Recurrent mutation in the human phenylalanine hydroxylase gene.

We report the identification of a missense mutation of Glu280 to Lys280 in the phenylalanine hydroxylase (PAH) gene of a phenylketonuria (PKU) patient in Denmark. The mutation is associated with haplotype 1 of the PAH gene in this population. This mutation has previously been found in North Africa, where it is in linkage disequilibrium with haplotype 38. While it is conceivable that this mutation could have been transferred from one haplotype background to another by a double crossover or gene conversion event, the fact that the mutation is exclusively associated with the two different haplotypes in the two distinct populations supports the hypothesis that these two PKU alleles are the result of recurrent mutations in the human PAH gene. Furthermore, since the site of mutation involves a CpG dinucleotide, they may represent hot spots for mutation in the human PAH locus.     

Evidence for the Glutamine Synthetase/Glutamate Synthase Pathway during the Photorespiratory Nitrogen Cycle in Spinach Leaves

Spinach leaf (Spinacia oleracea L.) discs infiltrated with [¹⁵N]glycine were incubated at 25°C in the light and in darkness for 0, 30, 60 and 90 minutes. The kinetics of ¹⁵N-incorporation into glutamine, glutamate, asparagine, aspartate, and serine from [¹⁵N]glycine was determined. At the beginning of the experiment, just after infiltration (0 min incubation) serine, and the amido-N of glutamine and asparagine were the only compounds significantly labeled in both light- and dark-treated leaf discs. Incorporation of ¹⁵N-label into the other amino acids was observed at longer incubation time. The per cent ¹⁵N-enrichment in all amino acids was found to increase with incubation. However, serine and the amido-N of glutamine remained the most highly labeled products in all treatments. The above pattern of ¹⁵N-labeling suggests that glutamine synthetase was involved in the initial refixation of ¹⁵NH₃ derived from [¹⁵N]glycine oxidation in spinach leaf discs.

The ¹⁵N-enrichment of the amino-N of glutamine was found to increase rapidly from 0 to 19% during incubation in the light. There was a comparatively smaller increase (4-9%) in the ¹⁵N-label of the amino-N of glutamine in tissue incubated in darkness. Furthermore the total flux of ¹⁵N-label into each of the amino acids examined was found to be greater in tissue incubated in the light than those in the dark. The above evidence indicates the involvement of the glutamine synthetase/glutamate synthase pathway in the recycling of photorespiratory NH₃ during glycine oxidation in spinach leaves.

     

Selective digestion of mouse metaphase chromosomes

Metaphase chromosomes prepared from colcemid-treated mouse L929 cells by non-ionic detergent lysis exhibit distinct heterochromatic centromere regions and associated kinetochores when viewed by whole mount electron microscopy. Deoxyribonuclease I treatment of these chromosomes results in the preferential digestion of the chromosomal arms leaving the centromeric heterochromatin and kinetochores apparently intact. Enrichment in centromere material after DNase I digestion was quantitated by examining the increase in 10,000xg pellets of the 1.691 g/cc satellite DNA relative to main band DNA. This satellite species has been localized at the centromeres of mouse chromosomes by in situ hybridization. From our analysis it was determined that DNase I digestion results in a five to six-fold increase in centromeric material. In contrast to the effect of DNase I, micrococcal nuclease was found to be less selective in its action. Digestion with this enzyme solubilized both chromosome arms and centromeres leaving only a small amount of chromatin and intact kinetochores.     

Variation of cell kinetic parameters in relation to the growth rate of the Ehrlich ascites tumor.

A multicompartmental model of the cell cycle and proliferation kinetics was used to analyse the time-course behavior of the cell cycle time, the growth fraction, and the cell loss rate during Ehrlich ascites tumor growth. The growth rate of Ehrlich ascites tumor cells as the tumor aged was significantly influenced by change in the cell cycle time.     

Regulation of casein messenger RNA during the development of the rat mammary gland.

