Vitamin K-dependent carboxylase: the carboxylation of exogenous substrates in different systems

Two types of solid-phase carboxylase, SPC-II and SPC-X, have been prepared from the livers of warfarin-treated cows. Their enzymatic activities were compared with substrate-free carboxylase in microsomes from normal cows and substrate-bound carboxylase in microsomes from warfarin-treated cows. A number of exogenous substrates for carboxylase have been purified and tested. We found that large substrates, such as descarboxyprothrombin, are carboxylated only by substrate-free carboxylase and not by the substrate-bound enzyme. No differences in apparent Km values between solid-phase carboxylases II and X were observed.     

Molecular species composition of membrane phosphatidylcholine influences the rate of cholesterol efflux from human erythrocytes and vesicles of erythrocyte lipid

The efflux of [3H]cholesterol from prelabelled human erythrocytes having modified phosphatidylcholine compositions was measured during 24-h incubations in the presence of unlabelled acceptor liposomes composed of equimolar amounts of egg phosphatidylcholine and cholesterol. The cells were modified by replacement of part of the native phosphatidylcholine with either dipalmitoylphosphatidylcholine, palmitoyloleoylphosphatidylcholine or dilinoleoylphosphatidylcholine catalyzed by phosphatidylcholine-specific transfer protein from bovine liver. The results indicated that the efflux of [3H]cholesterol was faster from erythrocytes in which the dipalmitoylphosphatidylcholine content was increased from 7 to 25% of the total, than from cells enriched in palmitoyloleoylphosphatidylcholine or dioleoylphosphatidylcholine. Incorporation of dilinoleoylphosphatidylcholine to a level of 13% of the total phosphatidylcholine slowed the rate of efflux of [3H]sterol. The phosphatidylcholine replacements produced no significant differences in cholesterol/phospholipid ratio before or after 24 h of incubation with the acceptor egg phosphatidylcholine-cholesterol vesicles. Using vesicles prepared from erythrocyte lipid, modified to reflect the changes in the phosphatidylcholine composition induced in the whole cells, the same influence of composition on the rate of cholesterol exchange was evident. Enhancement of the dipalmitoylphosphatidylcholine content from 7 to 25% of the total phosphatidylcholine pool increased the rate of [3H]cholesterol efflux, while the addition of the same amount of dilinoleoylphosphatidylcholine slowed it compared to controls. The magnitude of the effect was comparable in intact cells and erythrocyte lipid vesicles enriched in dipalmitoylphosphatidylcholine, while the influence of dilinoleoylphosphatidylcholine was more marked in the intact cells. These results demonstrate that changes in the molecular species composition of the phosphatidylcholine pool can influence the rate of exchange of cholesterol but not necessarily the cellular content of sterol in the human erythrocyte. The influence of this phospholipid appears to be expressed independently of the presence of membrane protein or an underlying cytoskeleton.     

Peroxisomal beta-oxidation enzyme proteins in the Zellweger syndrome

The absence of peroxisomes in patients with the cerebro-hepato-renal (Zellweger) syndrome is accompanied by a number of biochemical abnormalities, including an accumulation of very long-chain fatty acids. We show by immunoblotting that there is a marked deficiency in livers from patients with the Zellweger syndrome of the peroxisomal beta-oxidation enzyme proteins acyl-CoA oxidase, the bifunctional protein with enoyl-CoA hydratase and 3-hydroxyacyl-CoA dehydrogenase activities and 3-oxoacyl-CoA thiolase. Using anti-(acyl-CoA oxidase), increased amounts of cross-reactive material of low Mr were seen in the patients. With anti-(oxoacyl-CoA thiolase), high Mr cross-reactive material, presumably representing precursor forms of 3-oxoacyl-CoA thiolase, was detected in the patients. Catalase protein was not deficient, in accordance with the finding that catalase activity is not diminished in the patients. Thus in contrast to the situation with catalase functional peroxisomes are required for the stability and normal activity of peroxisomal beta-oxidation enzymes.     

Lipid molecular shape affects erythrocyte morphology: a study involving replacement of native phosphatidylcholine with different species followed by treatment of cells with sphingomyelinase C or phospholipase A2

In a previous report it was shown that the replacement of native erythrocyte phosphatidylcholine (PC) with different PC species which have defined acyl chain compositions can lead to morphological changes (Kuypers, F.A., W. Berendsen, B. Roelofsen, J. A. F. Op den Kamp, and L.L.M. van Deenen, 1984, J. Cell Biol., 99:2260-2267). It was proposed that differences in molecular shape between the introduced PC species and normal erythrocyte PC caused the membrane to bend outwards or inwards, depending on the shape of the PC exchanged. To support this proposal, two requirements would have to be fulfilled: the exchange reaction would take place only with the outer lipid monolayer of the erythrocyte, and the extent of lipid transbilayer movement would be restricted. If this theory is correct, any treatment causing unilateral changes in lipid molecular shape should lead to predictable morphological changes. Since this hypothesis is a refinement of the coupled bilayer hypothesis, but so far lacks experimental support, we have sought other means to change lipid molecular shape unilaterally. Shape changes of human erythrocytes were induced by the replacement of native PC by various PC species using a phosphatidylcholine-specific transfer protein: by hydrolysis of phospholipids in intact cells using sphingomyelinase C or phospholipase A2, and by the combination of both procedures. The morphological changes were predictable; additive when both treatments were applied, and explicable on the basis of the geometry of the lipid molecules involved. The results strongly support the notion that lipid molecular shape affects erythrocyte morphology.     

