Which of the following enzyme declines in specific activity during tail regression of frog metamorphosis ?

Question

Accepted Answer

The correct option is 3: $	ext{Mg}^{++}$ activated ATPase.

The fascinating process of frog metamorphosis, particularly the regression (reabsorption) of the tadpole tail, is a classic example of **programmed cell death** or **apoptosis**. This massive and coordinated tissue destruction is governed primarily by the hormone thyroxine. The core principle for answering this question lies in understanding the function of each enzyme in the context of a tissue that is actively dying versus one that is actively working. $\text{Mg}^{++}$ activated $\text{ATPase}$ is a crucial enzyme necessary for the active life and function of muscle tissue. Specifically, it is intimately associated with the muscle's contractile apparatus, including the myosin heads, where it catalyzes the hydrolysis of ATP to provide the energy for muscle contraction. Since the tadpole tail is a highly muscular, locomotory organ, its cells are rich in this enzyme. As the tail begins to regress, its muscle fibers are among the first cell types to enter apoptosis and are systemically broken down. This degradation of the muscle's structural and functional components leads directly to a sharp **decline** in the specific activity of $\text{Mg}^{++}$ activated $\text{ATPase}$. Its reduction is a clear biochemical signal that the muscle is losing its functional integrity and is being dismantled, contrasting sharply with the destructive enzymes whose activities rise.

The incorrect option is 1: Cathepsins.

Cathepsins are a family of highly potent lysosomal proteases, which are enzymes that specifically break down proteins. In the context of tail regression, Cathepsins play a critical and active role in dismantling the large stores of proteinaceous material within the tail cells, such as the contractile proteins in the muscle fibers and the structural proteins of the connective tissue matrix. The process of apoptosis is characterized by the release of these enzymes from the lysosomes into the cytoplasm, where they become active and digest the cell from within, leading to their specific activity being dramatically **increased** rather than declined. Therefore, Cathepsins are central agents of the tail's destruction, making this option incorrect.

The incorrect option is 2: Acid phosphatase.

Acid phosphatase is a highly-studied hydrolase enzyme and is a classic biochemical marker for lysosomes. During the thyroxine-induced apoptosis of the tadpole tail, the lysosomes within the tail cells undergo significant morphological and functional changes, preparing to digest the cellular components. The massive, coordinated reabsorption of the tail tissue requires the activation and upregulation of these lytic enzymes to break down and recycle the cellular material. Consequently, biochemical studies have repeatedly demonstrated that the specific activity of acid phosphatase exhibits a marked **increase** in the regressing tail tissue, peaking as the tail is maximally being broken down. This enzyme's rise is indicative of the intense lysosomal activity and autodigestion occurring, clearly contradicting a decline.

The incorrect option is 4: Ribonucleases.

Ribonucleases (RNases) are hydrolytic enzymes responsible for cleaving the phosphodiester bonds within RNA molecules, effectively breaking down ribosomal RNA, transfer RNA, and messenger RNA. The process of cell death, especially apoptosis, involves the systematic shutdown of cellular functions and the disposal of all non-essential molecular machinery, including the cessation of protein synthesis. To facilitate this and recycle the materials, the cellular RNA pool must be aggressively broken down. Just like the other lysosomal hydrolases, the specific activity of Ribonucleases is found to be significantly **elevated** in the regressing tadpole tail, providing the necessary machinery to degrade the genetic instructions and protein synthesis apparatus of the dying cells. Therefore, its activity profile is the opposite of what the question asks for.