Metabolic properties of an azaguanine-resistant variant of Chinese hamster ovary cells (azarts) with normal levels of hypoxanthine-guanine phosphoribosyltransferase activity

Heterogeneous precipitation mediated heterogeneous nanostructure enhances strength-ductility synergy in severely cryo-rolled and annealed CoCrFeNi2.1Nb0.2 high entropy alloy

Possibilities of enhancing mechanical properties of brittle intermetallic containing high entropy alloys (HEAs) using novel processing and microstructural design strategies were investigated in the present work. For this purpose, homogenized CoCrFeNi_2.1Nb_0.2 HEA consisting of FCC matrix and complex Laves phase particles was successfully processed by severe cold- or cryo-rolling to 90% reduction in thickness followed by annealing (800 °C/1 hour(h)). As compared to cold-rolling, cryo-rolling resulted in a finer lamellar nanostructure and decidedly greater fragmentation of the Laves phase. Upon annealing, the cold-rolled HEA showed a recrystallized FCC matrix dispersed with D0₁₉ structured ε nano-precipitates. In contrast, the finer nanostructure and greater driving force for accelerated precipitation of profuse nano-precipitates at the early stages of annealing inhibited recrystallization in the cryo-rolled HEA and resulted in the formation of heterogeneous microstructure consisting of retained deformed and recrystallized regions. The novel heterogeneous microstructure of the cryo-rolled and annealed HEA resulted in a remarkable enhancement in strength-ductility synergy. The present results indicated that cryo-rolling could be used as an innovative processing strategy for tailoring heterogeneous microstructure and achieving novel mechanical properties.

6-thioguanine resistance in a human colon carcinoma cell line with unaltered levels of hypoxanthine guanine phosphoribosyltransferase activity

Cell populations resistant to high doses (30 microM) of 6-thioguanine (6-TG, 6-TG(r) cells) were selected from a human colon carcinoma cell line, LoVo. This cell line, which lacks hMSH2, a component of the human mismatch binding heterodimer hMutSalpha, is resistant to low doses of 6-TG. The level of activity of hypoxanthine-guanine phosphoribosyltransferase, the enzyme responsible for the phosphoribosylation of the thiopurine, was comparable to that expressed in the parental cells. No significant difference was found in the levels of enzyme activities involved in the conversion of 6-TG or its derivatives into non-toxic compounds. In contrast, a significant difference was found in the uptake kinetics of 6-TG in the 2 cell types. Net uptake of 6-TG ceased after 100-sec incubation in the 6-TG(r) cells, while it appeared to continue throughout the 10-min incubation in the wild-type cells. As a consequence, after 10-min incubation, the total amount of 6-TG taken up by the parental LoVo cells was approximately 3 times higher than that present in the 6-TG(r) cells.

Alpha-5-phosphoribosyl-1-pyrophosphate-independent salvage of purines in cultured Chinese hamster lung fibroblasts

A variant clone of cultured chinese hamster lung fibroblasts (V79), selected for resistance to 8-azaguanine (V79 azagrst), although lacking hypoxanthine-guanine phosphoribosyltransferase (EC 2.4.2.8), is able to convert hypoxanthine into IMP via purine-nucleoside phosphorylase (EC 2.4.2.1) and nucleoside kinase. In addition to the phosphoribosylation pathway, we also present evidence for the occurrence of a kinase-mediated pathway of recovery of hypoxanthine in the wild-type cells. The lower rate of formation of IMP in the V79 azagrst cells, apparently correlated with the phosphorylation of the nucleoside, suggests possible differences in the catalytic and/or regulatory properties of nucleoside kinase in the two cell lines. This fact might be of particular relevance in evaluating the mechanisms of resistance to purine analogs displayed by several cell types.