Eukaryotic cell membranes were analyzed for hormone receptors. A membrane prepar

Question

Eukaryotic cell membranes were analyzed for hormone receptors. A membrane preparation was incubated with radiolabeled hormone (^3H-hormone) for five minutes. A similar incubation of membranes and^3H-hormone contained, in addition, a 1,000-fold excess of unlabeled hormone In both cases, the unbound hormone was removed by washing the preparation and amount of radioactivity remaining in the membrane preparation was determined. The following results were obtained. cpm is the unit of radioactivity of^3H.^3H-hormone Only 5,000cpm^3H-hormone plus Excess Unlabeled Hormone 1, 500 cpm The total amount of radioactivity bound in the absence of unlabeled hormone represents the total amount of the receptors at the surface of the cells. Most of the label is bound nonspecifically. The unlabeled hormone competes with^3H-hormone for the binding to the receptors. In the presence of unlabeled hormone, the only radioactivity detected is that of the hormone-receptor complex. Your answer: _____

Explanation / Answer

Answer is (b) Most of the label is bound nonspecifically.

Hormone receptors are detected by their ability to bind radioactive hormone to a membrane. When a increasing amount of labelled hormone added to a suspension , the amount that bind to the receptor intially increase and then decrease at heigher concentration. Most of the labeled hormone bind specifically to the receptor and some bound nonspecifically to other protein and phospholipid on the membrane. This nonspecific binding of labeled hormone can be measured by performing binding assay in the presence of large excess of unlabeled hormone. All of the binding site that is saturable filled with unlabeled hormone under this condition and bind no labeled hormone.

Here, in first option total amount of radioactivity represent total amount of binding not total amount of receptor at the surface of cell in the absence of unlabeled hormone.

In option c the unlabeled hormone present in heigher concentration then the labeled hormone, so it saturate the receptor without competition.

In option d in the presence of unlabeled hormone, the only radioactivity detected is because of nonspecific binding not due to hormone receptor complex formation.

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