Studies on Dihydropteridine Reductase Activity in Pheochromocytoma Cells |
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Authors: | Bruce T. Liang Karen K. Vaccaro Bernyce A. Perelle Robert L. Perlman |
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Affiliation: | Department of Physiology, Harvard Medical School, Boston, Massachusetts, U. S. A. |
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Abstract: | Abstract— The activity of dihydropteridine reductase (DPR) in pheochromocytoma cells has been studied. The activity of this enzyme in crude extracts of pheochromocytoma cells is approximately 50 nmol/min/mg protein. This activity is very much greater than the activity of tyrosine 3-monooxygenase (TH) in these extracts and the rate of conversion of tyrosine to DOPA in intact pheochromocytoma cells. Incubation of the cells with 56 m m -K+ or with cholera toxin has previously been shown to increase the rate of catecholamine synthesis and to cause a stable activation of TH in the cells. These treatments do not produce a stable activation of DPR, as assayed in vitro. Methotrexate inhibits DPR activity in vitro with an I50 of approximately 20 μ m , but has no effect on the rate of DOPA formation in intact pheochromocytoma cells. Therefore, DPR does not appear to be the rate-limiting enzyme in the pathway of catecholamine synthesis in pheochromocytoma cells. Moreover, the activities of DPR and of TH are not regulated coordinately in these cells. |
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Keywords: | Catecholamine synthesis Dihydropteridine reductase Tetrahydrobiop-terin Tyrosine 3-monooxygenase. |
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