The interaction of catecholamines and adrenal corticosteroids in the induction of phosphopyruvate carboxylase in rat liver and adipose tissue

Catecholamines induced an increase in the activity of rat adipose tissue and liver phosphopyruvate carboxylases that was maintained for 48h. The response of adipose tissue phosphopyruvate carboxylase was blocked by actinomycin D, corticosteroids and propranolol, whereas corticosteroids and propranolol did not affect the liver enzyme. Cortisol phosphate, like actinomycin D, interfered only with the initiation of the increase in enzyme activity caused by noradrenaline, but not with the process of enzyme accumulation. In contrast, cycloheximide was effective in blocking enzyme induction throughout the course of the catecholamine effect. Adrenocorticotrophic hormone caused a short-term induction of adipose tissue phosphopyruvate carboxylase, which could be blocked by propranolol. Hepatic phosphopyruvate carboxylase, but not the adipose tissue enzyme, was induced by dibutyryladenosine 3':5'-cyclic monophosphate and by glucagon. Both nicotinic acid and nicotinamide decreased the normal induction of adipose tissue phosphopyruvate carboxylase caused by starvation, but only nicotinamide increased the activity of the liver enzyme.

The initial synthesis of proteins during development. Phosphoenolpyruvate carboxylase in rat liver at birth

1. A specific antibody, prepared by immunizing rabbits with phosphoenolpyruvate carboxylase (EC 4.1.1.32) purified from adult rat liver, was used to study the appearance of this enzyme in livers from developing rats. 2. Although some inactive precursor of the enzyme may be present in foetal liver, the amount is not sufficient to account for the enzyme appearance at birth. 3. The rate of phosphoenolpyruvate carboxylase synthesis relative to other cytosol proteins increases 20-fold from the foetus to the 1-day-old rat. The high rate of synthesis was maintained at least until 3 days after birth. 4. There was no measurable degradation of phosphoenolpyruvate carboxylase during the first day after birth. During this period the hepatic enzyme content increased 12-fold. 5. When phosphoenolpyruvate carboxylase attained a constant activity in the liver of rats 2 days after birth the half-time of degradation was approx. 13h. 6. We suggest that the pattern of changes occurring during appearance of phosphoenolpyruvate carboxylase is similar to substrate-induced enzyme induction in bacteria.

Pathway of carbon flow during fatty acid synthesis from lactate and pyruvate in rat adipose tissue

The metabolism of pyruvate and lactate by rat adipose tissue was studied. Pyruvate and lactate conversion to fatty acids is strongly concentration-dependent. Lactate can be used to an appreciable extent only by adipose tissue from fasted-refed rats. A number of compounds, including glucose, pyruvate, aspartate, propionate, and butyrate, stimulated lactate conversion to fatty acids. Based on studies of incorporation of lactate-2-(3)H and lactate-2-(14)C into fatty acids it was suggested that the transhydrogenation sequence of the "citrate-malate cycle"(1) was not providing all of the NADPH required for fatty acid synthesis from lactate. An alternative pathway for NADPH formation involving the conversion of isocitrate to alpha-ketoglutarate via cytosolic isocitrate dehydrogenase was proposed. Indirect support for this proposal was provided by the rapid labeling of glutamate from lactate-2-(14)C by adipose tissue incubated in vitro, as well as the demonstration that glutamate can be readily metabolized by adipose tissue via reactions localized largely in the cytosol. Furthermore, isolated adipose tissue mitochondria convert alpha-ketoglutarate to malate, or in the presence of added pyruvate, to citrate. Glutamate itself can not be metabolized by these mitochondria, a finding in keeping with the demonstration of negligible levels of NAD-glutamate dehydrogenase activity in adipose tissue mitochondria. Pyruvate stimulated alpha-ketoglutarate and malate conversion to citrate and reduced their oxidation to CO(2). It is proposed that under conditions of excess generation of NADH malate may act as a shuttle carrying reducing equivalents across the mitochondrial membrane. Malate at low concentrations increased pyruvate conversion $$Word$$ citrate and markedly decreased the formation of CO(2) by isolated adipose tissue mitochondria. Malate also stimulated citrate and isocitrate metabolism by these mitochondria, an effect that could be blocked by 2-n-butylmalonate. This potentially important role of malate in the regulation of carbon flow during lipogenesis is underlined by the observation that 2-n-butylmalonate inhibited fatty acid synthesis from pyruvate, but not from glucose and acetate, and decreased the stimulatory effect of pyruvate on acetate conversion to fatty acids.

