Properties and biosynthesis of acetyl-CoA-carboxylase from chicken liver with administration of nicotinic acid during stimulation of lipogenesis

Acetyl-CoA-carboxylase is isolated and purified to a homogeneous state from the chicken liver with alimentary lipogenesis stimulation. Under the action of nicotinic acid in vivo the specific enzyme activity is shown to decrease considerably followed by some variations in its properties. According to the results obtained during ultracentrifugation and PAAG electrophoresis nicotinic acid causes partial enzyme deaggregation with simultaneous increase of its phosphorylation. The latter is accompanied by a rise in the content of phosphate labile to alkali on acetyl-CoA-carboxylase subunits. Nicotinic acid in vivo has practically no effect on acetyl-CoA-carboxylase synthesis and decay rate. Its inhibiting action is induced by stimulation of enzyme phosphorylation.     

Increased sensitivity of fat cell adenylate cyclase to stimulatory agonists during fasting is not related to impaired inhibitory coupling system

In rat adipocytes, inhibition of the forskolin-stimulated cyclic AMP response by nicotinic acid and N6-phenylisopropyladenosine was unaltered by a 72 h fasting. Under assay conditions favouring inhibition, basal and forskolin-stimulated adenylate cyclase responses to inhibition by GTP and nicotinic acid were also unimpaired by fasting. Under the same conditions, however, low GTP concentrations elicited a clear activatory effect in membranes from fasted but not from fed rats. Fasting failed to alter the incorporation of [32P]ADP ribose into the alpha i-subunit of Ni and the attenuation of nicotinic acid inhibitory action that are both induced by pertussis toxin. These results, demonstrating unimpaired inhibitory control of adenylate cyclase in starved rat adipocytes, suggest that the permissive effect of fasting on the action of stimulatory receptor agonists in fat cells reflects a specific increase in the activity of the adenylate cyclase stimulatory coupling system.     

Phosphorylation of acetyl-CoA-carboxylase in the chicken liver during intensification of lipogenesis and treatment with nicotinic acid

Sites of cAMP and ATP binding which regulate acetyl-CoA-carboxylase phosphorylation rate characterized under conditions of lipogenesis intensification and nicotinic acid action on this enzyme 1500 fold purified and containing proteinkinase activity. The acetyl-CoA-carboxylase preparation contains only one type of the cAMP binding sites which possess higher capacity under the action of nicotinic acid in vivo. A decrease of the cAMP binding under the conditions of lipogenesis intensification is induced by diminution of the cAMP binding site capacity without changing the binding constant value. It is established that [gamma-32P]ATP is incorporated in enzyme with Km value equal for two states under study. It this case the [gamma-32P]ATP incorporation rate is much higher for acetyl-CoA-carboxylase produced from chicken liver under the action of nicotinic acid.     

Identification of the prosthetic group of urocanase. The mode of its reaction with sodium borohydride and of its photochemical reactivation.

Urocanase from Pseudomonas putida and from beef liver were isolated by modifying described procedures. Both enzymes were inactivated and labeled on treatment with tritiated sodium borohydride and gave, upon subsequent hydrolysis, a radioactive acid. The previously reported identity of this acid as 2-hydroxybutanoic acid was disproved by several criteria. Other hydroxy acids were also proved to be different from the radioactive acid derived from urocanase. A large portion of the radioactive material from P. putida was found to be nicotinic acid by 1H NMR spectroscopy, gas-liquid chromatography of its methyl ester, and co-crystallization with authentic reference compounds both as the acid and as the hydrazide. A significant portion of the radioactive material derived from beef liver urocanase also co-crystallized with nicotinic acid. Sodium borohydride-treated inactive urocanase was partially reactivated by light. The action spectrum of the photoreactivation showed a maximum at 330 nm. Treatment of urocanase with sodium borodeuteride followed by hydrolysis afforded a sample of nicotinic acid which carried deuterium mainly in position 6. Both the reversible reducibility of urocanase and its action spectrum of photoreactivation suggest that urocanase contains an enzyme-bound nicotinamide nucleotide molecule which is essential for enzymic activity.     

Inhibition of rat and human adipocyte adenylate cyclase in the antilipolytic action of insulin, clofibrate, and nicotinic acid.

