrial DNA (mtDNA) lead to a number of human diseases characterized by neuromuscular degeneration. Accumulation of truncated mtDNA molecules (delta-mtDNA) lacking a specific 4977-bp fragment, the common deletion, leads to three related mtDNA diseases: Pearson's syndrome; Kearns-Sayre syndrome; and chronic progressive external ophthalmoplegia (CPEO). In addition, the proportion of delta-mtDNA present increases with age in a range of tissues. Consequently, there is considerable interest in the effects of the accumulation of delta-mtDNA on cell function. The 4977-bp deletion affects genes encoding 7 polypeptide components of the mitochondrial respiratory chain, and 5 of the 22 tRNAs necessary for mitochondrial protein synthesis. To determine how the accumulation of delta-mtDNA affects oxidative phosphorylation we constructed a series of cybrids by fusing a human osteosarcoma cell line depleted of mtDNA (rho0) with enucleated skin fibroblasts from a CPEO patient. The ensuing cybrids contained 0-86% delta-mtDNA and all had volumes, protein contents, plasma-membrane potentials and mitochondrial contents similar to those of the parental cell line. The bioenergetic consequences of accumulating delta-mtDNA were assessed by measuring the mitochondrial membrane potential, rate of ATP synthesis and ATP/ADP ratio. In cybrids containing less than 50-55% delta-mtDNA, these bioenergetic functions were equivalent to those of cybrids with intact mtDNA. However, once the proportion of delta-mtDNA exceeded this threshold, the mitochondrial membrane potential, rate of ATP synthesis, and cellular ATP/ADP ratio decreased. These bioenergetic deficits will contribute to the cellular pathology associated with the accumulation of delta-mtDNA in the target tissues of patients with mtDNA diseases.

The effect on mitochondrial function of the tRNA Ser(UCN)/COI A7445G mtDNA point mutation associated with maternally-inherited sensorineural deafness.

An A to G point mutation at nucleotide position 7445 in the mitochondrial DNA genome is associated with maternally-inherited sensorineural deafness in two separate human pedigrees. To determine whether this point mutation [tRNA(SER)UCN)/COI A7445G] led to sensorineural deafness by affecting cellular energy metabolism we investigated the bioenergetic function of mitochondria in lymphoblastoid cultures established from these patients. Even though essentially all of the mitochondrial DNA in these cells contained the mutation at nucleotide position 7445, there was no effect on a number of mitochondrial bioenergetic functions (mitochondrial content, membrane potential in both intact and digitonin-permeabilised cells, cellular ATP/ADP ratios and respiratory enzyme activity) when compared with control cells. The implications of these findings for both the aetiology of the sensorineural deafness associated with the A7445G mtDNA mutation, and the role of bioenergetic defects in mitochondrial DNA diseases in general are discussed.

Assignment of the PSST subunit gene of human mitochondrial complex I to chromosome 19p13.

The cDNA for the PSST subunit of human mitochondrial nicotinamide adenine dinucleotide (NADH): ubiquinone oxidoreductase [complex I; NADH dehydrogenase (ubiquinone), Fe-S (20 kDa); EC 1.6.5.3] was generated by polymerase chain reaction (PCR) amplification of human cDNA. The sequence of the mature protein deduced from the cDNA codes for a protein that is closely related to the bovine protein (93% homology). Nine conservative substitutions are found in the mature protein, mainly in the N and C terminal regions. The mature human protein is missing four amino acids (PAAL) close to the N terminus that are present in the bovine protein. The N terminus of the mature protein is preceded by a presequence of 38 amino acids that, although quite different from its bovine counterpart (52% homology), has properties that are characteristic of a mitochondrial import sequence. Southern hybridization analysis predicts an estimated gene size of 3.8 kb. Northern hybridization analysis of mRNA from fibroblasts of complex I-deficient patients revealed no size or transcript level abnormalities. The cDNA of the PSST protein was used to investigate tissue-specific expression and to localize the gene for this subunit to chromosome 19p13.

1H nuclear magnetic resonance assay of erythrocyte triosephosphate isomerase.

