Phosphate-Activated Glutaminase in Relation to Huntington''s Disease and Agonal State

Involvement of phosphate-activated glutaminase in Huntington's disease and agonal state was investigated in caudate nucleus and frontal cortex from postmortem brains. In Huntington's disease the activities of phosphate-activated glutaminase, glutamic acid decarboxylase, succinic dehydrogenase, choline acetyltransferase, and acetylcholinesterase were significantly reduced in the caudate nucleus, but not in the frontal cortex. The activity of phosphate-activated glutaminase, and to a lesser extent of glutamic acid decarboxylase, was reduced in cases of terminal illness, as compared with cases of sudden death. Succinic dehydrogenase and choline acetyltransferase were reduced only in the few cases of prolonged and severe terminal illness. Enzyme activities of the caudate nucleus were more affected by agonal state than were those of frontal cortex. Results indicate that phosphate-activated glutaminase could be a useful marker of neuronal damage due to agonal state, and that phosphate-activated glutaminase and succinic dehydrogenase are reduced in Huntington's disease.     

Enzyme Activities in Relation to pH and Lactate in Postmortem Brain in Alzheimer-Type and Other Dementias

Phosphate-activated glutaminase, glutamic acid decarboxylase, pyruvate dehydrogenase, succinic dehydrogenase, pH, and lactate were measured in frontal cortex and caudate nucleus of postmortem brains from cases of Alzheimer-type dementia (ATD), Down's syndrome, Huntington's disease, and one case of Pick's disease, as well as from sudden death and agonal controls. Lactate levels were higher and pH, phosphate-activated glutaminase, and glutamic acid decarboxylase levels were lower in the agonal controls than in the sudden death controls. Phosphate-activated glutaminase and glutamic acid decarboxylase were correlated with tissue pH and lactate, and also were reduced by in vitro acidification, suggesting that the low activities of these enzymes in agonal controls were related to decreased pH consequent upon lactate accumulation. Compared with control tissues at the same pH, phosphate-activated glutaminase and glutamic acid decarboxylase were unaltered in ATD and Down's frontal cortex and reduced in Huntington's caudate nucleus, and glutamic acid decarboxylase was reduced in Huntington's frontal cortex. These data suggest that GABAergic neurons are not affected in ATD and confirm the GABAergic defect in Huntington's disease. Pyruvate dehydrogenase and succinic dehydrogenase activities were the same in agonal controls and sudden death controls and were unaffected by acid pH and lactate in vitro, and pyruvate dehydrogenase was not correlated with pH or lactate. Reduced pyruvate dehydrogenase in frontal cortex of individual ATD, Down's, and Pick's cases, and in the caudate nucleus of Huntington's and Down's cases, was accompanied by gliosis/neuron loss. We conclude that decreased pyruvate dehydrogenase reflects neuronal loss.     

Glutamine synthetase activity in Huntington's disease

Glutamine synthetase activity was measured in seven brain areas post-mortem from control patients, and those with Huntington's disease. The activity of the enzyme was reduced in the frontal and temporal cortex, putamen and cerebellum, but not in the hippocampus, thalamus or olivary nucleus. The results do not suggest a generalised deficiency of glutamine synthetase in Huntington's disease. However, as this enzyme is localised to astrocytic cells, the reduction in activity in areas of neuronal devastation, where the ration of astrocytes to neurones is increased, may reflect a greater functional deficit. The enzyme plays a crucial role in cerebral ammonia assimilation and its inhibition in laboratory animals is known to produce neuronal toxicity. A reduction in its activity in Huntington's disease may well contribute to the neuronal pathology in certain areas.     

A role for thiamine in the regulation of fatty acid and cholesterol biosynthesis in cultured cells of neural origin.

