Control of choline oxidation by rat-liver mitochondria

The steady-state concentrations of choline and its reaction products in intact rat-liver mitochondria were determined under different conditions. From these measurements, it is concluded that in a sucrose medium choline dehydrogenation and betaine aldehyde dehydrogenation are the rate-limiting steps in overall choline oxidation under “State-3” or uncoupled conditions, respectively.Ageing of the mitochondria leads to changes in the mitochondrial membrane, resulting in a markedly different pattern of oxidation products. This finding explains why rotenone inhibits oxygen uptake with choline as substrate in fresh but not in aged mitochondria.     

Uncoupler-stimulated oxidation of choline by rat-liver mitochondria

The main product of uncoupler-stimulated oxidation of choline by rat-liver mitochondria is betaine, which is found almost exclusively extramitochondrially. The uncoupled oxidation of choline is stimulated by intramitochondrial phosphate. The effect of intramitochondrial phosphate is to induce adenine nucleotide efflux, which in its turn allows the efflux of betaine from the mitochondria.     

The oxidation of glutamate by rat-liver mitochondria

1. In rat-liver mitochondria both the dehydrogenase and transaminase routes participate in glutamate oxidation. However, the rate of ammonia production by the dehydrogenase pathway progressively decreases with the time of incubation. 2. Glutamate deamination is stimulated by blocking the transaminase pathway with arsenite or malonate. On the other hand, this process is completely suppressed by succinate, malate, pyruvate and oxaloacetate. Succinate and pyruvate inhibit, whereas malate and oxaloacetate stimulate, aspartate formation. 3. Glutamate deamination increases with increasing concentrations of 2,4-dinitrophenol from 0·05 to 0·2mm, and then becomes inhibited, together with the rate of oxygen consumption. Aspartate formation is progressively inhibited with increasing 2,4-dinitrophenol concentration from 0·05 to 0·8mm. In the presence of 0·20mm-2,4-dinitrophenol the rate of ammonia production is higher than in the presence of phosphate acceptors and decreases much slower and linearly with the time of incubation. 4. The addition of NAD⁺ enhances glutamate deamination without affecting oxygen uptake.     

Control of choline oxidation in rat kidney mitochondria

Choline is a quaternary amino cationic organic alcohol that is oxidized to betaine in liver and kidney mitochondria. Betaine acts as an intracellular organic osmolyte in the medulla of the kidney. Evidence is provided that kidney mitochondria have a choline transporter in their inner membrane. The transporter has a Km of 173 ± 64 μM and a Vmax of 0.4 ± 0.1 nmol/min/mg mitochondrial protein (at 10 °C). Uptake of choline is not coupled to betaine efflux. Transporter activity demonstrates a dependence on membrane potential and choline transport is inhibited by hemicholinium-3. Steady-state oxygen consumption due to choline oxidation in kidney mitochondria was measurable at 37 °C (125 ± 6 pmolO₂/min/mg mitochondrial protein), in the absence of other mitochondrial electron transport chain substrates and the choline transporter was shown to be the major site of control (96 ± 4%) over choline oxidation flux in isolated kidney mitochondria. We conclude that the choline transporter in rat kidney mitochondria is the major site of control over the production of the organic osmolyte, betaine.     

Transport and oxidation of choline by liver mitochondria

1. Rapid choline oxidation and the onset of P(i)-induced swelling by liver mitochondria, incubated in a sucrose medium at or above pH7.0, required the addition of both P(i) and an uncoupling agent. Below pH7.0, P(i) alone was required for rapid choline oxidation and swelling. 2. Choline oxidation was inhibited by each of several reagents that also inhibited P(i)-induced swelling under similar conditions of incubation, including EGTA, mersalyl, Mg(2+), the Ca(2+)-ionophore A23187, rotenone and nupercaine. None of these reagents had any significant effect on the rate of choline oxidation by sonicated mitochondria. There was therefore a close correlation between the conditions required for rapid choline oxidation and for P(i)-induced swelling to occur, suggesting that in the absence of mitochondrial swelling the rate of choline oxidation is regulated by the rate of choline transport across the mitochondrial membrane. 3. Respiratory-chain inhibitors, uncoupling agents (at pH6.5) and ionophore A23187 caused a loss of endogenous Ca(2+) from mitochondria, whereas nupercaine and Mg(2+) had no significant effect on the Ca(2+) content. Inhibition of choline oxidation and mitochondrial swelling by ionophore A23187 was reversed by adding Ca(2+), but not by Mg(2+). It is concluded that added P(i) promotes the Ca(2+)-dependent activation of mitochondrial membrane phospholipase activity in respiring mitochondria, causing an increase in the permeability of the mitochondrial inner membrane to choline and therefore enabling rapid choline oxidation to occur. Nupercaine and Mg(2+) appear to block choline oxidation and swelling by inhibiting phospholipase activity. 4. Choline was oxidized slowly by tightly coupled mitochondria largely depleted of their endogenous adenine nucleotides, suggesting that these compounds are not directly concerned in the regulation of choline oxidation. 5. The results are discussed in relation to the possible mechanism of choline transport across the mitochondrial membrane in vivo and the influence of this process on the pathways of choline metabolism in the liver.     

