Which of the following is not a form of vitamin D?

The D vitamins are a group of sterols that have a hormone-like function. The active molecule, 1,25-dihydroxycholecalciferol (1,25-diOH-D3), binds to intracellular receptor proteins. The 1,25-diOH-D3-receptor complex interacts with DNA in the nucleus of target cells in a manner similar to that of vitamin A (see Figure 28.20), and either selectively stimulates gene expression or specifically represses gene transcription. The most prominent actions of 1,25-diOH-D3are to regulate the plasma levels of calcium and phosphorusA. Distribution of vitamin D 1. Diet: Ergocalciferol(vitamin D2), found in plants, and cholecalciferol(vitamin D3), found in animal tissues, are sources of preformed vitamin D activity (Figure 28.22). Ergocalciferol and cholecalciferol differ chemically only in the presence of an additional double bond and methyl group in the plant sterol.2. Endogenous vitamin precursor:7-Dehydrocholesterol, an intermediate in cholesterol synthesis, is conveed to cholecalciferol in the dermis and epidermis of humans exposed to sunlight. Preformed vitamin D is a dietary requirement only in individuals with limited exposure to sunlight. B. Metabolism of vitamin D 1. Formation of 1,25-diOH-D3: Vitamins D2and D3are not biologically active, but are conveed in vivo to the active form of the D vitamin by two sequential hydroxylation reactions (Figure 28.23). The first hydroxylation occurs at the 25-position and is catalyzed by a specific hydroxylase in the liverRef: DM Vasudevan - Textbook of Biochemistry, 6th edition, page no: 386