Stimulation of thiamine diphosphate activity by ascorbic acid in rat brain microsomes

The effect of ascorbic acid on microsomal thiamine diphosphate activity in rat brain was examined. Ascorbic acid at 0.02–0.1 mM increased the thiamine diphosphate activity by 20–600% and produced a significant amount of lipid peroxide, which was measured with thiobarbiturate under the same conditions as the enzyme. A lag period of about 10 min was observed in the process of stimulation of enzyme activity by ascorbic acid. The stimulation of enzyme activity and the lipid peroxidation induced by ascorbic acid were blocked by metal-binding compounds (EDTA, α,α′-dipyridyl, o-phenanthroline) and an antioxidant (N,N′-diphenyl p-phenylenediamine). GSH significantly enhanced the stimulation of enzyme activity and formation of lipid peroxide by 0.02–0.05 mM ascorbic acid. The effect of GSH was due in part to maintenance of the concentration of ascorbic acid in the medium, since GSH could convert dehydroascorbic acid, an oxidized form of ascorbic acid, to ascorbic acid.     

Possible regulation of thiamine diphosphatase activity in rat brain microsomes by lipids.

The effects of various treatments, which affect membrane structure, on microsomal thiamine diphosphatase and thiamine triphosphatase activities of rat brain, were examined. The treatment of micorosomes at alkaline pH caused a 2-fold activation of the thiamine diphosphatase, this being related to a change in membrane structure which was evidenced by a decrease of the turbidity of the microsomal suspension. Repeated freezing and thawing after hypo-osmotic treatment also increased the activity of microsomal thiamine diphosphatase. In addition, the thiamine diphosphatase activity was enhanced by treatment of the microsomes with phospholipase C or acetone. This lipid depletion resulted in a marked reduction in the apparent Km value of the thiamine diphosphatase with a corresponding loss in heat stability of the enzyme. We found further that brain thiamine diphosphatase was solubilized by Triton X-100. This decreased the phospholipid content in the preparation, but did not affect the apparent Km value and heat stability of the enzyme. In contrast with thiamine diphosphatase, thiamine triphosphatase was inactivated by treatment at alkaline pH or with acetone. However, treatment with phospholipase C did not affect the activity of thiamine triphosphatase.     

Characteristics of an acid active thiamine diphosphatase from beef brain

That thiamine has a role in nerve conduction as well as synaptic transmission is suggested by the following observations. (1) Thiamine phosphate esters are hydrolyzed and released from nerve membranes during nerve conduction. (2) Ultraviolet radiation of single nerve fibers at the wavelength specific for thiamine destroys the ability of that nerve to conduct an impulse. (3) Thiamine diphosphatase (TDPase) is present on synaptosomes. Previous articles have characterized an alkaline active TDPase in brain; this report characterizes a pH 5 active TDPase and compares its properties to the pH 9 enzyme. Both enzymes require a divalent cation for optimal activity. The pH 5 enzyme is more sensitive to ATP. Myelin fractions of brain have the highest specific activity for the acid TDPase, and the nerve ending particles the highest total activity. No PO₄ ^3– inhibition was observed. Kinetic constants of this enzyme activity are reported.     

Enhancement of hydroperoxide-dependent lipid peroxidation in rat liver microsomes by ascorbic acid

Simultaneous addition of ascorbic acid and organic hydroperoxides to rat liver microsomes resulted in enhanced lipid peroxidation (approximately threefold) relative to incubation of organic hydroperoxides with microsomes alone. No lipid peroxidation was evident in incubations of ascorbate alone with microsomes. The stimulatory effect of ascorbate on linoleic acid hydroperoxide (LAHP)-dependent peroxidation was evident at all times whereas stimulation of cumene hydroperoxide (CHP)-dependent peroxidation occurred after a lag phase of up to 20 min. EDTA did not inhibit CHP-dependent lipid peroxidation but completely abolished ascorbate enhancement of lipid peroxidation. Likewise, EDTA did not significantly inhibit peroxidation by LAHP but dramatically reduced ascorbate enhancement of lipid peroxidation. The results reveal a synergistic prooxidant effect of ascorbic acid on hydroperoxide-dependent lipid peroxidation. The inhibitory effect of EDTA on enhanced peroxidation suggests a possible role for endogenous metals mobilized by hydroperoxide-dependent oxidations of microsomal components.     