Oxidative phosphorylation accompanying oxidation of short-chain fatty acids by rat-liver mitochondria

1. The factors concerned in the estimation of P/O ratios when fatty acids are oxidized by rat-liver mitochondria have been assessed. 2. The oxidation of butyrate, hexanoate and octanoate is accompanied by ATP synthesis. At low concentrations of the fatty acids, P/O ratios approximately 2.5 are obtained. 3. Oxidative phosphorylation is uncoupled, respiratory control ratios are lowered and respiration is inhibited when the concentration of the fatty acid in the incubating medium is raised (to 5-10mm); octanoate is a more potent uncoupler than either hexanoate or butyrate. 4. Serum albumin and carnitine, either singly or in combination, protect the mitochondria from the effect exerted by the fatty acids. 5. The rate of oxidation of short-chain fatty acids in the presence of ADP is increased in the presence of carnitine.     

Accumulation of carnitine esters of beta-oxidation intermediates during palmitate oxidation by rat-liver mitochondria

Rat-liver mitochondria were incubated with [14C]palmitate in the presence of L-malate, fluorocitrate, and L-carnitine. The specific activities of acetyl groups incorporated into citrate, ketone bodies and acetyl-L-carnitine were measured. During state-4 oxidation of [1--14C]palmitate the specific activity of the acetyl-CoA pool was 1.3-times higher than that of the average acetyl group of palmitate, indicating an incomplete breakdown of the palmitate molecule. Accumulation of carnitine esters was observed in this condition. The acyl moieties of carnitine esters formed during the state-4 oxidation of [U-14C]palmitate or [16(-14)C]palmitate were analysed by radioactive gas-chromatography. Substantial amounts of beta-oxidation intermediates were found. The accumulation of carnitine esters of C6-C14 intermediates can quantitatively explain the high specific activity of the acetyl-CoA pool during the state-4 oxidation of [1(-14)C] palmitate. The localization and control of beta-oxidation are discussed.     

Essentiality of ubiquinone for choline oxidation in rat liver mitochondria.

Rat liver mitochondria treated extensively with n-pentane are incapable of oxidizing choline. Choline oxidation is more sensitive than is succinate oxidation to serial n-pentane extraction of mitochondria. The ability to oxidize choline is restored by the addition of ubiquinone-2 or ubiquinone-10 to the oxidase assay medium.     

Compartmentation of acetyl-coA in rat-liver mitochondria.

The ratio of the specific radioactivities of 3-hydroxybutyrate: citrate was determined in rat liver mitochondria which were incubated in the presence of [1-14C]palmitate, pyruvate, bicarbonate, ATP, phosphate and malonate. Without compartmentation this ratio would maximally be 2, however, under our conditions values of 2.5-3.7 were observed. In further experiments with mitochondria, the sensitivity of pyruvate carboxylase for acetyl-CoA produced from various precursors was tested. It was found that acetyl-CoA produced from L-acetylcarnitine or by oxidation from either pyruvate, octanoate or palmitylcarnitine but not from leucine led to a stimulation of pyruvate carboxylation. These results demonstrate a compartmentation of acetyl-CoA in liver mitochondria. The further finding that different mitochondrial fractions showed varying ratios of specific radioactivities of 3-hydroxybutyrate:citrate indicates that the observed compartmentation may be explained by the existence of different types of mitochondria with varying enzyme patterns and acetyl-CoA pools.