Inhibition of mammalian ribonucleotide reductase by a dinucleotide produced in eucaryotic cells

Abstract

HS3, a highly phosphorylated dinucleoside originally purified from the fungus Achlya, has been isolated from Chinese hamster ovary cells undergoing glutamine starvation. The HS3 compounds obtained from the fungal and mammalian sources exhibited similar physical and chemical properties. This unusual dinucleotide may be an important regulator of eucaryotic ribonucleoside diphosphate reductase activity; for 50 micrometer HS3, isolated from either mammalian or fungal cells, significantly inhibited CDP reduction in Achlya or hamster cell preparations, but only marginally affected the activity of the enzyme from E. coli. Studies with HS3 isolated from Achlya and partially purified mammalian ribonucleotide reductase indicated that the compound noncompetitively inhibited the reduction of varying concentrations of the substrates CDP, ADP and GDP with Ki values of 23 micrometer, 14 micron and 16 micron respectively. These inhibitor concentrations are well below the estimated intracellular levels of HS3 in glutamine starved cells and suggest that HS3 inhibition of ribonucleotide reduction may be responsible for the rapid inhibition of DNA synthesis seen under these culture conditions.

Antioxidant properties of xanthones from Calophyllum brasiliense: prevention of oxidative damage induced by FeSO4

“…Several properties have been described for compounds derived from Calophyllum brasiliense Cambess [9,10,16]. Specially, xanthones exhibit diverse biological properties, including antihypertensive, antioxidative, antithrombotic and anticancer activities [37,38].…”

“…Calophyllum brasiliense Cambess is a big tree found in the tropical rain forests from Brazil to Mexico. It has been widely used in Latin American folk medicine to treat a variety of maladies including pain [9], inflammation, diabetes, hypertension [10,11], diarrhea, herpes and rheumatism [12]. Despite its extensive use, only a few biological activities have been reported in the scientific literature.…”

Altered Ribonucleotide Reductase Activity in Drug-Resistant Mammalian Cells Detected by an Assay Procedure for Intact Cells

“…Evidence suggests that ribonucleotide reduction is a rate-limiting step in the synthesis of DNA and that this activity plays a key role in the regulation of cell division in eucaryotic cells (1,3,7). The importance of this enzyme from a clinical point of view is emphasized by the finding that two immunodeficiency diseases in man appear to be caused by an imbalance of the allosteric control of the reductase (2,10).…”

“…Drug concentrations for selection of singleand double-step hydroxyurea-resistant cell lines C-labelled ADP (5,000 cpm/nmol), 0.4 mmol/1 0.67% Tween-80, and 0.167 mol/1 sucrose. The amount of product formed was determined as previ ously described (7). One unit of enzyme activity was defined as the amount of enzyme that would reduce 1 pmol of nucleotide in 30 min.…”

N‐Carbamoyloxyurea‐resistant Chinese hamster ovary cells with elevated levels of ribonucleotide reductase activity

“…For example, the specific activity of ribonucleotide reductase increases dramatically prior to DNA synthesis and then returns to low basal levels near the end of S phase; moreover, enhanced levels of the reductase are characteristic of rapidly proliferating tissues such as regenerating liver, embryonic organs, and malignant growths (Thelander and Reichard, 1979). In accord with the importance of this activity in the physiology of the cell, the enzyme exhibits complex and strict allosteric control by a variety of positive and negative nucleotide effectors (Eriksson et al, 1979;Lewis et al, 1977). This type of evidence suggests that ribonucleotide reductase is a crucial rate-limiting step in DNA synthesis and plays a key role in the regulation of cell division.…”