The excretion of I-malic acid in relation to the tricarboxylic acid cycle in the kidney

Relief of CoA sequestration and restoration of mitochondrial function in a mouse model of propionic acidemia

Propionic acidemia (PA, OMIM 606054) is a devastating inborn error of metabolism arising from mutations that reduce the activity of the mitochondrial enzyme propionyl-CoA carboxylase (PCC). The defects in PCC reduce the concentrations of nonesterified coenzyme A (CoASH), thus compromising mitochondrial function and disrupting intermediary metabolism. Here, we use a hypomorphic PA mouse model to test the effectiveness of BBP-671 in correcting the metabolic imbalances in PA. BBP-671 is a high-affinity allosteric pantothenate kinase activator that counteracts feedback inhibition of the enzyme to increase the intracellular concentration of CoA. Liver CoASH and acetyl-CoA are depressed in PA mice and BBP-671 treatment normalizes the cellular concentrations of these two key cofactors. Hepatic propionyl-CoA is also reduced by BBP-671 leading to an improved intracellular C3:C2-CoA ratio. Elevated plasma C3:C2-carnitine ratio and methylcitrate, hallmark biomarkers of PA, are significantly reduced by BBP-671. The large elevations of malate and α-ketoglutarate in the urine of PA mice are biomarkers for compromised tricarboxylic acid cycle activity and BBP-671 therapy reduces the amounts of both metabolites. Furthermore, the low survival of PA mice is restored to normal by BBP-671. These data show that BBP-671 relieves CoA sequestration, improves mitochondrial function, reduces plasma PA biomarkers, and extends the lifespan of PA mice, providing the preclinical foundation for the therapeutic potential of BBP-671.

Pantothenate kinase activation relieves coenzyme A sequestration and improves mitochondrial function in mice with propionic acidemia

[Figure: see text].

Effects of hypoxia on urinary organic acid and hypoxanthine excretion in fetal sheep

Severe birth asphyxia leads to a transient organic aciduria and increased hypoxanthine excretion. To investigate its origin and timing, we analyzed urine from 12 late gestation fetal sheep in utero subjected to moderately severe isocapnic hypoxia for 1 h. In six fetuses the carotid sinus nerves were cut to determine whether reflex peripheral vasoconstriction contributed to the changes in excretion. After a control period of 1 h, maternal inspired oxygen was reduced for 1 h so that fetal arterial oxygen tension fell significantly from 2.86 +/- 0.12 kPa (mean +/- SEM) to 1.55 +/- 0.04 kPa. The ewes were returned to normoxia, and monitoring was continued for 1 h. Fetal heart rate, arterial blood pressure, and femoral arterial blood flow (intact fetuses only) were recorded, and arterial pH, blood gases, and lactate were measured. Urine collected via a bladder catheter was analyzed for organic acids and hypoxanthine with gas chromatography-mass spectrometry. In intact fetuses, hypoxia increased excretion of hypoxanthine and several organic acids, notably lactic acid and intermediates of valine catabolism. Changes were apparent by 15 min, significant by 45 min, and maximal after reoxygenation. In denervated fetuses, there were small, significant, increases in organic acids and hypoxanthine by 45 min of hypoxia, but there was no surge in excretion posthypoxia. Hypoxia caused a large, significant, fall in femoral arterial blood flow in intact fetuses. We conclude that the extent of the reflex peripheral vasoconstriction, particularly in skeletal muscle, determines the amount of organic acid and hypoxanthine excretion and may explain similar biochemical disturbances after birth asphyxia. Urinary lactic acid measurement has a potential value for grading birth asphyxia.

Sodium-dependent dicarboxylate transport in rat renal basolateral membrane vesicles

Dicarboxylate transport in basolateral membrane vesicles prepared from rat kidney cortex was studied using 3H-methylsuccinate as a substrate. A sodium gradient (out greater than in) simulated methylsuccinate uptake and led to a transient overshoot. Lithium inhibited methylsuccinate uptake in the presence of sodium. The dependence of methylsuccinate uptake on sodium concentration indicated the interaction of more than one sodium ion with the transporter. Half-maximal stimulation was observed at 24 mmol/l sodium. Sodium-driven methylsuccinate uptake was electrogenic carrying a net positive charge. The basolateral dicarboxylate transport system exhibited an optimum at pH 7.0-7.5. In contrast, the sodium-dependent dicarboxylate transport system of brush border membranes depended much less on pH and had no optimum in the tested range. Cis-inhibition studies showed a preference of the system for dicarboxylates in the trans-configuration (fumarate) over cis-dicarboxylates (maleate). Citrate was accepted but oxalate and L-glutamate were not. DIDS exhibited a small inhibition. Among the monocarboxylates, gluconate and pyruvate inhibited methylsuccinate uptake whereas probenecid and p-aminohippurate (1 mmol/l) were without effect. The data indicate the presence of a sodium-dependent transport system in the basolateral membrane which accepts tricarboxylic acid cycle intermediates. This system is most likely not identical to the transport system responsible for organic anion secretion.

Secretion and contraluminal uptake of dicarboxylic acids in the proximal convolution of rat kidney

The transport of dicarboxylic acids in the proximal convolution was investigated by measuring: a) the zero net flux transtubular concentration difference of DL-methyl-succinate, b) its 2-s influx from the interstitium into tubular cells, and c) its 3.5-s efflux from the tubular lumen. With the first method a luminal concentration exceeding the peritubular concentration was observed, thus indicating a net active transtubular secretion of this slowly metabolized substance. All transport steps, luminal and contraluminal , as well as the overall transport, were Na+-dependent and inhibited by lithium (apparent Ki approximately equal to 1.8 mmol/l). The overall transport of methylsuccinate , as well as the contraluminal influx into proximal tubular cells, could be inhibited by paraaminohippurate and H2-DIDS with an apparent Ki of approximately equal to 1.8 mmol/l, by taurocholate with an apparent Ki approximately equal to 3.1 mmol/l and by pyruvate with an apparent Ki approximately equal to 5 mmol/l, but not by sulfate, thiosulfate, L-lactate, oxalate and urate. As judged from the inhibition of contraluminal methylsuccinate influx by 48 dicarboxylic acids (aliphatic and aromatic), a specificity pattern was observed similar to that of inhibition of luminal efflux of 2-oxoglutarate [22]: a preference of dicarboxylates in the transconfiguration with a chain length of 4-5 carbons; little change in the inhibitory potency with CH3-, OH-, SH- and O=, but strong reduction with a NH3+ in the 2 position; only a small reduction of inhibitory potency with 2,3 disubstituted SH and OH analogs; preference of the dicarboxylic benzene in the 1,4 position and of the diacetyl benzene in the 1,2 position. The data indicate a Na+-dependent dicarboxylic transport system at the contraluminal cell side of the proximal tubule which is very similar to the luminal transport system for dicarboxylic acids.

Renal metabolism of citrate

Our studies on renal handling of citrate have shown that: (1) citrate enters renal tubular cells from luminal fluid (reabsorption) and peritubular blood; (2) reabsorption becomes maximal, i.e., Tm-limited, at filtered loads 7 to 8 times the normal; (3) administration of malate stimulates net renal production of citrate, leading to release into urine (net secretion) and into peritubular blood; (4) acute metabolic alkalosis, induced while plasma citrate levels are above normal, depressess net citrate reabsorption, stimulates citrate release into peritubular blood and abolishes overall renal uptake of citrate; (5) essentially all citrate extracted by the kidney is converted to CO2 at endogenous circulating levels. This contribution is 15 per cent of the total renal CO2 production and is independent of chronic alterations in acid-base balance.