Clofibrate (Atromid-S), nicotinic acid, and insulin are known to be potent hypolipidemic and antilipolytic agents. The present study was undertaken to define the mechanism of action of this latter effect on isolated rat and human fat cells. Sodium clofibrate (0.42 mM), nicotinic acid (0.42 mM), and insulin (100 microU/mL) were shown to inhibit norepinephrine-stimulated lipolysis in rat and human adipose cells and this inhibition was associated with a reduction in intracellular 3',5'-cyclic AMP levels. A similar cyclic AMP lowering effect was demonstrated with insulin in the presence of procaine-HCL, which uncouples the adenylate cyclase system from lipolysis. This insulin effect was attributed to inhibition of adenylate cyclase. A direct and significant inhibition of adenylate cyclase in membrane fractions obtained from isolated human adipocytes was demonstrated for all three antilipolytic agents. The common membrane site of action of these agents whereby adenylate cyclase activity is depressed, thus decreasing cyclic AMP production and free fatty acid (FFA) mobilization from adipose stores, implies a central role for the adenylate cyclase system. These findings are consistent with the view that the hypotriglyceridemic effects of clofibrate, nicotinic acid, and insulin may be partly explained by deprivation of FFA substrate for hepatic very low density lipoprotein synthesis.     

A comparative study of the effects of nicotinamide and diethylnicotinamid on hepatic monooxygenase, glucuronyl and glutathione conjugating systems

The nicotinamide administration to rats (50 mg/kg, subcutaneously, over 5 days) increased the concentration of liver cytochrome b5, the activities of cytosol and microsomal glutathione S-transferase, UDP-glucuronosyltransferase and urinary excretion of bound glucuronic acid by 26.7, 33.1, 33.3, 53.0 and 31.0%, respectively. The chloral hydrate-induced sleep time in mice was reduced by 65%. Under similar experimental conditions the administration of equimolar amounts of diethylamide of nicotinic acid (75 mg/kg) exerted a more pronounced enzyme-stimulating effect. The cytochrome P-450 concentration, the activities of cytosol and microsomal glutathione S-transferase, UDP-glucuronosyltransferase as well as the sulphobromophthalein elimination from blood plasma and urinary excretion of bound glucuronic acid were increased by 37.0, 33.1, 54.6, 80.5, 24.5 and 49.0%, whereas the chloral hydrate-induced sleep time decreased by 75%. The nicotinamide and diethylamide of nicotinic acid stimulating effects on xenobiotic biotransformation in rat liver are assumed to be due to enhanced NADPH, glutathione and UDP-glucuronic acid biosynthesis as well as their antioxidant properties.     

Inhibition of activity of acetyl-CoA-carboxylase from chicken liver by nicotinic acid and its derivatives]

Nicotinic acid and nicotinamide inhibit in vitro the acetyl-CoA-carboxylase activity of partially purified enzyme from chicken liver. The incorporation of 10, 20, 50 and 100 mkmoles of nicotinic acid or nicotinamide into the incubation medium (0,9 ml) leads to the inhibition of the enzyme activity by 19, 45, 70 and 100% and by 39, 51, 60 and 78%, respectively. NADH+ and NADP+ at concentrations by one order of magnitude lower than those of nicotinic acid and nicotinamide decrease the enzyme activity in a similar manner. The constants of inhibition by the above-mentioned compounds were calculated with respect to ATP, acetyl-CoA and citrate.     

The effect of pyrazines on the metabolism of tryptophan and nicotinamide adenine dinucleotide in the rat Evidence of the formation of a potent inhibitor of aminocarboxy-muconate-semialdehyde decarboxylase from pyrazinamide

The intraperitoneal or oral administration of pyrazinamide and pyrazinoic acid (pyrazine 2-carboxylic acid) resulted in a marked increase of the NAD content in rat liver. The injections of pyrazine and pyrazine 2,3-dicarboxylic acid exhibited no significant effect on the hepatic NAD content. The boiled extract obtained from liver and kidney of rat injected with either pyrazinamide or pyrazinoic acid exhibited a potent inhibitory effect on the aminocarboxymuconate-semialdehyde decarboxylase (EC 4.1.1.45) activity in either liver or kidney, although pyrazinamide or pyrazinoic acid per se did not inhibit the enzyme activity. The unknown inhibitor of aminocarboxymuconate-semialdehyde decarboxylase was dialysable and heat-stable, and mostly excreted in urine by 6 and 12 h after injection of pyrazinoic acid and pyrazinamide, respectively. Pyrazine 2,3-dicarboxylic acid, pyrazine, nicotinamide, nicotinic acid, tryptophan, anthranilic acid, 5-hydroxyanthranilic acid and quinolinic acid exhibited no significant effect on the aminocarboxymuconate-semialdehyde decarboxylase activity in liver and kidney at the concentration of 1 mM in the reaction mixture. The expired ¹⁴CO₂ from l-[benzen ring-U-¹⁴C]tryptophan was markedly decreased by the pyrazinamide injection, while the urinary excretion of ¹⁴C-labeled metabolites from l-tryptophan, mainly quinolinic acid, was markedly increased. These results suggest that the glutarate pathway of l-tryptophan was strongly inhibited by the inhibitor produced after the administration of pyrazinoic acid and pyrazinamide. Pyrazinamide but not pyrazinoic acid also exhibited a significant inhibition of the nuclear enzyme poly(ADP-ribose) synthetase in rat liver.     

