Calcitroic acid (1α-hydroxy-23-carboxy-24,25,26,27-tetranorvitamin D3) is a major metabolite of 1α,25-dihydroxyvitamin D3 (calcitriol).[1] Around 1980, scientists first reported the isolation of calcitroic acid from the aqueous extract of radioactively treated animals' livers and intestines. Subsequent researches confirmed calcitroic acid to be a part of enterohepatic circulation.[1] Often synthesized in the liver and kidneys, calcitroic acid is generated in the body after vitamin D is first converted into calcitriol, an intermediate in the fortification of bone through the formation and regulation of calcium in the body.[1] These pathways managed by calcitriol[2] are thought to be inactivated[3] through its hydroxylation by the enzyme CYP24A1, also called calcitriol 24-hydroxylase.[4] Specifically, It is thought to be the major route to inactivate vitamin D metabolites.[3] The hydroxylation and oxidation reactions will yield either calcitroic acid via the C24 oxidation pathway or 1,25(OH2)D3-26,23-lactone via the C23 lactone pathway.[5] However, the only scientifically known formation of calcitroic acid is through an oxidative reaction of the 1ɑ,25-dihydroxy vitamin D3. The positions of C24 and C23 undergo multiple oxidative reactions. Thus, causing the large and small side chains of 1ɑ,25-dihydroxy vitamin D3 to cleave off and form calcitroic acid.[6]
The compound has been prepared in the laboratory.[2]
Hydroxylation and further metabolism of calcitriol in the liver and the kidneys yields calcitroic acid, a water-soluble compound that is excreted in bile.[1]
In case where a higher concentration of this acid is used in vitro, studies determined that calcitroic acid binds to vitamin D receptor (VDR) and induces gene transcription.[1]
There is an x-ray co-crystal structure of calcitroic acid that justifies that the calcitroic acid and vitamin D receptor have agonistic confirmation properties. Calcitroic acid has two side chains, the smaller side chain consists of a hydrogen bond with His333 and a single water molecule. In addition, the longer side chain consists of His333 and His423 interacting with 1,25(OH)2D3.[7]
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