Abstract— Cultured glial (C-6) and neuronal (neuroblastoma) cells were utilized to define the role of thiamine in the regulation of fatty acid and cholesterol biosynthesis. Glial cells subjected to thiamine deficiency exhibited rates of fatty acid synthesis that were only 13% of the rates in thiamine-supple-mented cells. The decrease in fatty acid synthetic rate was accompanied by a comparable decrease in the activities of fatty acid synthetase and acetyl-CoA carboxylase, the two critical enzymes in the pathway. Immunochemical techniques demonstrated that the decrease in activity of fatty acid synthetase reflected a decrease in enzyme content and that this change in content was caused by a decrease in enzyme synthesis. The disturbance of fatty acid synthesis was exquisitely sensitive to thiamine–i.e. marked improvement was evident within hours of replenishment with only 0.01 μ/ml of thiamine. Total recovery occurred in 1–2 days. Thiamine-deficient glia also exhibited reduced rates of cholesterol biosynthesis, i.e. 60% of the rates in thiamine-supplemented cells. This effect was accompanied by a comparable reduction in activity of 3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) reductase, the rate-limiting step in cholesterol biosynthesis. Unlike the glial cells, the neuronal cells exhibited either no or only a slight reduction in lipid synthesis under similar conditions of thiamine deficiency. The data have important implications for the genesis of the neuropathology in states of altered thiamine homeostasis and for the mechanisms of regulation of lipid synthesis.     

Hormonal regulation of fatty acid synthetase, acetyl-CoA carboxylase and fatty acid synthesis in mammalian adipose tissue and liver.

The major objectives of this study were to define the roles of adrenal glucocorticoids and glucagon in the long-term regulation of fatty acid synthetase and acetyl-CoA carboxylase of mammalian adipose tissue and liver. Particular emphasis was given to elucidation of the mechanisms whereby these hormones produce their regulatory effects on enzymatic activity. To dissociate mental manipulation, nutritional conditions were ridgidly controlled in the experiments described. Administration of glucocorticoids to adult rats led to a marked reductionin activities of fatty acid synthetase and carboxylase in adipose in adipose tissue but no change occurred in liver. Adrenalectomy produced an increase in activities of these lipogenic enzymes in adipose tissure, but, again, no change was noted in liver. The decrease in enzymatic activities in adipose tissue with glucocorticoid administration correlated well with a decrease in fatty acid synthesis, determined in vivo by the 3-H2O method. The mechanisms whereby glucocorticoids led to a decrease in fatty acid synthetase activity were elucidated by the use of immunochemical techniques. Thus, the decrease in fatty acid synthetase activity observed in adipose tissue was shown to reflect a decrease in content of enzyme, and not a change in catalytic efficiency. The mechanism underlying the decrease in enzyme content is a decrease in synthesis of the enzyme. The relation of the effects of glucocorticoids to the effects of certain other hormones involved in regulation of lipogenesis was investigated in hypophysectomized and in diabetic animals. Thus, the observation that the glucocorticoid effect on synthetase and carboxylase occurred in adipose tissue of hypophysectomized rats indicated that alterations in levels of other pituitary-regulated hormones were not necessary for the effect. That glucocorticoids play some role in regulation of synthetase and carboxylase in liver, at lease in the diabetic state, was shown by the observation that the low activities of these enzymes in diabetic animals could be restored to normal by adrenalectomy. An even more pronounced restorative effect was apparent in adipose tissue of adrenalectomized, diabetic animals. Administration of glucagon during the refeeding of starved rats resulted in a marked reduction in the induction of fatty acid synthetase, acetyl-CoA carboxylase and in the rate of incorporation of 3-H from 3-H2O into fatty acids in liver, but no change in these parameters occurred in adipose tissue. Administration of theophylline resulted in intermediate reduction in liver. The mechanisms whereby glucagon led tto a decrease in fatty acid synthetase activity were elucidated by the use of immunochemical techniques. Thus, the changes in fatty acid synthetase activity were shown to reflect reductions in content of enzyme. The mechanism underlying these reductions in content is reduced synthesis of enzyme.     

Acetyl-CoA Synthesizing Enzymes in Cholinergic Nerve Terminals

The activities of five enzymes involved in acetyl-CoA synthesis, pyruvate dehydrogenase complex, ATP citrate lyase, carnitine acetyltransferase, acetyl-CoA synthetase, and citrate synthase, were determined in normal nucleus interpeduncularis and nucleus interpeduncularis in which cholinergic terminals were removed following lesion of the habenulointerpeduncular tract. The activities of aspartate transaminase, fumarase, and GABA transaminase also were determined to compare the effect of lesion on other mitochondrial enzymes which are not linked to the biosynthesis of ACh. In normal nucleus interpeduncularis the activities of carnitine acetyltransferase and pyruvate dehydrogenase complex were higher than the activity of ChAT (choline acetyltransferase), whereas the activities of acetyl-CoA synthetase and citrate synthase were considerably lower than that of ChAT. The effect of the lesion separated the enzymes into two groups: the activities of pyruvate dehydrogenase complex, carnitine acetyltransferase, fumarase and aspartate transaminase decreased by 30--40%, whereas the activities of the other enzymes descreased 5--15%. ChAT activity was in all cases less than 15% of normal. It could be concluded that none of the acetyl-CoA synthesizing enzymes decreased to the degree that ChAT did. Only pyruvate dehydrogenase complex and carnitine acetyltransferase seem to be localized in cholinergic terminals to a significant degree. ATP citrate lyase as well as acetyl-CoA synthetase seem to have less significance in supporting acetyl-CoA formation in cholinergic nerve terminals.     