The effect of pyrazines on the metabolism of tryptophan and nicotinamide adenine dinucleotide in the rat. Evidence of the formation of a potent inhibitor of aminocarboxy-muconate-semialdehyde decarboxylase from pyrazinamide

The intraperitoneal or oral administration of pyrazinamide and pyrazinoic acid (pyrazine 2-carboxylic acid) resulted in a marked increase of the NAD content in rat liver. The injections of pyrazine and pyrazine 2,3-dicarboxylic acid exhibited no significant effect on the hepatic NAD content. The boiled extract obtained from liver and kidney of rat injected with either pyrazinamide or pyrazinoic acid exhibited a potent inhibitory effect on the aminocarboxymuconate-semialdehyde decarboxylase (EC 4.1.1.45) activity in either lier or kidney, although pyrazinamide or pyrazinoic acid per se did not inhibit the enzyme activity. The unknown inhibitor of aminocarboxymuconate-semialdehyde decarboxylase was dialysable and heat-stable, and mostly excreted in urine by 6 and 12 h after injected of pyrazinoic acid and pyrazinamide, respectively. Pyrazine 2,3-dicarboxylic acid, pyrazine, nicotinamide, nicotinic acid, tryptophan, anthranilic acid, 5-hydroxyanthranilic acid and quinolinic acid exhibited no significant effect on the aminocarboxymuconate-semialdehyde decarboxylase activity in liver and kidney at the concentration of 1 mM in the reaction mixture. The expired 14CO2 from L-[benzen ring-U-14C]tryptophan was markedly decreased by the pyrazinamide injection, while the urinary excretion of 14C-labeled metabolites from L-tryptophan, mainly quinolinic acid, was markedly increased. These results suggest that the glutarate pathway of L-tryptophan was strongly inhibited by the inhibitor produced after the administration of pyrazinoic acid and pyrazinamide. Pyrazinamide but not pyrazinoic acid also exhibited a significant inhibition of the nuclear enzyme poly(ADP-ribose) synthetase in rat liver.     

Localization of the cyclic ADP-ribose-dependent calcium signaling pathway in hepatocyte nucleus

CD38 is a type II transmembrane glycoprotein found on both hematopoietic and non-hematopoietic cells. It is known for its involvement in the metabolism of cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate, two nucleotides with calcium mobilizing activity independent of inositol trisphosphate. It is generally believed that CD38 is an integral protein with ectoenzymatic activities found mainly on the plasma membrane. Here we show that enzymatically active CD38 is present intracellularly on the nuclear envelope of rat hepatocytes. CD38 isolated from rat liver nuclei possessed both ADP-ribosyl cyclase and NADase activity. Immunofluorescence studies on rat liver cryosections and isolated nuclei localized CD38 to the nuclear envelope of hepatocytes. Subcellular localization via immunoelectron microscopy showed that CD38 is located on the inner nuclear envelope. The isolated nuclei sequestered calcium in an ATP-dependent manner. cADPR elicited a rapid calcium release from the loaded nuclei, which was independent of inositol trisphosphate and was inhibited by 8-amino-cADPR, a specific antagonist of cADPR, and ryanodine. However, nicotinic acid adenine dinucleotide phosphate failed to elicit any calcium release from the nuclear calcium stores. The nuclear localization of CD38 shown in this study suggests a novel role of CD38 in intracellular calcium signaling for non-hematopoietic cells.     

