Calcitriol (1,25-dihydroxyvitamin D) promotes intestinal calcium absorption by inducing synthesis of which calcium-binding protein in enterocytes?

• A Calsequestrin
• B Calbindin-D9k (and calbindin-D28k in kidney) ✓
• C Parvalbumin
• D S100 proteins

Explanation

1,25-(OH)2D3 (calcitriol) binds the vitamin D receptor (VDR), which heterodimerises with RXR and binds vitamin D response elements (VDREs) in the promoters of intestinal genes. The principal genomic target induced is calbindin-D9k (CALB3) in intestinal enterocytes, a calcium-binding protein that facilitates transcellular calcium transport from apical TRPV6 channels to the basolateral PMCA1 pump, raising calcium absorption efficiency from ~10% to 30–40%. Calcitriol also induces TRPV6 and PMCA. Calsequestrin stores calcium in the sarcoplasmic reticulum of muscle; parvalbumin buffers calcium in fast-twitch muscle and neurons; S100 proteins have diverse regulatory roles not specific to intestinal calcium absorption.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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