UDP-galactose hydrolysis in brain and its effect on cerebroside synthesis

d-Galactose-Esterification of a Fungal Polyketide Catalyzed by a Carnitine Acyltransferase Domain

While sugar-containing natural products are commonly biosynthesized via glycosyltranferases using sugar-UDP as the electrophile, nature has evolved alternative strategies of glyco-modification to expand the diversity of natural products. Hydroxyl groups on sugars can serve as nucleophiles in the release of polyketide products from polyketide synthases. Herein, we demonstrate a highly reducing polyketide synthase (HRPKS) from the biocontrol fungus Trichoderma afroharzianum T22, which is terminated with a carnitine acyltransferase (cAT) domain, catalyzes the biosynthesis of a d-galactose esterified polyketide named as trichogalactin. Structure-guided enzymatic assays showed that the sugar nucleophile in the esterification reaction catalyzed by cAT is α-d-galactose-1-phosphate (Gal-1-P) instead of free d-galactose. The released product, trichogalactin phosphate, is subsequently dephosphorylated by a host alkaline phosphatase to complete the biosynthesis of trichogalactin. The cAT domain is highly specific for Gal-1-P and does not accept α-d-glucose-1-phosphate or α-d-mannose-1-phosphate. Our study expands the inventory of natural products from an agriculturally important fungus and demonstrates the potential of mining cAT-containing HRPKSs to discover new glyco-esterified natural products.

Dinochromosome Heterotermini with Telosomal Anchorages

Dinoflagellate birefringent chromosomes (BfCs) contain some of the largest known genomes, yet they lack typical nucleosomal micrococcal-nuclease protection patterns despite containing variant core histones. One BfC end interacts with extranuclear mitotic microtubules at the nuclear envelope (NE), which remains intact throughout the cell cycle. Ultrastructural studies, polarized light and fluorescence microscopy, and micrococcal nuclease-resistant profiles (MNRPs) revealed that NE-associated chromosome ends persisted post-mitosis. Histone H3K9me3 inhibition caused S-G2 delay in synchronous cells, without any effects at G1. Differential labeling and nuclear envelope swelling upon decompaction indicate an extension of the inner compartment into telosomal anchorages (TAs). Additionally, limited effects of low-concentration sirtinol on bulk BfCs, coupled with distinct mobility patterns in MNase-digested and psoralen-crosslinked nuclei observed on 2D gels, suggest that telomeric nucleosomes (TNs) are the primary histone structures. The absence of a nucleosomal ladder with cDNA probes, the presence of histone H2A and telomere-enriched H3.3 variants, along with the immuno-localization of H3 variants mainly at the NE further reinforce telomeric regions as the main nucleosomal domains. Cumulative biochemical and molecular analyses suggest that telomeric repeats constitute the major octameric MNRPs that provision chromosomal anchorage at the NE.

Ribose 1-phosphate and inosine activate uracil salvage in rat brain

The purpose of this study was to determine the mechanism by which inosine activates pyrimidine salvage in CNS. The levels of cerebral inosine, hypoxanthine, uridine, uracil, ribose 1-phosphate and inorganic phosphate were determined, to evaluate the Gibbs free energy changes (deltaG) of the reactions catalyzed by purine nucleoside phosphorylase and uridine phosphorylase, respectively. A deltaG value of 0.59 kcal/mol for the combined reaction inosine+uracil <==> uridine+hypoxanthine was obtained, suggesting that at least in anoxic brain the system may readily respond to metabolite fluctuations. If purine nucleoside phosphorolysis and uridine phosphorolysis are coupled to uridine phosphorylation, catalyzed by uridine kinase, whose activity is relatively high in brain, the three enzyme activities will constitute a pyrimidine salvage pathway in which ribose 1-phosphate plays a pivotal role. CTP, presumably the last product of the pathway, and, to a lesser extent, UTP, exert inhibition on rat brain uridine nucleotides salvage synthesis, most likely at the level of the kinase reaction. On the contrary ATP and GTP are specific phosphate donors.

Characterization and developmental expression of lactotriosylceramide:galactosyltransferase for the synthesis of neolactotetraosylceramide in the nervous system

