Lipid biosynthesis from DL (2- 14 C) mevalonic acid in intact mice and rabbits

Starvation induced cholesterogenesis in hepatic and extra hepatic tissues of climbing Perch, Anabas testudineus (Bloch)

Cholesterol is a structural lipid, which may be differentially utilized or synthesized in response to stress or during insulin deficient states such as starvation. In the present investigation we estimated the levels of cholesterol in Anabas testudineus, which was subjected to brief (15 days) and prolonged fasting (60 days). Tissues such as liver, kidney, brain, accessory respiratory organ, pectoral and lateral line muscle were selected for the study. Cholesterol content was estimated by the Crawford method (1958). Both the starvation regimes showed a significant increase in cholesterol levels in almost all the tissues, but for liver, which strangely showed an insignificant decline during the short-term starvation. This overall upsurge in cholesterol levels observed in all extra hepatic tissues may be attributed to the synthesis of stress hormones such as glucocorticoids, which may promote gluconeogenesis and adrenocorticoids, which may help the animal to combat the stressful condition of starvation. Anabas adapted well to starvation stress and survived all throughout the experimental period.

High-density lipoprotein cholesteryl ester and protein catabolism in hypercholesterolemic rats induced by copper deficiency

High-density lipoprotein (HDL) catabolism induced by copper deficiency was examined in vivo in hypercholesterolemic Sprague-Dawley rats. Doubly labeled HDL was used to trace the catabolic pathways of both cholesteryl ester and protein moieties of HDL particles. The catabolic rate of removal from the plasma, as well as uptake by various tissues, was determined for each HDL component. Copper-deficient rats exhibited a 30% increase in HDL cholesterol concentration, confirming hypercholesterolemia. In addition, plasma volume was enlarged 38% in deficient animals, resulting in a significantly increased intravascular pool of all HDL components of at least 60%. These data emphasize the importance of determining plasma volume and total pool size of pertinent plasma components in this hypercholesterolemic model. The absolute catabolic rate (ACR) of HDL protein removal from the plasma was 369 +/- 22 and 278 +/- 12 micrograms/h in copper-deficient and control rats, respectively. The ACR of HDL cholesteryl ester was 647 +/- 37 micrograms/h in deficient animals and 321 +/- 13 micrograms/h in controls, suggesting that the mechanisms of selective clearance of HDL cholesteryl ester (compared with protein) were increased threefold by copper deficiency. Virtually all of the increased removal of HDL cholesteryl ester in deficient rats occurred in the liver. Since previous studies indicate that increased hepatic cholesterol excretion may not occur in copper deficiency, the present results suggest that cholesterol delivered to the liver as HDL cholesteryl ester is possibly reassembled into new HDL particles at an increased rate in copper-deficient rats.

Role of the kidneys in the metabolism of plasma mevalonate. Studies in humans and in rhesus monkeys

Studies were carried out in humans and in rhesus monkeys to determine the role of the kidneys in the metabolism of circulating mevalonic acid (MVA). Following intravenous infusion of [14C]MVA and [3H]cholesterol, there was a rapid appearance of [14C]squalene in the kidneys that exhibited a significantly longer half-life than plasma or hepatic squalene. In man and in rhesus monkeys there was a rapid equilibration between newly synthesized cholesterol from MVA and exogenously administered cholesterol in all tissues except the kidneys, where the specific activity ratio of newly synthesized to exogenous cholesterol was significantly higher. Estimates of the quantitative metabolism of intravenously infused radiolabeled MVA in the monkey demonstrated that 23% was excreted in the urine, 67% metabolized to cholesterol (58% in nonrenal tissues and 9% in the kidneys), and 10% catabolized to CO2 and nonsteroid products. Measurements of MVA metabolism in anephric and uninephric patients demonstrate that, in the absence of renal uptake of MVA, exogenous and newly synthesized cholesterol achieve almost instantaneous equilibrium in the plasma; whereas in control subjects with normal renal function, this equilibration required at least 21 d for the two cholesterol decay curves to become parallel. These results suggest that the kidneys are solely responsible for the observed disequilibrium between newly synthesized and exogenous cholesterol; we suggest that this was due to the delayed release of newly synthesized cholesterol from the kidneys into the plasma compartment following intravenous infusion with radiolabeled MVA. The data document the importance of the kidneys in the metabolism of circulating MVA. However, calculation of the quantitative significance of this pathway in relation to whole body MVA metabolism indicates that renal metabolism of MVA accounts for approximately 0.1% of daily MVA turnover, and that alterations in this pathway due to any form of renal pathology would not result in significant changes in hepatic or whole body sterol synthesis rates. We urge caution in the use of radiolabeled MVA in long-term kinetic studies of sterol metabolism because our data show that the plasma compartment of MVA is not necessarily in isotopic equilibrium with tissue MVA.