Stimulation by unsaturated fatty acid of squalene uptake in rat liver microsomes

Supernatant protein factor (SPF) and anionic phospholipids such as phosphatidylglycerol (PG) stimulate squalene epoxidase activity in rat liver microsomes by promoting [3H]squalene uptake as well as substrate translocation (Chin, J., and K. Bloch. 1984. J. Biol. Chem. 259: 11735-11738). This process is postulated to be membrane-mediated and not carrier-mediated. Here we show that treatment of PG with phospholipase A2 in the presence of bovine serum albumin abolishes the stimulatory effect of SPF on epoxidase activity. Disaturated fatty acyl-PGs are not as effective as egg yolk lecithin PG in the SPF effect. These findings suggest an important role for the unsaturated fatty acid moiety of PG. We also show that at submicellar concentrations, cis-unsaturated fatty acids stimulate microsomal epoxidase activity whereas saturated fatty acids do not. This effect is due to an increase in substrate uptake which in turn may facilitate substrate availability to the enzyme.     

Stimulation of acyl-CoA:cholesterol acyltransferase activity in rat liver microsomes by phosphatidylcholine

Acyl-CoA:cholesterol acyltransferase (ACCAT) activity of rat liver microsomes was stimulated by phosphatidylcholine. The stimulatory effect varied with the composition of the phosphatide: dimyristyl-, dipalmityl-, distearyl- and dioleylphosphatidylcholine were stimulatory, whereas dicaproyl- and dilinoleylphosphatidylcholine were not. The results suggest that increased fluidity of the membrane induced by phosphatide is probably not involved in the stimulation of cholesterol esterification. Phosphatide exerted its effect directly on the microsomes and did not extract cholesterol or ACCAT from the microsomes to an appreciable extent.Hydrolysis of microsomal phosphatide suppressed ACCAT activity. Enztme activity was restored with the addition of phosphatidylcholine. The results suggest that phosphatide may be required for cholesterol esterification.     

Increase in nucleoside diphosphatase in rat brain striatum lesioned with kainic acid

The activity of ammoniagenesis from guanine nucleotides was found to increase significantly in rat brain after infusion of kainic acid into the striatum. Among the enzymes involved in degrading guanine nucleotides, nucleoside diphosphatase was markedly increased in the lesioned striatum. The enzyme activity began to increase 2 days after the infusion, and reached the maximum on the 13th day, the level being 4 times as high as that of the intact contralateral region. The increased activity was due to Type L enzyme, judging from its substrate specificity. Puromycin and cycloheximide inhibited this increase, indicating that the increased activity resulted from an increase in the net synthesis of the enzyme. These findings suggest that Type L NDPase might play some important roles in gliosis after neuronal lesion.     

Characterization of fatty acid desaturase activity in rat lung microsomes.

Preparations of rat lung microsomes containing 0.030-0.050 nmole of cytochromes P-450 and b5 per mg microsomal protein have been observed to contain significant levels of fatty acid desaturase activity. Both stearoyl CoA and palmitoyl CoA are desaturated to their monounsaturated analogues, oleic acid and palmitoleic acid, respectively. Activity (per mg microsomal protein) of the lung preparations varied according to the diet of the animals prior to killing in the order: fat free diet greater than normal rat chow greater than starvation. All preparations exhibited approximately 50% inhibition when incubated in the presence of 0.10 mM CN-. Maximal activity was obtained with the 0.50 mM NADH less activity with equal amounts of NADPH, and there was no synergistic interaction of NADH and NADPH together. The rate of desaturation was linear with protein concentrations between 0.15-1.5 mg microsomal protein/incubation at incubation times up to 8 min. A pH optimum range of 7.0-7.4 was observed. For all variables of fatty acid desaturase activity which were examined, the rate of desaturation of stearoyl CoA was approximately twice that for palmitoyl CoA. These results indicate that the same fatty acid desaturation system which is functional in the liver is also present in significant amounts in mammalian lungs.     

Thiamine pyrophosphatase and nucleoside diphosphatase in rat brain

Two types of nucleoside diphosphatase were found in rat brain. One (Type L) had similar properties to those of the liver microsomal enzyme with respect to its isoelectric point, substrate specificity, Km values, optimum pH, activation by ATP and molecular weight. The other (Type B), which separated into multiple forms on isoelectric focusing, had lower Km values and a smaller molecular weight than the Type L enzyme, and was inhibited by ATP. The Type B enzyme catalyzed the hydrolysis of thiamine pyrophosphate as well as those of various nucleoside diphosphates at physiological pH, while Type L showed only nucleoside diphosphatase activity at neutral pH. These findings suggest that the two enzymes play different physiological roles in the brain.     

