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Biomedical subjects

H Follmann

Publications and source records attributed to H Follmann.

At least 55 records · Page 3Linked to original sources
PubMed1979

Deoxyribonucleotide synthesis and DNA polymerase activity in plant cells (Vicia faba and Glycine max).

Enzymes of deoxyribonucleotide and DNA biosynthesis, which are little known in plants, were studied in root tips of germinating broad beans (Vicia faba) and in fast-growing cultures of soybean cells (Glycine max). The plant cells contain a ribonucleoside 5'-diphosphate reductase which is detected in vitro only during a limited period of growth, viz. 30--32 h after inhibition of Vicia seeds, and between the second and third day after inoculation of soybean cultures. In both species ribonucleotide reductase activity precedes maximum DNA synthesis. The reductases could be precipitated with ammonium sulfate but were not purified further due to the extremely low enzyme content of the plant extracts. Therefore the reductive pathway of deoxyribotide formation was also established in Vicia root tips by efficient labeling of the plant DNA with a ribonucleoside, [5-3H]cytidine, which reaches a maximum at the same time as the reductase activity measured in vitro. Cycloheximide inhibits this process, indicating the need for de novo enzyme induction. In contrast, DNA polymerase is present in the tissue throughout the entire development and rises only 2-fold in activity during the S phase. The soluble polymerases were partially characterized in both legume species and were found very similar to the DNA polymerase of pea seedlings. Ribonucleotide reductase is more likely a limiting component of DNA formation during the plant cell cycle than DNA polymerase.

DNA↗
PubMed1978

Age-dependent DNA labeling and deoxyribonucleotide synthesis in wheat seeds.

Utilisation of ribonucleosides as precursors of DNA biosynthesis was studied in germinating wheat embryos because the reductive pathway leading to deoxyribonucleotides is very difficult to demonstrate in extracts of higher plants in vitro. [5-3H]Cytidine and [6-3H]uridine are incorporated into wheat DNA (RNA-free) via ribonucleotide reduction without intermediate scission of the glycosidic bond. This reaction is observed at 20-30 h after the onset of germination only in aged (2-4-year-old) seeds while the embryos isolated from fresh grains show very little cytidine incorporation; in contrast, thymidine incorporation into DNA between 10 and 18 h of germination is not age dependent. Fresh wheat contains a soluble, heat-stable inhibitor fraction, most probably a modified oligonucleotide, which efficiently prevents cytidine incorporation when added to old embryos together with the labeled nucleoside. This material also inhibits purified Escherichia coli ribonucleotide reductase and is thought to be part of the control system for ribonucleotide reduction in wheat; it may gradually decay during storage of the seeds. Dry wheat embryos do not contain deoxyribonucleoside triphosphates. Pool sizes of dATP and dTTP in germinating embryos were found to reach 1 pmol/microgram DNA at 10-15 h of germination (i.e. before ribonucleotide reduction) and were independent of the age of seeds. These data suggest that wheat contains other preformed dexoyribonucleoside derivatives which are phosphorylated at an early time and can initially sustain DNA synthesis. Induction of measurable ribonucleotide reductase activity in fresh winter wheat was for the first time accomplished by 15 days of vernalization of the seeds at +2 degrees C.

Cytidine↗
PubMed1975

Adenine nucleosides in solution: circular dichroism studies and base conformation.

Adenosine, AMP, S-adenosylhomocysteine, S-adenosylmethionine, aristeromycin and 25 other synthetic adenosine analogs modified in the 4' or 5' positions show certain groups of different circular dichroism (CD) spectra. Both positive and negative Cotton effects can occur in the long-wavelength part (250-270 nm) of the spectra. Molar ellipticities [theta] range from -6000 (in adenosine 5'-carboxylate) to +4000 deg. cm2 dmol-1 (in 5'-deoxy-5'iodoadenosine), including some compounds with small, polar 5'-substituents in which low-intensity bands are found in signed pairs. Most of these adenosine derivatives that have the same adenine chromophore and a ribofuranose moiety unsubstituted in the 2' and 3' positions prefer an anti-conformation of the adenine base, as evidenced by proton magnetic resonance spectroscopy. In the majority of cases, electronic perturbations of the chromophore or major alterations of the assymmetric sugar residue can be excluded as sources of the CD variations. Therefore a correlation of the long-wavelength CD bands with the glycosyl torsion angle phiCN is suggested, where the gauche, gauche/anti combination which is typical of AMP in the crystal and in solution (phiCN approximately -40degrees, [theta] negative) is one reference point and a region for phiCN = 0degrees ([theta] positive) is assigned to compounds with space-filling substituents such as S-adenosylmethionine. Both negative and positive Cotton effects can be associated with the anti conformation range. Within this series, the base conformation of novel nucleoside structures could be predicted from CD measurements. The CD spectrum gives no indication, however, of whether a certain torsion angle is the result of a rigid structure (as in AMP) or the average value of a molecule with high rotational freedom (as in 5'-deoxyadenosine). The conformations of aristeromycin and 4'-thioadenosine are discussed in relation to adenosine, and a structure-determining effect of the 4' bridge atom is noted.

Adenosine↗