Glutamine synthetase and fructose-1, 6-diphosphatase activity in the putamen of control and Huntington's disease brain post mortem

There is a linear negative correlation between the activities of glutamine synthetase and fructose-1, 6-diphosphatase in normal Human putamen autopsy samples, and also in the Huntington's disease putamen. However, glutamine synthetase activity is reduced in choreic brain samples, while fructose-1, 6-diphosphatase activity is normal. The ratio of fructose-1, 6-diphosphatase to glutamine synthetase is therefore increased in Huntington's disease. The products of the two reactions, glutamine and fructose-6-phosphate, are the starting substrates for glycolipid and glycoprotein biosynthesis, via the glutamine:fructose-6-phosphate aminotransferase catalysed formation of glucoseamine-6-phosphate. The alternative metabolic route of fructose-6-phosphate leads to glycogen. The availability of glutamine, and the activity of glutamine synthetase may control fructose-6-phosphate metabolism, and the increased ratio of fructose-1,6-diphosphatase to glutamine synthetase in Huntington's disease may explain the accumulation of glycogen, and the reduction in ganglioside levels reported in this state.     

Nutritional and hormonal control of lung and liver fatty acid synthesis.

During starvation and in streptozotocin-induced diabetes, the total activities of rat lung acetyl CoA carboxylase and fatty acid synthetase are reduced to one-third of the normal values. Refeeding of the starved animals or administration of insulin to diabetic animals restores the levels to the original values. The insulin effect is dose and time dependent. These data contrast with those in the liver, where a 30- to 50-fold depression of these enzymes is observed in the diabetic state and administration of insulin is actually followed by doubling of the activity over normal controls. Fat-free high-fructose diet (containing 60% fructose by weight) enhances the activities of liver enzymes 3- to 6-fold over the values of controls on laboratory diet but has no effect on the lung enzymes. Long-term feeding of fructose diet also increases the activities of liver enzymes from diabetic animals to twice the value of normal controls on laboratory diet. Insulin administration to fructose-fed diabetic animals restores the enzyme activities to those obtained with fructose-fed normal controls. However, the stimulation of lung enzymes of diabetic animals can be effected either by fructose or by insulin. Antigen-antibody titrations and measurements of the rate of protein synthesis show that the increased activity of the lung and liver fatty acid synthetase is due to enhanced content rather than increased specific activity. These data suggest that insulin or fructose effects on fatty acid-synthesizing enzymes are mediated through intermediate(s) whose concentration is affected in the experimental diabetes. Furthermore, all tissues may not have stringent insulin requirements since the lung enzymes can be stimulated by fructose alone.     

Regulation of fatty acid synthesis.

Acetyl-CoA carboxylase and fatty acid synthetase are the two major enzymes involved in the synthesis of fatty acids in animals. The activities of both enzymes are affected by nutritional manipulations. Although acetyl-CoA carboxylase is considered generally to be the rate-limiting step in lipogenesis, there is evidence that suggests that fatty acid synthetase may become rate limiting under certain conditions. The principal support for the view that acetyl-CoA carboxylase is the rate-limiting enzyme for lipogenesis is that the activity of the enzyme is controlled by allosteric effectors that change the catalytic efficiency of the enzyme. Until recently, the only known control of fatty acid synthetase was through changes in rate of enzyme synthesis. Data are reviewed that show that fatty acid synthetase can exist in forms possessing different catalytic activities. Thus fatty acid synthetase appears to be subject to the type of control necessary for an enzyme to serve as a regulator of the rate of a biological process over a short term.     