Immunochemical studies with soluble and mitochondrial pyruvate carboxylase activities from rat tissues

1. Pyruvate carboxylase (EC 6.4.1.1), purified from rat liver mitochondria to a specific activity of 14 units/mg, was used for the preparation of antibodies in rabbits. 2. Tissue distribution studies showed that pyruvate carboxylase was present in all rat tissues that were tested, with considerable activities both in gluconeogenic tissues such as liver and kidney and in tissues with high rates of lipogenesis such as white adipose tissue, brown adipose tissue, adrenal gland and lactating mammary gland. 3. Immunochemical titration experiments with the specific antibodies showed no differences between the inactivation of pyruvate carboxylase from mitochondrial or soluble fractions of liver, kidney, mammary gland, brown adipose tissue or white adipose tissue. 4. The antibodies were relatively less effective in reactions against pyruvate carboxylase from sheep liver than against the enzyme from rat tissues. 5. Pyruvate carboxylase antibodies did not inactivate either propionyl-CoA carboxylase or acetyl-CoA carboxylase from rat liver. 6. It is concluded that pyruvate carboxylase in lipogenic tissues is similar antigenically to the enzyme in gluconeogenic tissues and that the soluble activities of pyruvate carboxylase detected in many rat tissues do not represent discrete enzymes but are the result of mitochondrial damage during tissue homogenization.

Dietary protein and the control of fatty acid synthesis in rat adipose tissue

Fatty acid synthesis in adipose tissue normally proceeds at a high rate when fasted animals are refed a diet containing carbohydrate, protein, and low levels of fat. This study investigated the effect of omitting protein from the refeeding diet. Rats were fasted for 48 hr and refed either a protein-free diet or a balanced diet, and the rate of fatty acid synthesis from glucose, pyruvate, lactate, and aspartate was measured. Refeeding the animals a diet devoid of protein resulted in a low rate of fatty acid synthesis from each of these substrates as well as a reduction in carbon flow over the citrate cleavage pathway. The activities of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, NADP-malate dehydrogenase, and ATP-citrate lyase were also reduced in epididymal fat pads from these rats. On the other hand, adipose tissue phosphoenolpyruvate carboxykinase activity was five times as great as that in tissue from animals refed a balanced diet. This difference could be eliminated if actinomycin D was injected coincident with refeeding. Refeeding rats diets high in carbohydrate is not, therefore, capable of inducing high rates of fatty acid synthesis in adipose tissue in the absence of dietary proteins. Thus, liver and adipose tissue respond differently to dietary protein.

Measurement of adipose-tissue metabolites in vivo

1. The concentrations of glucose, pyruvate, lactate, citrate, glutamate, malate and aspartate were measured in epididymal adipose tissue from starved, fed and starved-re-fed rats. 2. To measure these intermediates it was necessary to correct for their concentration in the extracellular tissue space, which was considered to be most satisfactorily equated with the glucose space. This space in vivo was 7.42, 4.90 and 7.54ml./100g. wet wt. of tissue in adipose tissue taken from starved, fed and starved-re-fed rats respectively. After correction for the glucose space, the concentrations of metabolites (nmoles/g. of cells) in epididymal adipose tissue of fed rats were: pyruvate, 8.5; lactate, 50.3; citrate, 18.5; glutamate, 100.0; malate, 6.4; aspartate, 34.2. 3. Starvation for 72hr. resulted in a fall in pyruvate and aspartate concentrations to 3.57 and 25.1nmoles/g.; starvation for 72hr. followed by re-feeding for 72hr. caused an increase in glutamate and aspartate concentrations to 140 and 67.6nmoles/g. 4. These changes are interpreted with regard to the simultaneous alteration in lipogenesis that occurs during the starvation-re-feeding cycle.