You Are Studying the Effect of a Secreted Signal Molecule

You are studying the effect of a secreted signal molecule on the activity of a cytoplasmic enzyme.Prior studies have shown the signal molecule is required for enzyme activity.You add the signal molecule to a test tube solution containing both the enzyme and its substrate.After incubating the solution,you assay for the reaction product as a measure of enzyme activity,however,no product was detected.By comparison,when you add the signal molecule to a vial containing intact cells,you are able to detect the reaction product.Which of the following statements is inconsistent with these experimental results?

A) The cells express a receptor for the signal molecule.
B) There is no direct interaction between the signal molecule and enzyme.
C) The cells facilitate a direct protein-protein interaction between the signal molecule and enzyme.
D) Binding of the signal molecule to its receptor initiates a signal transduction pathway leading to enzymatic activity.

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