Isolation of astrocytes,neurons, and synaptosomes of rat brain cortex

A simplified method was developed for the bulk separation of neuronal perikarya and astroglial celis from adult rat brain without the involvement of density gradients. Activities of various enzymes involved in glutamate metabolism were estimated and compared with those of synaptosomes. The activities of glutamate dehydrogenase and aspartate aminotransferase were higher in synaptosomes than in neuronal perikarya or glia. Glutamine synthetase was distributed in all the three fractions while glutaminase activity was higher in astrocytes than in synaptosomes and was not detectable in neuronal perikarya. The significance of these results in relation to metabolic compartmentation was discussed.     

Stimulation of the synthesis of hepatic fatty acid synthesizing enzymes of hypophysectomized rats by 3,5,3'-l-triiodothyronine.

The levels of hepatic fatty acid synthesizing enzymes, acetyl-CoA carboxylase and fatty acid synthetase, are lowered to about one-tenth of the controls in hypophysectomized animals, whereas the lung enzymes decrease by only 25–30%. Administration of 3,5,3′-l-triiodothyronine to the hypophysectomized animals returns the hepatic and lung enzyme activities to the control values. Optimum levels are achieved at a dose of about 150 μg/100 g body wt and 3–4 days after triiodothyronine administration. The triiodothyronine response can be reduced by 80% with actinomycin-D or cycloheximide but not with hydrocortisone hemisuccinate. Antibody-antigen titrations and measurements of the rate of synthesis of fatty acid synthetase are indicative of increased synthesis of fatty acid synthetase and not of activation of the preexisting inactive species. These measurements provide evidence for the involvement of hormones other than insulin in the control of synthesis of fatty acid synthesizing enzymes.     

Coordinate regulation of acetyl coenzyme A carboxylase and fatty acid synthetase in liver cells of the developing chick in vivo and in culture.

The activities of hepatic acetyl-CoA carboxylase and fatty acid synthetase undergo two distinct types of development in the perinatal chick. The first increase begins prior to hatching, continues after hatching in the starved chick, and is independent of feeding. The second increase is caused by feeding and is reversed by starvation (A. G. Goodridge (1973) J. Biol. Chem.248, 1932–1938). We have purified these enzymes to homogeneity and raised antibodies to them in rabbits. Using immunochemical techniques we have established that the activity changes in both types of development were a function of changes in the concentrations of enzyme proteins. All activity changes were accompanied by similar changes in the relative rates of synthesis of the two enzymes. Regulation of the activities of acetyl-CoA carboxylase and fatty acid synthetase was further characterized in liver cells from 19-day-old embryos maintained in culture in a chemically defined medium. After 3 days in culture in the absence of hormones, the activities of the enzymes increased significantly with respect to the activities of the freshly prepared cells. Addition of either insulin or triiodothyronine alone caused additional small increases. Insulin plus triiodothyronine caused 8- and 15-fold increases in acetyl-CoA carboxylase and fatty acid synthetase, respectively, relative to cells incubated without hormones. In the presence of insulin alone glucagon had no effect on the activity of either enzyme. In the presence of insulin plus triiodothyronine, glucagon inhibited the increase in enzyme activities by about 75%. The results of quantitative immunoprecipitin tests indicated that activity changes caused by the various hormones were functions of changes in the concentrations of the enzyme proteins. The effects of the hormones on enzyme activities were accompanied by comparable or larger changes in the relative rates of synthesis of the enzymes. Under a wide variety of experimental conditions, both in vivo and in culture, the relative rates of synthesis of acetyl-CoA carboxylase and fatty acid synthetase are regulated coordinately. Under some of these conditions, synthesis of malic enzyme also is regulated coordinately with the syntheses of acetyl-CoA carboxylase and fatty acid synthetase. The common intracellular mechanisms underlying the coordinate control remain to be elucidated.     

Enzymes of glutamine metabolism in testa-pericarp and endosperm of developing wheat grain

Glutamate dehydrogenase, glutamine synthetase, glutamate synthase, glutamate puruvate transaminase and glutamate oxaloacetate transaminase have been assayed in developing testa-pericarp and endosperm of two wheat varieties, namely Shera (11.6% protein) and C-306 (9.8% protein). On per organ basis, activities of all the enzymes studied, except glutamine synthetase, increased during development. Glutamine synthetase activity decreased during development in the testa-pericarp, whereas, no glutamine synthetase activity could be detected in endosperm of either variety at any stage of development. Compared to testa-pericarp, endosperm had higher activities of glutamate synthase and glutamate pyruvate transaminase. On the whole, enzyme activities in Shera were higher, as compared to C-306. Developmental patterns and relative levels of enzyme activities in the two varieties were more or less the same, when expressed on dry weight basis or as specific activities. The results suggest that ammonia assimilation in developing wheat grain takes place by the glutamate dehydrogenase pathway in the endosperm; and both by the glutamate dehydrogenase and glutamine synthetase—glutamate synthase pathways in the testa-pericarp.     

