Advances in the knowledge of the metabolism of vitamin D.

The secosteroid, vitamin D, functions in calcium homoeostasis as a hydroxylated derivative: 1,zS-dihydroxy-vitamin D (I ,z~-DOHD). This molecule becomes localized in the nuclei of the mucosal cells of the small intestine (Lawson, Wilson & Kodicek, 1969) where it appears to induce messenger RNA (Emtage, Lawson & Kodicek, 1973) for proteins which will subsequently increase the transport of Ca by these cells. Thus vitamin D seems to act in a manner similar to the classical steroid hormones of the adrenal cortex and the gonads. It is notable that the steroid hormones all have oxygen functions at the extremities of the long axis of the molecule. For example, corticosterone, cortisol and aldosterone have 3-ketoand 2 I -hydroxy-groups while progesterone has ketogroups on both C-3 and C-20. This polar oxygenation enables these steroids to interact, with high affinity, with specific receptor proteins in target cells. The specificity of the interaction is enhanced by the presence of functional groups deployed elsewhere on the steroid structure. In the instance of I,z~-DOHD the polar oxygens are the hydroxy-groups on C-3 and -25 and the group giving the functional specificity is the hydroxyl on GI.