Neolactoglycolipids are derived from neolactotetraosylceramide (nLcOse4Cer). They are found during the embryonic and neonatal developmental periods in the rat cerebral cortex and disappear shortly after birth. These glycolipids are, however, abundant in the adult cerebellum. Lactotriosylceramide (LcOse3Cer):galactosyltransferase (GT), which catalyzes the terminal step in the biosynthesis of nLcOse4Cer, was characterized in mammalian brain. The enzyme was highly specific for LcOse3Cer, with a terminal GlcNAc beta 1-3Gal-residue, and it did not catalyze the transfer of galactose to other glycolipids studied with alternate carbohydrate residues. The microsomal membrane enzyme required Mn2+ and a detergent for in vitro activity. The optimal pH was 7.4, and the Km value for LcOse3Cer was 34 microM (Vmax = approximately 2 nmol/mg/h). The LcOse3Cer:GT was shown to be different from the GM2:GT and the soluble enzyme lactose synthase A. The specific activity of LcOse3Cer:GT was enriched fivefold higher in the white matter than in the gray matter of young adult rat brain, whereas GM2:GT was enriched only about 1.5-fold higher in the white matter. The developmental expression of LcOse3Cer:GT in the cerebral cortex and cerebellum was not correlative with the levels of nLcOse4Cer in these neural areas. Despite the complete absence of nLcOse4Cer in the cerebral cortex of animals older than 5 days, significant activity of the LcOse3Cer:GT was found even in the adult cortex. In cerebellum, the levels of nLcOse4Cer increased with development, but the specific activity of the enzyme was reduced by 50% soon after birth and then remained practically the same with development.(ABSTRACT TRUNCATED AT 250 WORDS)

Glucosyceramide synthase of mouse kidney: further characterization with an improved assay method

The synthesis of glucosylceramide from ceramide and UDP-[3H]glucose by mouse kidney homogenates is very sensitive to the concentration of tissue. This was shown to be due to the presence of a UDP-glc pyrophosphatase, which could be blocked by adding NAD to the medium. A new solvent partitioning system is described, containing t-butyl methyl ether, isopropyl alcohol, and aqueous sodium sulfate, which separates the original substrate (UDP-[3H]glc) from the enzyme product, [3H]cerebroside. A particular advantage of the solvent system is that only a single partitioning step is needed, without backwashes, and the enzyme product appears in the upper phase, making transfer to a counting vial more reliable. A novel incubation device, a thermostatically controlled ultrasonic bath, is used to produce highly uniform enzyme reaction rates. Ca2+, as well as Mg2+ and Mn2+, was found to be a good stimulator of the glucosyltransferase. The enzyme activity in kidney of 22-day old mice, approximately 240 pmol/h/mg tissue, is significantly greater than previously demonstrated. The enzyme was stable in intact kidneys stored at -70 degrees C but unstable at 4 degrees C. The enzyme, when acting on endogenous ceramides, showed no demonstrable glucosylation of the C24 family of ceramides although this family is the predominant one in kidney.

Synthesis of gluco- and galactocerebrosides in bovine neurones and oligodendroglia

Oligodendroglial and neuronal perikarya have been isolated from fresh and frozen bovine brain and used to investigate the synthesis of cerebrosides from UDP-hexoses and ceramides. The radioactive cerebrosides produced have been identified by TLC of the intact lipids on borate-silica gel and by a combination of acid hydrolysis and ion-exchange chromatography of the liberated hexoses. Incubation of either neuronal or oligodendroglial homogenates with UDP-galactose (UDP-gal) and mixed ceramides (hydroxy plus nonhydroxy fatty acids) leads to the synthesis of hydroxy fatty acid galactocerebroside. This lipid is also formed by both neuronal and oligodendroglial homogenates if the UDP-gal is replaced by UDP-glucose (UDP-glu). Formation of glucocerebroside has been observed only in the presence of neuronal homogenates and UDP-glu. Electron microscopic examination of the isolated cell preparations has shown that, although the oligodendroglia are relatively pure, the neurones are contaminated with oligodendroglia, which may account for some of the hydroxy fatty acid galactocerebroside synthesis. When unlabelled UDP-gal is included in the incubations containing cell homogenates, mixed ceramides, and labelled UDP-glu, galactocerebroside formation is greatly reduced, but there is comparatively little effect on glucocerebroside synthesis. In addition, experiments using D-[14C]galactose 1-phosphate and UDP D-[6-3H]glucose simultaneously have shown that the rate of formation of tritiated cerebroside is much greater than that of the 14C-labelled compound.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmental changes in glycolipid synthesizing enzymes in the brain of a myelin-deficient mutant Wistar rat

Changes in the activities of UDP-galactose:ceramide galactosyltransferase (CGalT, EC 2.4.1.45), UDP-glucose:ceramide glucosyltransferase (CGlcT, EC 2.4.1.80) and 3'-phosphoadenosine-5'-phosphosulfate (PAPS): galactosylceramide 3'-sulfotransferase (EC 2.8.2.11) over the myelinating period between 12 and 25 days were studied in the brains of control and myelin-deficient rats. Although the activity of galactosyltransferase with ceramides containing hydroxy fatty acids quadrupled in normal male littermates between 14 and 20 days, hardly any increase was observed in the mutant and the activity was less than 10% of control above 20 days of age. With normal fatty acid containing ceramides as acceptors, the activity decreased from 83% of the control at 12 days to approximately 30% after 20 days. Sulfotransferase activity also did not show the normal increase during the 3rd week of life and declined from 60% to 22%. Glucosyltransferase and lysosomal hydrolases in brain and ceramide galactosyltransferase in sciatic nerves appeared to be normal. These results suggest close similarities to the jimpy mutant mouse in which myelin deficiency is also inherited as an x-linked recessive trait.