The metabolism of pyruvate by bovine-adrenal-cortex mitochondria

The role of mitochondrial Ca(2+) and NAD(P)H in the control of aldosterone secretion

The mineralocorticoid hormone aldosterone is synthesized in the zona glomerulosa of the adrenal cortex. Glomerulosa cells respond to the physiological stimuli, elevated extracellular [K(+)] and angiotensin II, with an intracellular Ca(2+) signal. Cytosolic Ca(2+) facilitates the transport of the steroid-precursor cholesterol to mitochondria and, after a few hours, it also induces the transcription of aldosterone synthase. Therefore, the cytosolic Ca(2+) signal is regarded as the most important short and long-term mediator of aldosterone secretion. However, cytosolic Ca(2+) is also taken up by mitochondria and, in turn, the mitochondrial Ca(2+) response activates mitochondrial dehydrogenases resulting in stimulation of respiration and increase in reduced pyridine nucleotides. Since both cholesterol side-chain cleavage and all of the hydroxylation steps of steroid synthesis require NADPH as a cofactor, the importance of cytosolic Ca(2+) - mitochondrial Ca(2+) coupling and of appropriate NADPH supply in respect to hormone production can be assumed. However, the importance of the mitochondrial factors has been neglected so far. Here, after summarizing earlier findings we provide new results obtained through modifying mitochondrial Ca(2+) uptake by knocking down p38 MAPK or OPA1 and overexpressing S100G, supporting the notion that mitochondrial Ca(2+) and reduced pyridine nucleotides are facilitating factors for both basal and stimulated steroid production.

Metabolism of malate in bovine adrenocortical mitochondria studied by 13C-NMR spectroscopy

13C-NMR spectroscopy was used to study the metabolism of [13C]malate in bovine coupled adrenocortical mitochondria. The most apparent difference between the mitochondria from steroidogenic tissues and mitochondria from other tissues is the presence, in addition to the normal respiratory chain, of a second electron-transport system responsible for steroid hydroxylation. [13C]malate was synthesized from [13C]succinate by isolated adrenocortical mitochondria. The basic functional suspension consisted of oxygenated mitochondria to which were added ADP, inorganic phosphate (Pi) and [13C]malate, both in the absence or presence of the steroid substrate, deoxycorticosterone. These mitochondria synthesized [13C]citrate and [13C]pyruvate from [13C]malate. The 13C labeling of these two metabolites demonstrated an important role of the malic enzyme and the kinetics depended on the presence of the steroid substrate; the citric acid cycle was stopped during the hydroxylation pathway. The addition of cyanide, a strong inhibitor of the respiratory chain, confirmed an increased malic enzyme activity when hydroxylation occurred, since pyruvate was trapped by formation of a cyanohydrin. The relative enzymic activities of malic enzyme and isocitrate dehydrogenase were compared, both in the absence or presence of the steroid substrate, by supplementing the basic suspension with unlabeled exogenous metabolites, such as pyruvate or oxaloacetate.

Pyridine nucleotide redox state parallels production of aldosterone in potassium-stimulated adrenal glomerulosa cells

Extracellular potassium ions (K+) raise the intracellular concentration of free Ca2+ ([Ca2+]i) by gating voltage-dependent Ca2+ channels and stimulate aldosterone production in adrenal glomerulosa cells. The pathway leading from calcium influx to increased steroid synthesis has not been completely elucidated. In the present study we demonstrate that the reduction of pyridine nucleotides known to be required for steroid hydroxylation is enhanced by K+ (4.1-8.4 mM) in single rat glomerulosa cells. The action of K+ was strictly dependent on the presence of extracellular Ca2+. Amytal, a blocker of site I of the mitochondrial respiratory chain, abolished the K+ effect, indicating a mitochondrial origin for the recorded changes. Supraphysiological K+ concentration (18 mM) resulted in a further increase in [Ca2+]i, while steroidogenesis was decreased as measured in cell suspensions. However, a possible explanation for this dichotomy is provided by the finding that the level of reduced pyridine nucleotides also decreased at supraphysiological K+ concentration.

The role of malic enzyme in the malate dependent biosynthesis of progesterone in the mitochondrial fraction of human term placenta

Mitochondria isolated from human term placenta were able to form citrate from malate as the only added substrate. While mitochondria were incubated in the presence of Mn2+ the citrate formation was stimulated significantly both by NAD+ and NADP+ and was inhibited by hydroxymalonate, arsenite, butylmalonate and rotenone. It is concluded that NAD(P)-linked malic enzyme is involved in the conversion of malate to citrate in these mitochondria. It has also been shown that the conversion of cholesterol to progesterone by human term placental mitochondria incubated in the presence of malate was stimulated by NAD+ and NADP+ and inhibited by arsenite and fluorocitrate. This suggests that the stimulation by malate of progesterone biosynthesis depends not only on the generation of NADPH by NAD(P)-linked malic enzyme, but also on NADPH formed during further metabolism of pyruvate to isocitrate which is in turn efficiently oxidized by NADP+-linked isocitrate dehydrogenase.

