Identification of N5-methyl-N5-formyl-2,5,6-triamino-4-hydroxypyrimidine as a major adduct in rat liver DNA after treatment with the carcinogens, N,N-dimethylnitrosamine or 1,2-dimethylhydrazine

Human Tamm-Horsfall urinary glycoprotein from an individual of the blood group Sd(a+) phenotype was tritium-labelled by treatment with galactose oxidase and sodium boro[3H]hydride and was then digested with endo-beta-galactosidase. A series of dialysable, labelled fragments was released from which a pentasaccharide was isolated that strongly inhibited the agglutination of Sd(a+) red cells by human anti-Sda serum and hence contained the Sda determinant structure. Reduction, methylation analysis and sequential exo-glycosidase digestion established the structure of the pentasaccharide as: GalNAc beta(1 leads to 4)[NeuAc(2 leads to 3)]Gal beta(1 leads to 4)GlcNAc beta(1 leads to 3)Gal     

Specific silver staining of experimentally undercondensed chromosome regions

Treatment of human and mouse cell cultures with DNA binding AT-specific compounds and with some base analogues induced distinct undercondensations in several heterochromatic chromosome regions. All those heterochromatic regions undercondensed by AT-specific DNA ligands (distamycin A, DAPI, Hoechst 33258) could be heavily labeled with the silver(Ag)-staining technique; but the heterochromatic regions undercondensed with the cytidine analogue 5-azacytidine were Ag-negative. In metaphase chromosomes from BrdU-treated human cell cultures, the bifilarly substituted chromatids, which show a slight undercondensation, were also Ag-negative. Cytochemical analyses of the Ag-stained undercondensed heterochromatic regions showed that the Ag-stainable material consisted of nonhistone proteins. The mechanism of Ag staining in the undercondensed heterochromatic regions was compared with Ag staining of the nucleolus organizer regions.     

The role of methylation in chemotaxis. An explanation of outstanding anomalies

The role of methylation in chemotaxis is understood generally, but several anomalies exist which bring into question the timing of methylation relative to sensing. A double mutant bacterium, deficient in both methyltransferase and methylesterase (Tr-Es-) is capable of chemotaxis even though the respective single mutants (Tr- and Es-) are not. This Tr-Es- mutant will accumulate in capillaries containing aspartic acid but not in capillaries containing serine despite the fact that both the aspartate and serine receptors are part of the methylation-dependent pathway. To understand these anomalies, a combination of theoretical analyses and experimental studies was performed. A mathematical analysis of the gradients of aspartate and serine in the capillary assay shows that outside the capillary the gradients are shallow, but just inside the mouth of the capillary they are very steep. Also, when the number of bacteria accumulated in the capillary is at a maximum, the range of attractant concentrations in the steep gradient just inside the mouth of the capillary is optimal for response and partial adaptation by the Tr-Es- mutant. We postulate that random motion brings the Tr-Es- mutant into the capillary, where it is able to move up the steep gradient. The difference in timing of the responses to serine and aspartate explains why the Tr-Es- mutant accumulates in aspartate- but not in serine-containing capillaries. A simple diffusion-capture model incorporating these concepts can account for experimental values of the number of Tr-Es- bacteria accumulating in the capillary. These studies provide a rational explanation for all of the apparent anomalies and lead to the conclusion that methylation/demethylation plays a crucial role in sensing as well as setting the zero point of the receptor.     

Characteristics to Consider when Choosing an Animal Model for the Study of Lead Bioavailability

Abstract

Most animal studies conducted to determine the bioavailability of lead have, in the past, employed rodents or lagomorphs as experimental models. In this paper issues and data are presented which raise questions and uncertainties about employing rodents or lagomorphs for investigations into the bioavailability of lead. These issues include: (1) the possible role of coprophagy and feeding behavior in reducing estimates of lead bioavailability; (2) anatomical and physiological differences related to coprophagy which may influence estimates of lead bioavailability derived in rats or rabbits; (3) evidence for relatively high biliary excretion of lead by rats and rabbits; (4) the possibility of a strong developmental component to the active transport of lead. The importance of addressing these and other questions in studies designed to determine the bioavailability of lead is discussed.