Researchers quantified the amount of protein produced by cultured cells during a one-hour period by detecting the radioisotope ³⁵S. Prior to this period, the cells were most likely provided with radiolabeled:

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11ecba2d_0f15_4720_a3bf_2f6114b2b2b6_MD0008_00 Cells continuously produce proteins by combining free amino acids into polypeptide chains. Once they are produced, proteins typically remain intact for hours to days before being degraded. Researchers interested in comparing the amounts of protein produced by cells in a given period must disregard any proteins that were already present before the assay began. One way to quantify the amount of protein produced during a specified period is to provide the cells with amino acids that contain a radioactive atom for the duration of the assay. Proteins that are synthesized during the specified time frame will incorporate the radiolabeled amino acids into their primary structure. These proteins can then be detected by measuring radioactive emissions, whereas proteins that were synthesized prior to the period of interest will contain no radioisotopes and will not be detected. Of the 20 standard amino acids, only cysteine and methionine contain sulfur atoms. Either of these amino acids could be labeled with ³⁵S to facilitate detection. However, methionine is the only amino acid among the given choices that contains sulfur, so the cells were most likely provided with radiolabeled methionine. (Choices A, C, and D) Asparagine, threonine, and serine contain no sulfur atoms, so they cannot contain the radioisotope ³⁵S. Educational objective: Protein synthesis may be quantified by detection of radiolabeled amino acids. ³⁵S is a radioactive isotope that is commonly used to label proteins, typically as a component of the amino acid methionine.

Researchers Quantified the Amount of Protein Produced by Cultured Cells