Purification and enzymatic characterization of CMP-sialic acid: beta-galactosyl1----3-N-acetylgalactosaminide alpha 2----3-sialyltransferase from human placenta

A CMP-NeuAc:Gal beta 1----3GalNAc-R alpha 2----3-sialyltransferase has been purified over 20,000-fold from a Triton X-100 extract of human placenta by affinity chromatography on concanavalin A-Sepharose and CDP-hexanolamine-Sepharose in a yield of 10%. Sodium dodecyl sulfate-gel electrophoresis under reducing conditions revealed that the enzyme consists of a major polypeptide species with a molecular weight of 41,000 and some minor forms with molecular weights of 40,000, 43,000, and 65,000, respectively, which can be resolved partially by gel filtration on Sephadex G-100. Isoelectric focusing revealed that the enzyme occurs in a major and a minor charged form with pI values of 5.0-5.5 and 6.0, respectively. Acceptor specificity studies indicated that the enzyme catalyzes the incorporation of sialic acid from CMP-NeuAc into glycoproteins, glycolipids, and oligosaccharides which possess a terminal Gal beta----3GalNAc unit. Analysis of the structure of the product chain by high-pressure liquid chromatography and thin layer chromatography as well as methylation analysis revealed that a NeuAc alpha 2----3Gal beta 1----3GalNAc sequence is elaborated. The best glycoprotein acceptors are antifreeze glycoprotein and porcine submaxillary asialo/afucomucin. The disaccharide Gal beta 1----3GalNAc-Thr shows values for Km and V which are close to those of the latter glycoprotein. Lactose as well as oligosaccharides in which galactose is linked beta 1----3 or beta 1----4 to N-acetylglucosamine are less efficient acceptors. Of the glycolipids tested only gangliosides GM1 and GD1b served as an acceptor. The enzyme does not show an absolute aglycon specificity, and attaches sialic acid regardless the anomeric configuration of the N-acetylgalactosaminyl residue in the accepting Gal beta 1----3GalNAc unit. By use of specific acceptor substrates it could be demonstrated that the purified enzyme is free from other known sialyltransferase activities. Studies with rabbit antibodies raised against a partially purified sialyltransferase preparation indicated that the enzyme is immunologically unrelated to a Gal beta 1----4GlcNAc-R alpha 2----3-sialyltransferase, which previously had been identified in human placenta (Van den Eijnden, D.H., and Schiphorst, W. E. C. M. (1981) J. Biol. Chem. 256, 3159-3162). Initial-rate kinetic studies suggest that the sialyltransferase operates through a mechanism involving a ternary complex of enzyme, sugar donor, and acceptor. This is the first report on the extensive purification and characterization of a sialyltransferase from a human tissue.     

Presence of markers for virulence in the unique short region or repeat region or both of pseudorabies hybrid viruses.

The unique short region and part of the repeat region of virulent pseudorabies virus strain NIA-3 was replaced by the corresponding region of the avirulent NIA-4 strain by transfection with subgenomic DNA fragments. The resulting hybrid virus showed a reduced virulence in both mice and pigs. Therefore, important markers for virulence are located in the unique short or repeat region or both of pseudorabies virus. We provide evidence that the terminally located repeat is not required for the generation of progeny with intact pseudorabies virus genomes. Apparently, the terminal repeat is regenerated from the internal repeat.     

Free space iron pools in roots: generation and mobilization

A rapid and simple method for the determination of a ferric iron pool in the free space of roots is described. Formation of this pool depended on the source of iron in the nutrient solution. During growth in water culture at pH 5 to 6 with Fe-ethylenediaminetetraacetate, a free space pool of 500 to 1000 nanomoles Fe per gram fresh weight was formed in the roots of bean (Phaseolus vulgaris L. var. Prélude), maize (Zea mays L. var. Capella), and chlorophytum (Chlorophytum comosum [Thunb.] Jacques). No significant pool (less than 100 nanomoles per gram fresh weight) was formed with ferrioxamine. Upon impending Fe deficiency, bean and chlorophytum were able to mobilize this pool. Fe-deficient bean plants mobilized iron from the free space iron pool of another plant in the same vessel.     

Purification and Characterization of the Pea Chloroplast Pyruvate Dehydrogenase Complex : A Source of Acetyl-CoA and NADH for Fatty Acid Biosynthesis

The pyruvate dehydrogenase complex has been purified 76-fold, to a specific activity of 0.6 μmoles per minute per milligram protein, beginning with isolated pea (Pisum sativum L. var Little Marvel) chloroplasts. Purification was accomplished by rate zonal sedimentation, polyethyleneglycol precipitation, and ethyl-agarose affinity chromatography. Characterization of the substrates as pyruvate, NAD⁺, and coenzyme-A and the products as NADH, CO₂, and acetyl-CoA, in a 1:1:1 stoichiometry unequivocally established that activity was the result of the pyruvate dehydrogenase complex. Immunochemical analysis demonstrated significant differences in structure and organization between the chloroplast pyruvate dehydrogenase complex and the more thoroughly characterized mitochondrial complex. Chloroplast complex has a higher magnesium requirement and a more alkaline pH optimum than mitochondrial complex, and these properties are consistent with light-mediated regulation in vivo. The chloroplast pyruvate dehydrogenase complex is not, however, regulated by ATP-dependent inactivation. The properties and subcellular localization of the chloroplast pyruvate dehydrogenase complex are consistent with its role of providing acetyl-CoA and NADH for fatty acid synthesis.