Demonstration of plasma-membrane adenosine diphosphatase activity in rat lung

The adenosine diphosphatase (ADPase) activity of rat lung has been investigated. Subcellular fractionation of lung tissue homogenates by sucrose density gradient centrifugation has shown the ADPase activity to be associated with the plasma membrane. ADPase was solubilised from the membranes and fractionated by ammonium sulphate precipitation to separate a specific, low-Km ADPase from non-specific alkaline phosphatase activity. The solubilised ADPase has a Km of 50 microM at pH 7.5 and appears to be distinct from ATPase.     

Tissue distribution of adenosine diphosphatase activity in the rat

The specific activity of adenosine diphosphatase (ADPase) was determined in rat tissue homogenates using a specific radioassay. 14 different tissues were sampled and activity was found in most homogenates examined. The levels ranged from very little activity in the pancreas to high levels in the duodenum and ileum. High levels of ADPase activity found in the gastrointestinal tract could be partially attributed to non-specific alkaline phosphatase. It is concluded that ADPase activity is widely distributed in rat tissues.     

Hydrolysis of neutral glycerides by lipases of rat brain microsomes.

The hydrolysis of monoacylglycerol and diacylglycerol by rat brain microsomes was followed by measuring the release of glycerol and monooleylglycerol from dispersions of water insoluble glyceryl esters of oleic acid. The microsomes showed three lipolytic activities. One activity, optimal at pH 4.8, catalyzed the hydrolysis of diacylglycerol but not monoacylglycerol. Two other lipolytic activities, optimal at pH 8.0-8.6, catalyzed the hydrolysis of both diacylglycerol and monoacylglycerol. The pH 8.0-8.6 activities were sensitive to heat and SH-reagents. Detergents were inhibitory in all cases. Extraction of the microsomes with KCl, KSCN, urea or Triton X-100 did not change the ratio of diacylglycerol hydrolysis at pH 4.8 and 8.0. The results of subcellular fractionation studies showed that there was no significant enrichment of the acid lipase in any fraction.     

Regeneration of ascorbic acid by rat colon

Plants and animals alike use ascorbic acid in a variety of reactions that result in net generation of dehydro-L-ascorbic acid. The ability to reduce dehydro-L-ascorbic acid back to ascorbic acid would conserve "total ascorbate" and would help to maintain the toxic oxidized form of the molecule at a low level. This study evaluated the rate of dehydro-L-ascorbic acid reduction either by following the rate of NADPH consumption or by analysis of the amount of 14C-labeled dehydro-L-ascorbic acid converted to ascorbic acid. A large percentage of the NADPH consumed by a semipurified preparation of rat colonic mucosa in vitro was dependent on the presence of dehydro-L-ascorbic acid. The tissue factor active in regenerating ascorbic acid is intermediate in size between cytochrome c and blue dextran. The present results indicate that the mucosa reduced dehydro-L-ascorbic acid by a cytosolic enzyme that uses NADPH as a hydrogen donor. Subsequent to precipitation by ammonium sulfate, the 55-70% fraction contains most of the reductase activity while consisting of only 17% of the cellular soluble protein.     

Effect of ascorbic acid on heme metabolism in hepatic microsomes

Hepatic microsonal cytochrome P-450 levels are significantly decreased (46–68%) in ascorbic acid-deficient guinea pigs. Studies attempting to elucidate the mechanism responsible for decreased cytochrome P-450 demonstrated that ascorbic acid status did not influence the turnover $\text{(}\text{t}\text{1}\text{2}\text{)}$ or the degradation of hepatic cytochrome P-450 heme. Urinary excretion of Δ-aminolevulinic acid (ALA) and coproporphyrin was significantly decreased (30 and 69% respectively) in ascorbic acid-deficient guinea pigs. Injections (ip) of ALA into ascorbic acid-deficient guinea pigs were not effective in returning cytochrome P-450 levels to values found in ascorbic acid-supplemented guinea pigs. In addition, plasma and hepatic iron and blood heme were related directly, while hepatic copper and plasma copper or ceruloplasmin were related inversely, to the ascorbic-acid status of the guinea pig. These data, along with past investigations on heme synthesis in the ascorbic acid-deficient guinea pig, are consistent with mechanisms proposing that ascorbic acid may influence: 1) apocytochrome P-450 synthesis, 2) binding of heme and apo-cytochrome P-450 to form active cytochrome P-450, and/or 3) incorporation of Fe++ into the heme moiety of cytochrome P-450, perhaps via changes in copper metabolism.