Selenium Metabolism in Neptunia amplexicaulis

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Abstract
ATP sulfurylase (EC 2.7.7.4), cysteinyl-tRNA synthetase (EC 6.1.1.1.16) and methionyl-tRNA synthetase (EC 6.1.1.10) from N. amplexicaulis were purified .apprx. 162-, 140- and 185-fold, respectively. Purified ATP sulfurylase in the presence of purified inorganic pyrophosphatase catalyzed the incorporation of So3 into adenosine 5''-phosphosulfate; evidence of an analogous reaction with selenate is presented. Crude extracts catalyzed both the sulfate- and the adenosine 5''-phosphosulfatedependent NADH oxidation in the adenosine 5''-phosphosulfate kinase assay of Burnell and Whatley, but an analogous reaction with selenate was not detected. Both purified cysteinyl-tRNA synthetase and methionyl-tRNA synthetase used Se-containing analogs as substrates in both the ATP-pyrophosphate exchange and the aminoacylation assays. Se-containing amino acids apparently are excluded from proteins by a mechanism(s) other than substrate discrimination at the amino acid activation stage of protein synthesis.