The in vivo incorporation of acetate into different non-saponifiable lipids by neonatal chick tissues

The in vivo incorporation of [l-14C]acetate into non-saponifiable lipids was higher in neonatal chick liver than in intestinal mucosa, brain and kidneys, and proportional to the amount of substrate injected (2-20 mumole). 14CO2 expired in the breath was also proportional to the dose of acetate. Radioactivity from [l-14C]acetate accumulated by liver was maximal 30 min after the injection of acetate and decreased afterwards. Acetate was mainly incorporated into cholesterol by all the tissues assayed, although small percentages of lanosterol and squalene were obtained in liver. In this tissue, distribution of radioactivity was practically independent from the dose of substrate injected while in intestinal mucosa, brain and kidneys the percentage of cholesterol increased with this dose. The time course of the in vivo formation of different non-saponifiable lipids by neonatal chick tissues was also studied. More than 90% of radioactivity in this fraction obtained 15 min after the acetate injection was recovered as cholesterol in liver and kidneys, while in brain and intestinal mucosa this percentage was about 50% at this time, increasing afterwards. A high percentage of lanosterol was found in brain and intestinal mucosa 15 min after the injection of acetate.

Incorporation of mevalonate into squalene, lanosterol and cholesterol by different neonatal chick tissues

The role of neonatal chick liver and kidneys in the incorporation of mevalonic acid into squalene, lanosterol and cholesterol was studied. Differences between the synthesizing ability of these and other tissues and the influence of the in vivo or in vitro conditions were also examined. In the in vivo experiments, distribution of radioactivity among the nonsaponifiable lipids was not dependent of the doses of mevalonic acid injected. About 80-95% of radioactivity was recovered as cholesterol in liver and brain, whereas in kidneys this percentage was only about 35%. Squalene and lanosterol were formed by kidneys in a high percentage, higher than in liver and other tissues. 12 hr after mevalonate injection, the percentage of cholesterol formed by kidneys increased until more than 50%. In the in vitro experiments carried out in the presence of 0.045-4.0 mM mevalonate, cholesterol was also the main nonsaponifiable identified, but in a lesser percentage than in vivo. In the same conditions, the incorporation of mevalonic acid by kidneys was maximal into squalene. After in vitro incubations for 2 hr, the percentage of cholesterol in kidneys also increased.

The in vitro metabolism of mevalonate by sterol and nonsterol pathways in neonatal chick

1. Incorporation of mevalonate into nonsaponifiable lipids by neonatal chick liver and kidney slices was studied as a function of weight of tissue and incubation time. In the same conditions, more nonsaponifiable lipids were produced in kidney than in liver. 2. CO2 production from mevalonate increased with the weight of tissue and with the incubation time. Over 80% of CO2 produced from mevalonate resulted from the shunt pathway in kidney, while in liver this route was quantitatively insignificant. 3. 2-14C from mevalonate was also incorporated into saponifiable (acidic) fraction by kidney slices. 4. Incorporation into nonsaponifiable lipids and total CO2 increased with the concentration of mevalonate. This increase was more pronounced in kidney than in liver especially at low mevalonate concentrations. In the presence of 0.1-8.0 mM mevalonate over 80% of the CO2 produced from this substrate resulted from the shunt pathway, while liver was practically ineffective whatever be the concentration of mevalonate. 5. The percentage of mevalonate metabolized in the kidney by shunt pathway increased with the incubation time and reached over 15% in the presence of 0.1-8.0 mM mevalonate.

Mevalonate metabolism: role of kidneys

More than one-half of the amount of mevalonate that is metabolized by pathways not leading to sterols is accounted for by the action of the kidneys. Conversion of mevalonate in vivo to squalene and sterols in the kidneys is confined almost entirely to the proximal and distal convoluted tubules in the cortex. More sterol than squalene is synthesized from mevalonate not only in the liver but also in the kidney.