In an animal experiment, mice proerythroblasts are cultured in 2 different growth media; the first medium is folate deficient, whereas the second (control) is supplemented with folic acid. Both media contain high concentrations of erythropoietin. Over 48 hours, cells in the control media proliferate and differentiate into reticulocytes, whereas in the folate-deficient media, cell proliferation is minimal, with the majority of cells undergoing apoptosis. In another experiment, a substance is added to the folate-deficient media, which prevents apoptosis and permits proliferation of the proerythroblasts. Which of the following is the most likely substance added to the growth medium?

Question

In an animal experiment, mice proerythroblasts are cultured in 2 different growth media; the first medium is folate deficient, whereas the second (control) is supplemented with folic acid.  Both media contain high concentrations of erythropoietin.  Over 48 hours, cells in the control media proliferate and differentiate into reticulocytes, whereas in the folate-deficient media, cell proliferation is minimal, with the majority of cells undergoing apoptosis.  In another experiment, a substance is added to the folate-deficient media, which prevents apoptosis and permits proliferation of the proerythroblasts.  Which of the following is the most likely substance added to the growth medium?

Quizplus · Accepted Answer

11ec160b_36d1_4c03_9249_e9efc302d70c_MD0001_00 Folate derivatives are crucial in the synthesis of nucleic acids (particularly thymine, but also purine bases), conversion of homocystine to methionine, and generation of one-carbon carriers (eg, S-adenosylmethionine) for methylation reactions. All biochemical functions of folate require the reduced tetrahydrofolate form of the vitamin, which is synthesized by dihydrofolate reductase (a rate-limiting step in folate metabolism). The enzyme thymidylate synthase is responsible for converting deoxyuridine monophosphate (dUMP) to deoxythymidine monophosphate (dTMP]). Although most enzymes involved in one-carbon metabolism maintain folate in its active tetrahydrofolate form, thymidylate synthase is unique in that it oxidizes 5,10-methylenetetrahydrofolate to dihydrofolate. This makes de novo thymidine synthesis particularly susceptible to folate-deficient conditions, as tetrahydrofolate must be continuously regenerated by dihydrofolate reductase. In this experiment, inhibition of thymidylate synthase due to low folate conditions increases the ratio of dUMP to dTMP, causing the incorporation of uracil into DNA in place of thymidine. This leads to excessive activation of DNA repair mechanisms, resulting in double-stranded DNA breaks and apoptosis. Thymidine supplementation increases the amount of available thymidine and is capable of inhibiting apoptosis in folate-deficient cells. (Choice A) Cobalamin is a necessary cofactor for methionine synthase, the enzyme that transfers a methyl group from 5-methyltetrahydrofolate to homocysteine, forming methionine and regenerating tetrahydrofolate. Cobalamin deficiency can cause folate to become trapped in 5-methyltetrahydrofolate form, resulting in secondary folate deficiency. However, cobalamin supplementation would not be of significant benefit in this patient as the primary folate deficiency prevents synthesis of tetrahydrofolate in the first place. (Choice B) Cytosine is a pyrimidine base. Unlike thymine, it is not dependent on folate and can be synthesized in sufficient quantities under folate-deficient conditions. (Choice C) Glutamine is a major source of nitrogen in the synthesis of purine and pyrimidine bases. However, glutamine supplementation would not ameliorate the cellular apoptosis induced by folate deficiency. (Choice D) Homocysteine is an amino acid associated with endothelial cell injury and vascular inflammation. Levels are elevated in folate deficiency due to impaired conversion into methionine. Educational objective: Folate deficiency inhibits the synthesis of nucleic acids, particularly the formation of deoxythymidine monophosphate (dTMP). This leads to defective DNA synthesis that characteristically causes increased apoptosis of hemopoietic cells and megaloblastic anemia. Thymidine supplementation bypasses this enzyme and can reduce erythroid cell apoptosis. __________ References: Apoptosis mediates and thymidine prevents erythroblast destruction in folate deficiency anemia. (http://www.ncbi.nlm.nih.gov/pubmed/8171036)

In an Animal Experiment, Mice Proerythroblasts Are Cultured in 2