A direct method for measuring the activity of erythrocyte triosephosphate isomerase using 1H NMR spectroscopy was developed. NMR spectroscopy allows the simultaneous monitoring of the substrate and the product of the reaction by virtue of the differences in the NMR spectrum of each chemical species. The assay conditions were based on a modification of a conventional spectrophotometric method. The enzymatic activity measured using NMR gave results comparable to those obtained in a standard assay. The results were used in the kinetic characterization of triosephosphate isomerase in hemolysates from subjects with homozygous or heterozygous deficiency of the enzyme. In general, NMR spectroscopy has the potential for wide application in the rapid development of new enzyme assays.

Stimulation of human erythrocyte 2,3-bisphosphoglycerate phosphatase by vanadate.

The rates of vanadate-stimulated hydrolysis of 2,3-bisphosphoglycerate in metabolically competent erythrocytes and in hemolysates were determined from data on time courses up to 35 min employing 31P nuclear magnetic resonance spectroscopy. The enhanced rate of hydrolysis of the bisphosphate was attributed principally to the activation of the phosphatase activity of 2,3-bisphosphoglycerate synthase both in cell suspensions and in hemolysates. Information on the concentrations of vanadate and vanadyl present in the preparations was obtained employing 51V nuclear magnetic resonance spectroscopy and electron paramagnetic resonance spectroscopy. Redox reactions involving vanadium ions appeared to be important in establishing the final equilibrium concentrations of the oxy- and oxo-ions (vanadate and vanadyl, respectively), but the data suggested that the activation of the enzyme resulted from direct action of the vanadium ions on the enzyme and not as a consequence of the alteration in the equilibrium of intracellular oxidants and reductants.

gamma-Glutamylcyclotransferase: inhibition by D-beta-aminoglutaryl-L-alanine and analysis of the solvent kinetic isotope effect.

Spin-echo NMR spectroscopy was used to record the cleavage of a gamma-glutamyl--amino-acid by (5-L-glutamyl)-L-amino-acid 5-glutamyltransferase (cyclizing) (gamma-glutamylcyclotransferase) in human erythrocyte hemolysates. The Michaelis-Menten steady-state kinetic parameters were obtained by fitting the integrated Michaelis-Menten equation to the reaction time curves. The product, L-5-oxoproline, was shown to be an inhibitor of the reaction. The active site of the enzyme was probed by studies of the inhibition by D- and L-beta-aminoglutaryl-L-alanine which are the beta-amino-acid isomers of D- and L-gamma-glutamyl-L-alanine (the latter being a natural substrate of the enzyme); the D-isomer was the more potent inhibitor (Ki = 0.30 +/- 0.02 mmol/l water). When the alanyl alpha-carboxyl of the inhibitor was reduced to a hydroxyl (i.e. to give D-beta-aminoglutaryl-L-alaninol) the potency of inhibition was reduced. The previously reported kinetic isotope effect of solvent 2H2O on the enzyme-catalyzed reaction has been further studied using a proton inventory. We propose that the solvent kinetic isotope effect is due to an intramolecular proton transfer between the glutamyl amino group and the peptide bond nitrogen.

No evidence for bradykinin hydrolysis in human erythrocyte suspensions: 1H NMR studies.

In view of their permeability to small peptides, it has been postulated that human erythrocytes may play a role in terminating the action of some circulating peptide hormones. Work using classical paper chromatographic techniques for detecting free amino acids indicated that the octapeptide, des-(Arg9)-bradykinin, enters these cells and its amino-terminal arginine residue is released by cytosolic aminopeptidase-P. We have used 1H NMR to monitor directly the release of arginine from bradykinin. The hydrolytic reaction rate in hemolysates, with an initial peptide concentration of 13.0 mmol l-1 was 6.5 mmol (1 packed red cell)-1 h-1. But no reaction was evident after a 4.5-h incubation with intact cells, thus contradicting the earlier suggestion that erythrocytes are involved in the primary inactivation of this hormone. This is consistent with our previous findings that the pentapeptide leu-enkephalin fails to enter human erythrocytes but that its lower-order degradation products may do so.