Effects of aging on contributions of dietary fat and triiodothyronine treatment to lipogenic enzyme induction

Although lipogenic enzyme inductions are reduced by fat feeding, this reduction decreases with aging and is particularly detectable in the case of acetyl-CoA carboxylase and fatty acid synthetase activities. On the other hand, the fat-dependent reductions of malic enzyme and acetyl-CoA carboxylase were consistently relieved by triiodothyronine (T3) treatment. The effects of T3 treatment on these enzyme inductions were greater in 10-month-old rats than in 1-month-old rats, while the carbohydrate-dependent induction and the fat-dependent reduction of the enzymes decreased with aging. In these animals, alterations in malic enzyme mRNA translational activities were roughly in parallel to the enzyme activities. Therefore, the age-dependent alterations in effects of T3 treatment and fat on malic enzyme induction do not appear to occur in post-translation.     

Biosynthetic mechanism of ribulose-1,5-bisphosphate carboxylase in the purple photosynthetic bacterium,Chromatium vinosum: I. Inducible formation

The nature of the inducible formation of enzymes engaged in the photosynthetic CO₂ fixation was examined in Chromatium vinosum during its autotropic development. Although the activity of RuBP carboxylase was the lowest among several enzyme activities examined, it was enhanced 2.5 times during a 5-hr incubation, while other enzyme activities were little altered. The enhancement of the RuBP carboxylase activity was dependent on the presence of reduced sulfur compounds in the incubation medium and illumination (>100 lx). The increase in enzyme activity, however, was repressed by CO₂ or pyruvate. Furthermore, O₂ markedly reduced the enzyme activity. In order to prove whether or not the enhancement of RuBP carboxylase activity was attributable to the biosynthesis of the enzyme, the incorporation of [³⁵S]methionine into RuBP carboxylase was followed by immunoprecipitation analysis. The incorporation was dependent on the reduced sulfur compounds, and was repressed by elevating the CO₂ level.     

Effects of dietary nutrients on substrate and effector levels of lipogenic enzymes, and lipogenesis from tritiated water in rat liver

When fasted rats were refed for 4 days with a carbohydrate and protein diet, a carbohydrate diet (without protein) or a protein diet (without carbohydrate), the effects of dietary nutrients on the fatty acid synthesis from injected tritiated water, the substrate and effector levels of lipogenic enzymes and the enzyme activities were compared in the livers. In the carbohydrate diet group, although acetyl-CoA carboxylase was much induced and citrate was much increased, the activity of acetyl-CoA carboxylase extracted with phosphatase inhibitor and activated with 0.5 mM citrate was low in comparison to the carbohydrate and protein diet group. The physiological activity of acetyl-CoA carboxylase seems to be low. In the protein diet group, the concentrations of glucose 6-phosphate, acetyl-CoA and malonyl-CoA were markedly higher than in the carbohydrate and protein group, whereas the concentrations of oxaloacetate and citrate were lower. The levels of hepatic cAMP and plasma glucagon were high. The activities of acetyl-CoA carboxylase and also fatty acid synthetase were low in the protein group. By feeding fat, the citrate level was not decreased as much as the lipogenic enzyme inductions. Comparing the substrate and effector levels with the Km and Ka values, the activities of acetyl-CoA carboxylase and fatty acid synthetase could be limited by the levels. The fatty acid synthesis from tritiated water corresponded more closely to the acetyl-CoA carboxylase activity (activated 0.5 mM citrate) than to other lipogenic enzyme activities. On the other hand, neither the activities of glucose-6-phosphate dehydrogenase and malic enzyme (even though markedly lowered by diet) nor the levels of their substrates appeared to limit fatty acid synthesis of any of the dietary groups. Thus, it is suggested that under the dietary nutrient manipulation, acetyl-CoA carboxylase activity would be the first candidate of the rate-limiting factor for fatty acid synthesis with the regulations of the enzyme quantity, the substrate and effector levels and the enzyme modification.     

