Cellular distribution of nicotinamide adenine dinucleotide glycohydrolase in the central nervous system

Latex beads phagocytosis capacity and ecto-NAD+ glycohydrolase activity of rat brain microglia cells in vitro

Conditions are described which allow the preparation in vitro of pure (greater than 95%) microglial cell cultures isolated from newborn rat brain. Such ameboid cells cultivated in vitro can efficiently phagocytize opsonized latex beads and are capable of ingesting more (100-200 beads of 1.1 micron diameter per cell) and larger (6.4 microns) particles than other nerve cells, such as oligodendrocytes and astroglia. The microglial cells also show an important ecto-NAD+ glycohydrolase activity which is characteristic of phagocytic cells. We noted that the phagocytic capacity and ecto-NAD+ glycohydrolase of these cells were correlated and increased notably during the in vitro culture. Microglia cultivated in vitro appear to be a good model to study the activation of phagocytic properties in the central nervous system and corresponding modulation by natural or pharmacological immunomodulators.

Topography, purification and characterization of thyroidal NAD+ glycohydrolase

Subcellular fractionation of bovine thyroid tissue by differential pelleting and isopycnic gradient centrifugation in a zonal rotor indicated that NAD(+) glycohydrolase is predominantly located and rather uniformly distributed in the plasma membrane. Comparison of NAD(+) glycohydrolase activities of intact thyroid tissue slices, functional rat thyroid cells in culture (FRT(l)) and their respective homogenates indicated that most if not all of the enzyme (catalytic site) is accessible to extracellular NAD(+). The reaction product nicotinamide was predominantly recovered from the extracellular medium. The diazonium salt of sulphanilic acid, not penetrating into intact cells, was able to decrease the activity of intact thyroid tissue slices to the same extent as in the homogenate. Under the same conditions this reagent almost completely abolished NAD(+) glycohydrolase activity associated with intact thyroid cells in culture. The triazine dye Cibacron Blue F3GA and its high-M(r) derivative Blue Dextran respectively completely eliminated or caused a severe depression in the NAD(+) glycohydrolase activity of FRT(l) cells. The enzyme could be readily solubilized from bovine thyroid membranes by detergent extraction, and was further purified by gel filtration and affinity chromatography on Blue Sepharose CL-6B. The overall procedure resulted in a 1940-fold purification (specific activity 77.6mumol of nicotinamide released/h per mg). The purified enzyme displays a K(m) of 0.40mm for beta-NAD(+), a broad pH optimum around pH7.2 (0.1 m-potassium phosphate buffer) and an apparent M(r) of 120000. Nicotinamide is an inhibitor (K(i) 1.9mm) of the non-competitive type. The second reaction product ADP-ribose acts as a competitive inhibitor (K(i) 2.7mm). The purified enzyme splits beta-NAD(+), beta-NADP(+), beta-NADH and alpha-NAD(+) at rates in the relative proportions 1:0.75:<0.02:<0.02 and exhibits transglycosidase (pyridine-base exchange) activity. Anionic phospholipids such as phosphatidylinositol and phosphatidylserine inhibit the partially purified enzyme. A stimulating effect was observed upon the addition of histones.

NAD+ glycohydrolase, an ecto-enzyme of calf spleen cells

By using a sensitive fluorimetric assay of NAD+ glycohydrolase (EC 3.2.2.6), we showed that calf spleen cells are able to hydrolyse 1,N6-etheno-NAD+ given in the medium. The observed rates of substrate hydrolysis and product accumulation in the medium are equivalent. Moreover, the splenocytes are able to cleave the nicotinamide-ribose bond of a water-soluble polymer of NAD+, and their NAD+ glycohydrolase activity is fully inhibited by a high-molecular-weight Blue Dextran. Selective permeation of the cellular membrane digitonin revealed an intracellular pool of NAD+ glycohydrolase, which accounts for 25% of the total activity. We conclude that NAD+ glycohydrolase associated with the splenocytes has the characteristics of an ecto-enzyme.

NAD glycohydrolase: enzyme characterization using intact mammalian erythrocytes

1. NADase activity has been determined on intact erythrocytes of several species. 2. Although a wide range in maximum velocity exists across species, Michaelis constants observed are very similar. 3. The enzyme is found on the outer surface of the erythrocyte plasma membrane. 4. It is inhibited by substrate after an apparent permanent modification. 5. This modification may be due to self ADP-ribosylation. 6. We have also demonstrated the presence of an ADP-ribosyltransferase on the outer surface of the sheep erythrocyte membrane.

Studies on the association of NAD glycohydrolase with membranes in calf spleen

The subcellular distribution of NAD glycohydrolase was studied by fractionation of calf spleen homogenates using differential and discontinuous density gradient centrifugations. The highest amount of NAD glycohydrolase activity was associated with microsomes, which in this tissue were found to contain, in addition to endoplasmic reticulum, a large proportion of vesicles derived from plasma membranes. The distribution pattern of NAD glycohydrolase was found to parallel that of plasma membrane markers. When microsomal vesicles were treated with digitonin, NAD glycohydrolase activity and plasma membranes specifically increased in density. We conclude that in calf spleen the bulk of NAD glycohydrolase is associated with plasma membranes. Microsomal NAD glycohydrolase was associated with sealed vesicles; its activity could not be increased by disruption of the sidedness of the vesicles. This result and further observations based on the known restricted permeability of biological membranes to charged substances, and on the activity of the enzyme with non-penetrating substrates and inhibitors, indicate that the NAD glycohydrolase active site is located on the exterior side of the vesicles. It is proposed that calf spleen NAD glycohydrolase is an ecto-enzyme.