Casein mRNA was isolated and partially purified from RNA extracts of rat lactating mammary glands and translated in a teterologous cell-free protein synthesizing system derived from wheat germ. Casein mRNA activity was assayed by immunoprecipitation using a specific antiserum prepared against a mixture of the purified rat caseins. Properties of rat casein mRNA were examined using a variety of sizing techniques, including chromatography on Sepharose 4B, sedimentation on sucrose gradients after heat denaturation, and electrophoresis on 2.5% agarose gels in 6 M urea. Casein mRNA activity was found in an 8-16S region after gradient centrifugation with the peak occurring at 10.5 S. In addition, the binding of rat casein mRNA to dT-cellulose was examined. Only 40% of the total casein mRNA activity was selectively retained. A partial purification of casein mRNA was accomplished by a combination of these sizing and affinity chromatography techniques. In the purified preparations casein mRNA activity comprises approximately 90% of the total mRNA activity. Characterization of this material by agarose gel electrophoresis revealed two main bands of RNA at approximately 12 and 16 S, both containing casein mRNA activity. These mRNAs were of the correct size to code for two of the principal rat caseins of approximately 25,000 and 42,000 molecular weights. Casein mRNA and total mRNA activities were then compared in total RNA extracts at various stages of normal mammary gland development in the rat, i.e. during pregnancy, lactation, and involution following weaning. A selective induction of casein mRNA activity compared to total mRNA activity was found to occur during pregnancy and lactation. Moreover, a selective loss of activity was also observed during mammary gland involution. A surprisingly high level of casein mRNA activity was found in RNA extracts from early and midpregnant mammary glands.     

Molecular Modeling Study on Tunnel Behavior in Different Histone Deacetylase Isoforms

Histone deacetylases (HDACs) have emerged as effective therapeutic targets in the treatment of various diseases including cancers as these enzymes directly involved in the epigenetic regulation of genes. However the development of isoform-selective HDAC inhibitors has been a challenge till date since all HDAC enzymes possess conserved tunnel-like active site. In this study, using molecular dynamics simulation we have analyzed the behavior of tunnels present in HDAC8, 10, and 11 enzymes of class I, II, and IV, respectively. We have identified the equivalent tunnel forming amino acids in these three isoforms and found that they are very much conserved with subtle differences to be utilized in selective inhibitor development. One amino acid, methionine of HDAC8, among six tunnel forming residues is different in isoforms of other classes (glutamic acid (E) in HDAC10 and leucine (L) in HDAC 11) based on which mutations were introduced in HDAC11, the less studied HDAC isoform, to observe the effects of this change. The HDAC8-like (L268M) mutation in the tunnel forming residues has almost maintained the deep and narrow tunnel as present in HDAC8 whereas HDAC10-like (L268E) mutation has changed the tunnel wider and shallow as observed in HDAC10. These results explained the importance of the single change in the tunnel formation in different isoforms. The observations from this study can be utilized in the development of isoform-selective HDAC inhibitors.     

Control of the pericentrosomal H2O2 level by peroxiredoxin I is critical for mitotic progression

Proteins associated with the centrosome play key roles in mitotic progression in mammalian cells. The activity of Cdk1-opposing phosphatases at the centrosome must be inhibited during early mitosis to prevent premature dephosphorylation of Cdh1—an activator of the ubiquitin ligase anaphase-promoting complex/cyclosome—and the consequent premature degradation of mitotic activators. In this paper, we show that reversible oxidative inactivation of centrosome-bound protein phosphatases such as Cdc14B by H₂O₂ is likely responsible for this inhibition. The intracellular concentration of H₂O₂ increases as the cell cycle progresses. Whereas the centrosome is shielded from H₂O₂ through its association with the H₂O₂-eliminating enzyme peroxiredoxin I (PrxI) during interphase, the centrosome-associated PrxI is selectively inactivated through phosphorylation by Cdk1 during early mitosis, thereby exposing the centrosome to H₂O₂ and facilitating inactivation of centrosome-bound phosphatases. Dephosphorylation of PrxI by okadaic acid–sensitive phosphatases during late mitosis again shields the centrosome from H₂O₂ and thereby allows the reactivation of Cdk1-opposing phosphatases at the organelle.