THE REGIONAL DISTRIBUTION OF CYTIDINE 5''-MONOPHOSPHO-N-ACETYL-NEURAMINIC ACID SYNTHETASE IN CALF BRAIN

—The enzyme cytidine 5′-monophospho-N-acetylneuraminic acid synthetase was studied in different parts of the calf brain. Characterization of partial purified enzyme preparations from cortical grey matter and corpus callosum by means of pH optima and K_m values, showed the enzyme of grey and white brain areas to be identical. Unexpectedly the regional differences of the enzyme activities per g wet tissue and per mg protein were very slight. From the presence of the enzyme in pure white brain areas, which are known to be poor in neuronal perikarya, and the fact that the enzyme is localized in the cell nucleus, we concluded that cytidine 5′-monophospho-N-acetylneuraminic acid is produced in glia cell nuclei and that it is very likely that biosynthesis of sialo-glycoproteins and/or ganglio-sides occurs within glia cells. The enzyme activity per μmol DNA-P is somewhat higher in grey than in white regions, indicating a slightly higher activity per neuronal than per glial nucleus. The regional differences of lipid and protein-bound sialic acid and RNA show a striking similarity and contrast to those of the enzyme. These differences are interpreted in terms of a differential content in neurons and glia cells.     

Reduction of ferric leghemoglobin in soybean root nodules

Callus tissue cultures were developed from apical meristem regions of tumor-like ineffective root nodules of alfalfa. Callus growth was a function of tissue source and hormone composition and concentration. Callus derived from ineffective nodules also were shown not to contain Rhizobium meliloti.

Glutamate dehydrogenase, glutamine synthetase, glutamate synthase, glutamate oxaloacetate transaminase and phosphoenolpyruvate carboxylase activities were present in callus cultures and in the respective nodule source used for callus induction. The mean specific activity of all enzymes evaluated was higher in callus cultures than in ineffective nodules. Quantitative but not qualitative differences in enzyme activities were evident between ineffective nodules and callus derived from these nodules. Tissue cultures derived from ineffective nodules may provide a model system to evaluate host plant-Rhizobium interactions.

     

Increase in Kynurenic Acid in Huntington''s Disease Motor Cortex

Huntington's disease is a neurological disorder characterised by a progressive chorea and dementia. Recent evidence has suggested that dysfunction involving endogenous excitatory amino acids may be important in the pathogenesis of this disease. Following the recent demonstration that kynurenic acid is present in the brain, we examined the levels in various areas of brain from patients who died with Huntington's disease and from age/sex-matched controls. Blocks (100-500 mg) of cortex (Brodmann's areas 4 and 10) and caudate nucleus and globus pallidus (lateral and medial parts) were obtained from the Cambridge Brain Bank. The tissue was then processed for the extraction and analysis of kynurenic acid. Whereas no differences in the content of kynurenic acid were observed in the caudate nucleus, lateral or medial globus pallidus, or prefrontal cortex (area 10) between controls' brains and those from patients who died with Huntington's disease, there was a 94% (p less than 0.01; n = 5) increase in the kynurenic acid content in the motor cortex (area 4) from Huntington's disease brains, relative to those of controls. Some time ago we suggested that a subtle change in the relative concentrations of quinolinic and kynurenic acids might be important in the pathogenesis of neurodegeneration. It is possible that the observation of raised kynurenic acid levels supports this supposition. Further work is now in progress to determine whether the change in kynurenic acid is a primary effect or a compensatory response to an increase in excitatory activity.     

Citrate carrier and lipogenic enzyme activities in lead nitrate-induced proliferative and apoptotic phase in rat liver.

After in vivo administration of lead nitrate, functional changes of the mitochondrial tricarboxylate carrier and of the cytosolic lipogenic enzymes acetyl-CoA carboxylase and fatty acid synthetase have been detected in rat liver. The rate of citrate transport was greatly reduced in rats during both the proliferative phase (3 days after the lead nitrate administration) and the involutive phase (5 days after the metal injection), which follows hepatic hyperplasia and corresponds to the peak of hepatocyte apoptosis. In both phases, a decrease of the lipogenic enzyme activities has been detected. In treated animals, an alteration of mitochondrial lipid composition has also been found. The modified lipid microenvironment could be responsible for the decreased carrier activity which, in turn, may account for the reduced activities of the lipogenic enzymes.