Stimulation of glucose transport in rat adipocytes by insulin, adenosine, nicotinic acid and hydrogen peroxide. Role of adenosine 3′:5′-cyclic monophosphate

Glucose transport into adipocytes of the rat was measured by monitoring the conversion of [1-(14)C]glucose into (14)CO(2). Glucose transport was made rate-limiting by increasing the flux through the pentose phosphate pathway with phenazine methosulphate, an agent that rapidly reoxidizes NADPH. Under these conditions, the observed rate of glucose disappearance from the incubation medium was about 20% higher than the rate of conversion of the C-1 of glucose into (14)CO(2). Apparent rates of glucose transport were significantly increased by insulin, H(2)O(2), adenosine and nicotinic acid. Stimulation of the apparent rate of glucose transport by insulin was dependent on adipocyte concentration, the hormone being most effective at relatively high cell concentrations. Adenosine and nicotinic acid further enhanced the maximum stimulation of glucose transport by insulin. Potentiation of insulin action by adenosine was more pronounced at lower cell concentrations. At relatively high cell concentrations the stimulatory action of insulin was markedly decreased by adenosine deaminase. Stimulation of apparent rates of glucose transport by the compounds noted above were antagonized by agents that increased intracellular cyclic AMP concentrations (theophylline and isoprenaline) and by dibutyryl cyclic AMP. Intracellular concentrations of cyclic AMP were significantly lowered when adipocytes were incubated with insulin, H(2)O(2), adenosine or nicotinic acid. These effects were observed under basal conditions or when intracellular cyclic AMP concentrations were elevated by theophylline or isoprenaline. On the basis of the above data, we suggest that insulin, H(2)O(2), adenosine and nicotinic acid may all stimulate glucose transport in rat adipocytes by lowering the intracellular cyclic AMP concentration. These data therefore support the hypothesis that cyclic AMP inhibits glucose transport in rat adipocytes.     

Cloning and characterization of the hamster and guinea pig nicotinic acid receptors

In this study, we present the identification and characterization of hamster and guinea pig nicotinic acid receptors. The hamster receptor shares approximately 80-90% identity with the nucleotide and amino acid sequences of human, mouse, and rat receptors. The guinea pig receptor shares 76-80% identity with the nucleotide and amino acid sequences of these other species. [(3)H]nicotinic acid binding affinity at guinea pig and hamster receptors is similar to that in human (dissociation constant = 121 nM for guinea pig, 72 nM for hamster, and 74 nM for human), as are potencies of nicotinic acid analogs in competition binding studies. Inhibition of forskolin-stimulated cAMP production by nicotinic acid and related analogs is also similar to the activity in the human receptor. Analysis of mRNA tissue distribution for the hamster and guinea pig nicotinic acid receptors shows expression across a number of tissues, with higher expression in adipose, lung, skeletal muscle, spleen, testis, and ovary.     

Regulation of rat and bovine adrenal metabolism of polycyclic aromatic hydrocarbons by adrenocorticotropin and 2,3,7,8-tetrachlorodibenzo-p-dioxin

The effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) on polycyclic aromatic hydrocarbon (PAH) metabolism and steroidogenesis in primary cultures of bovine adrenal cortical (BAC) and rat adrenal cortical (RAC) cells have been examined. Remarkably TCDD is an ineffective inducer (15-50%) of PAH metabolism in confluent BAC cells and completely antagonizes a 5-fold induction by benz[alpha]anthracene (BA). In the same concentration range (EC50 5 X 10(-11) M) TCDD suppresses steroidogenesis through an effect on cholesterol metabolism. Adrenocorticotropin (ACTH) and cAMP also suppress PAH metabolism at concentrations which stimulate steroidogenesis (10(-7) M). In RAC cells ACTH potently induces PAH metabolism (7-fold) at a comparable concentration to the stimulation of steroidogenesis. Parallel stimulation of PAH metabolism and steroidogenesis by cAMP suggest that ACTH induction of PAH metabolism is mediated by cAMP. TCDD induces PAH metabolism (2.8-fold, EC50 8 X 10(-11) M) at similar concentrations to the inhibitory effect in BAC cells and this action is additive with ACTH induction. In male rats in vivo TCDD induces adrenal microsomal PAH metabolism (72%) and is more effective in this respect than 3-methylcholanthrene (3MC). Rabbit antibodies against rat liver cytochrome P-450c (the major TCDD-inducible liver form) inhibited the TCDD-induced adrenal metabolism of 7,12-dimethylbenz[alpha]anthracene (DMBA), which also exhibited regioselectivity typical of metabolism by P-450c. Constitutive adrenal microsomal metabolism, which exhibited regioselectivity of DMBA metabolism comparable to the ACTH-sensitive cellular metabolism, was not affected by anti-P-450c. It is concluded that ACTH and TCDD induce distinct forms of cytochrome P-450 in RAC cells and that the latter represents a typical Ah-receptor mediated response. The anomalous effect on PAH metabolism in BAC cells that parallels inhibition of steroidogenesis may derive from repression of a distinct adrenal form of P-450 by the TCDD-Ah-receptor complex.     

