Correct option is B
Correct Answer: (2) B, C and E
Parathyroid Hormone (PTH) is critical in regulating calcium homeostasis in humans. Let's evaluate each statement based on known physiology:
A."It is a 108 amino acid (aa) residue long hormone whose 1-42 aa exhibits full biological activity."
Incorrect
Human PTH is 84 amino acids long.
The 1–34 amino acid segment (not 1–42) is biologically active and retains full physiological effects.
Hence, statement A is incorrect.
B."It is an 84 aa hormone whose 1-34 aa exhibits full biological activity."
Correct
Human PTH is 84 amino acids in length.
The N-terminal 1–34 fragment is sufficient for full biological activity — this is the basis for the synthetic analog teriparatide (PTH 1–34) used in osteoporosis treatment.
C."An acute decrease of Ca²⁺ results in a marked increase of PTH mRNA, followed by increased rate of PTH synthesis."
Correct
Low blood calcium levels are sensed by the calcium-sensing receptor (CaSR) on parathyroid cells.
This leads to upregulation of PTH gene transcription, increasing PTH mRNA and PTH synthesis.
D."Rate of degradation of pro-PTH increases when Ca²⁺ concentrations are low."
Incorrect
When Ca²⁺ is low, PTH secretion and synthesis are increased.
In fact, pro-PTH degradation is reduced, allowing more PTH to be processed and secreted.
So this statement is incorrect.
E."Cathepsin B cleaves PTH into two fragments."
Correct
Cathepsin B, a lysosomal enzyme, is involved in intracellular proteolysis of PTH.
It cleaves intact PTH into inactive fragments, helping regulate circulating PTH levels.