Transaminative pathway of glutamate oxidation in adrenal-cortex mitochondria

Mitochondria isolated from adrenal cortex of beef do oxidize glutamate if the amino group acceptor-oxaloacetate (or its precursor-malate) is present in the incubation medium. The glutamate (plus oxaloacetate) oxidation was enhanced by ADP or deoxycorticosterone, indicating that this respiration can support both oxidative phosphorylation and 11 beta-hydroxylation of deoxycorticosterone to corticosterone. Avenaciolide (inhibitor of glutamate entry into the mitochondria), aminooxyacetate (inhibitor of aspartate aminotransferase activity) and arsenite (inhibitor of 2-oxoglutarate dehydrogenase) when introduced into the incubation media before respirating substrates, inhibited the ability of ADP or deoxycorticosterone to stimulate the rate of glutamate (plus oxaloacetate) oxidation.

2,3,7,8-Tetrachlorodibenzo-p-dioxin-mediated depression of rat testicular heme synthesis and microsomal cytochrome P-450

Exposure to 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) produces hirsutism, alopecia, and chloracne, symptoms that suggest a possible alteration of endocrine function. Therefore, the effects of TCDD on rat testicular cytochrome P-450 content were investigated. Forty-eight hours after a single, oral dose of TCDD (25 microgram/kg) testicular microsomal cytochrome P-450 levels were depressed by approximately 24%. Microsomal cytochrome P-450 continued to decrease to 62% of control levels at 4 days and remained at approximately the same levels 7 days following treatment. Testicular microsomal heme content exhibited a similar pattern after administration of TCDD. No alterations in testicular delta-aminolevulinic acid (ALA) synthase were detected. The incorporation of [14C]ALA into microsomal heme was decreased to approximately 36% of control values at 24 hr after TCDD administration. Testicular weights were not altered during the 7-day experimental period. These data suggest that TCDD depresses cytochrome P-450 levels in the rat testis through an inhibition of the synthesis of testicular heme.

2-bromopalmitate inhibition of stimulatory effect on LH in isolated luteal cells

The effect of 2-bromopalmitate, an inhibitor of fatty acids oxidation, on progesterone synthesis in isolated luteal cells was studied. Incubation of the cells with increasing concentrations (0 - 2 mM) of the inhibitor resulted in an initial enhancement of progesterone synthesis, both in the presence and absence of LH (1 microgram/ml). In the LH-treated cells, this stimulation in steroid synthesis becomes markedly impaired at 2 mM 2-bromopalmitate. In the control cells, however, progesterone synthesis was sustained at the elevated level. At high concentrations of 2-bromopalmitate, the stimulatory effect of LH (relative to the controls) on steroidogenesis was progressively diminished, until it was completely abolished at an inhibitor concentration of 2 mM. The oxidation of labelled palmitic acid by luteal cells was also effectively inhibited by 2-bromopalmitate (1 mM). The results indicate that the steroidogenic effect of LH is mediated, in part, by fatty acid oxidation, and were explained in terms of the interaction between carbohydrate and fatty acid oxidation in supporting ovarian steroidogenesis.

The effect of azastene, cyanoketone and trilostane upon respiration and cleavage of the cholesterol side chain in mitochondria from bovine adrenal cortex

Mitochondria prepared from bovine adrenal cortex and incubated with ADP and phosphate respired at about 45% of the rat observed in the presence of an uncoupler of oxidative phosphorylation; there was, however, little inefficiency in the reactions involved in the phosphorylation of ADP. Three inhibitors of the 3 beta-hydroxysteroid dehydrogenase (azastene, cyanoketone and trilostane) were employed with a view to preventing pregnenolone metabolism and thus aiding the assay of cholesterol side-chain cleavage. Freshly made solutions of these inhibitors did not modify mitochondrial respiratory rates, at concentrations of 10 microM. In contrast, solutions maintained at 0-4 degrees C for one week subsequently inhibited the respiratory rate of uncoupled mitochondria. When fresh solutions of the inhibitors were used in the assays of cholesterol side-chain cleavage, 10 microM azastene did not significantly inhibit pregnenolone metabolism. Cyanoketone and trilostane were both significant inhibitors of pregnenolone metabolism, but 10 microM cyanoketone reduced the initial rate of cholesterol side-chain cleavage by 50% in the presence of 10 mM malate, although this inhibition did not occur in the presence of 10 mM DL-isocitrate. Thus trilostane may be the preferred inhibitor of the 3 beta-hydroxysteroid dehydrogenase during studies of cholesterol side-chain cleavage in vitro.

Citrate formation by rat lung mitochondrial preparations

Rat lung mitochondrial preparations were incubated in the preasence of pyruvate and malate. The principal metabolic products measured were citrate and CO2. Citrate formation from pyruvate was found to be dependent on the presence of malate. Significant citrate was formed in the presence of isocitrate and the rate of citrate formation was increased by the addition of pyruvate. Small amounts of citrate were formed by lung mitochondrial preparations in the presence of 2-oxoglutarate and succinate only after the addition of pyruvate. The level of acetyl-CoA was significantly greater in the presence of pyruvate than in the presence of pyruvate plus malate. The addition of malate to lung mitoochondrial preparations increased 14CO2 production from [2-14C] pyruvate into malate and citrate. A low level of pyruvate-dependent H14CO3-incorporation into acid-stable products was observed, principally citrate and malate, but this rate did not exceed 5% of the rate of net citrate formation in the presence of malate and pyruvate. The capacity of rate lung mitochondria to form oxaloacetate from pyruvate alone in vitro is very limited, and would appear to cast doubt on a major role of pyruvate carboxylase in citrate formation. It is concluded that the rate of citrate formation from pyruvate is limited by the availability of intramitochondrial oxaloacetate and the rate of citrate efflux across the mitochondrial membrane.