Properties of Glycomacropeptide and Para-K-casein Derived from Human K-Casein and Comparison of Human and Bovine K-Caseins as to Susceptibility to Chymosin and Pepsin

The nutritional efficiency of quinolinic acid as a substitute for nicotinic acid was investigated using weanling rats Of the Sprague Dawley strain (3-weeks old) fed a nicotinic acid-free, tryptophan-limited diet containing various amounts of nicotinic acid or quinolinic acid. Judging from the growth response, food efficiency ratio, levels of NAD activity in the blood, liver, brain and upper small intestine, and urinary excretion of niacin we have concluded that exogenous quinolinic acid is approximately 1/9 as active as nicotinic acid. As many foods contain quinolinic acid, dietary quinolinic acid cannot be ignored from the standpoint of tryptophan and nicotinic acid replacement.     

Hormonal regulation of acid cholesterol ester hydrolase activity: effects of triiodothyronine and 17 alpha-ethynylestradiol

Cholesterol ester hydrolase activity was measured in isolated rat hepatocytes and adipocytes. Administration of triiodothyronine to rats resulted in a specific and selective increase in lysosomal acid (pH 4.5) cholesterol ester hydrolase activity in hepatocytes. Since the majority of lipoprotein degradation occurs in liver parenchymal cells (hepatocytes), the stimulation of liver (hepatocyte) acid cholesterol ester hydrolase activity by triiodothyronine could contribute to the hypocholesterolemic action of thyroid hormones. Treatment of rats with 17 alpha-ethynylestradiol to increase the hepatic degradation of lipoprotein did not change acid cholesterol ester hydrolase activity in liver, indicating that the thyroid hormone induced stimulation of acid cholesterol ester hydrolase activity in hepatocytes is not a secondary effect owing to the increased hepatic catabolism of low density lipoproteins (LDL). In contrast to the results with hepatocytes, hyperthyroidism did not increase acid cholesterol ester hydrolase activity in rat adipocytes.     

Nicotinic acid effect on acetyl-CoA-carboxylic activity in chicken liver

Changes of acetyl-CoA-carboxylase (EC 6.4.1.2) activity and the NAD content in the liver tissue were studied in dynamics after excessive administration of nicotinic acid to chickens. It is established that in chickens, which were given a high-carbohydrate diet after fasting, administration of nicotinic acid at first causes a fall of the acetyl-CoA-activity in the liver tissue, followed by its gradual rise against a background of the NAD content drop and by the 24th hour its level approaches the initial values. The maxima of NAD accumulation and of the acetyl-CoA-carboxylase activity decrease coincide in time. The administration of nicotinic acid to these chickens causes both a decrease in the intensity of 2-14C acetate incorporation into free fatty acids and a drop in their content.     

Influence of insulin on lysosomal activity and urea production in isolated parenchymal cells from rat liver.

Non-latent (free) activities of two lysosomal enzymes (acid phosphatase and beta-glucuronidase) and urea production were measured in purified rat liver parenchymal cells incubated in the presence and absence of insulin. Non-latent enzyme activity was measured by including 0.25M sucrose in the assay mixtures to provide osmotic protection to the lysosomes. Total enzyme activity was estimated with Triton X-100 in the homogenates. Insulin was found to inhibit ureogenesis and to reduce non-latent lysosomal enzyme activity in the hepatocytes in vitro. Our data support the idea that insulin inhibits autophagy in rat liver parenchymal cells. Such an effect of insulin may also explain the inhibitory action of insulin on urea production in the rat liver.     

Stimulation of phosphoenolpyruvate carboxykinase (guanosine triphosphate) activity by low concentrations of circulating glucose in perfused rat liver.

1. After nicotinic acid treatment, rat liver glycogen is depleted and phosphoenolpyruvate carboxykinase activity increased, to about twice the initial value. 2. The increase in phosphoenolpyruvate carboxykinase activity promoted by nicotinic acid is prevented by cycloheximide or actinomycin D, suggesting that this effect is produced by synthesis of the enzyme de novo. 3. Despite the enhancement of phosphoenolpyruvate carboxykinase activity and glycogen depletion, which occurs 5h after the injection of nicotinic acid, the gluconeogenic capacity of liver is low and considerably less than the values found in rats starved for 48h. 4. When the livers of well-fed rats are perfused in the presence of low concentrations of glucose, the activity of phosphoenolpyruvate carboxykinase significantly increases compared with the control. 5. This increase is not related to the glycogen content, but seems to be also the result of synthesis of the enzyme de novo, since this effect is counteracted by previous treatment with cycloheximide or actinomycin D. 6. Phosphoenolpyruvate carboxykinase activity is not increased in the presence of low concentrations of circulating glucose when 40 mM-imidazole (an activator of phosphodiesterase) is added to the perfusion medium. 7. Addition of dibutyryl cyclic AMP to the perfusion medium results in an increase in phosphoenolpyruvate carboxykinase activity, in spite of the presence of normal concentrations of circulating glucose. On the other hand, the concentration of cyclic AMP in the liver increases when that of glucose in the medium is low. 8. These results suggest that, in the absence of hormonal factors, the regulation of phosphoenolpyruvate carboxykinase can be accomplished by glucose itself, inadequate concentrations of it resulting in the induction of the enzyme. The mediator in this regulation, as in hormonal regulation, seems to be cyclic AMP.     

Effect of nicotinic acid on zinc and iron metabolism

Nicotinic acid has functional groups capable of forming complexes with trace metals. The present study examines the effect of nicotinic acid supplementation on absorption and utilization of zinc and iron. In vitro zinc uptake by human erythrocytes was studied using blood samples of 10 healthy subjects. It was found that 8 moles nicotinic acid or NADP increased ⁶⁵Zn uptake by 38.9% and 43.1% in fasting, and by 70.9% and 28.1% in postprandial conditions. In animal experiments, nicotinic acid supplementation to finger millet based diet resulted in significant enhancement of percent zinc absorption, liver zinc and growth of weanling mice (P < 0.05). When mice were fed with nicotinic acid-deficient, -adequate and -excess synthetic diets for four weeks it was observed that, in comparison with the nicotinic acid-deficient diet, percent zinc absorption, intestinal zinc, percent haeomoglobin and liver iron increased significantly under nicotinic acid-adequate and -excess conditions. The results obtained suggested that nicotinic acid, in addition to its known effect on growth and metabolism, may be playing an important role in enhancing zinc and iron utilization.     

Quinolinic Acid Phosphoribosyltransferase in Rat Brain

Because of the possible participation of quinolinic acid in brain function and/or dysfunction, the characteristics of its catabolic enzyme, quinolinic acid phosphoribosyltransferase (QPRTase; EC 2.4.2.19), were examined in rat brain tissue. For this purpose, a sensitive radiochemical assay method, based on the conversion of quinolinic acid to nicotinic acid mononucleotide (NAMN), was developed. For brain QPRTase, the Mg2+ dependency, substrate specificity, and optimal assay conditions were virtually identical to those of the liver enzyme. Kinetic analyses of brain QPRTase revealed a Km of 3.17 +/- 0.30 microM for quinolinic acid and Km = 65.13 +/- 13.74 microM for the cosubstrate phosphoribosylpyrophosphate. The respective Vmax values were: 0.91 +/- 0.08 pmol NAMN/h/mg tissue for quinolinic acid and 11.65 +/- 1.55 fmol NAMN/h/mg tissue for phosphoribosylpyrophosphate. All kinetic parameters measured for the brain enzyme were significantly different from those determined for liver QPRTase, indicating structural differences or distinct regulatory processes for the brain and liver enzymes. Phthalic acid was a potent competitive inhibitor of brain QPRTase. Examination of the regional distribution of QPRTase in the rat CNS and retina indicated a greater than 20-fold difference between the area displaying the highest activity (olfactory bulb) and those of only moderate activity (frontal cortex, striatum, retina, hippo-campus). Enzyme activity was present at the earliest age tested, 2 days, and tended to increase in older animals. Brain QPRTase activity was preferentially located in the nerve-ending (synaptosomal) fraction. Enzyme activity was stable over extensive periods of storage at -80 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)