amino-terminus of PTH, which is conserved among vertebrates, exhibits all known biological actions of PTH. The carboxyl-terminal region of PTH does not bind the PTH1R but has biological actions in some assays. The biological significance of the C-terminal region of PTH is unclear, and a C-terminal PTH receptor has yet not been identified [6].
Biochemistry and Metabolism
PTH is initially produced as preproPTH, a 115-amino acid precursor peptide, that later matures intracellularly into full-length PTH containing 84 amino acids. PTH is stored in secretory granules in parathyroid cells and is released when serum calcium is low. This circulating PTH comprises full-length PTH(1–84) peptides as well as several forms of truncated, mostly carboxyl-terminal fragments, the majority being PTH (34–84) and PTH (37–84) [7, 8]. These fragments cannot bind and activate the classic PTH1R.
While the plasma half-life of intact PTH (1–84) is only a few minutes, renal clearance of PTH fragments is slower. Therefore, under normocalcemic conditions, up to 80% of circulating PTH is inactive fragments, while only about 20% is intact, biologically active PTH (1–84) [9]. PTH fragments are produced by the parathyroid glands. These glands contain proteolytic enzymes such as cathepsins B and D, and therefore release fragments together with intact PTH (1–84) [10]. Under hypocalcemic conditions, the percentage of intact PTH (1–84) released in the circulation increases, and under hypercalcemic conditions, it decreases. Fragments are also formed by proteolytic cleavage of intact PTH (1–84) in the periphery, mainly in the Kupffer cells of the liver.
The fact that circulating PTH mainly comprises biologically inactive fragments makes the measurement of plasma PTH challenging. The first-generation PTH assays, reported in 1963, were radioimmunoassays [11] which, for the first time, enabled the measurement of PTH, but their utility was limited as they detected not only intact PTH (1–84) but also circulating fragments.
The introduction of an improved double antibody immunoassay in 1987, the intact PTH assay, greatly improved the accuracy and clinical utility [12]. This sandwich assay uses a carboxyl-terminal capture antibody linked to a solid phase, and an amino-terminal detection antibody, which made this assay more specific and, for example, able to distinguish primary hyperparathyroidism from hypercalcemia of malignancy.
A “third generation” “whole PTH” or “biointact PTH” assay [13], which uses an amino-terminal detection antibody specific to the extreme amino-terminus PTH (1–6) did not prove to be superior, but studies are limited [14].
In summary, currently used PTH assays are second-generation assays, which can be relied on to make the diagnosis of hyper- and hypoparathyroidism.
Vitamin D
Vitamin D Production
Vitamin D3 (cholecalciferol) is produced in the epidermal layer of the skin from 7-dehydrocholesterol (Fig. 1). This is a nonenzymatic process by which, under the influence of solar or UVB irradiation (optimal wavelength 280–320 μm), the B ring of 7-dehydrocholesterol is opened to form pre-D3, lumisterol and tachisterol. Pre-D3 is then isomerized in a thermo-sensitive process to form D3. The production rate of D3 depends upon the intensity of the UVB (time of day, season of the year, latitude), aging, sun screen use, and degree of skin pigmentation [15, 16]. African Americans may need 5–10 times longer UVB light exposure compared to Caucasians to produce the same amount of vitamin D in the skin, thus explaining why they are at much higher risk for vitamin D deficiency. Continued exposure to sunlight would increase the production of D3 without reaching toxic amounts because once a maximum level is achieved Pre-D3 will be converted into lumisterol and tachisterol.
Vitamin D2 (ergocalciferol) is produced by UVB irradiation of ergosterol in plants and fungi (Fig. 1). The chemical structure of D2 differs from that of D3 because of a double bond between C22 and C23 and a methyl group at C24 in the side chain.
Foods, with the exception of wild caught salmon and other oily fish, cod liver oil, and mushroom, contain very little vitamin D unless fortified.
Fig. 1. Synthesis and metabolism of vitamin D. Upon exposure to solar ultraviolet B (UBV) radiation, ergosterol and 7-dehydrocholesterol are converted to previtamin D2 (PreD2) and previtamin D3 (PreD3), respectively, and immediately after to vitamin D2 and D3 in a heath-dependent reaction. In the liver, vitamin D is hydroxylated in position 25 by the CYP2R1 enzyme to form 25-hydroxyvitamin D 25(OH)D, which in the kidney is further hydroxylated in position 1α by the CYP27B1enzyme to form 1,25(OH)2D. The CYP27B1 activity is stimulated (+) by PTH and inhibited (–) by calcium (Ca), phosphorus (P), fibroblast growth factor 23 (FGF23), and 1,25(OH)2D itself. 1,25(OH)2D decreases its synthesis by inhibiting the CYP27B1 and increases its catabolism to 1,24,25(OH)3D by stimulating the activity of CYP24A1. Ca, P, and FGF23 also stimulate the CYP24A1 enzyme, thus shunting the substrate 25(OH)D away from the CYP27B1 enzyme.
Vitamin D Metabolism
Vitamin D (D refers to either D2 or D3) produced in the skin or ingested with food reaches the circulation where it binds to the serum vitamin D binding protein (DBP) and reaches the sites of storage (mainly fat and muscle) and other tissues, especially the liver, where it is converted to 25-hydroxyvitamin D (25[OH]D; calcifediol; Fig. 1) by the action of the CYP2R1 25-hydroxylase enzyme, a member of the cytochrome P450 oxidase superfamily. The production of 25(OH)D is largely dependent upon the amount of its substrate, vitamin D. 25(OH)D is the major circulating form of vitamin D and is biologically inactive unless its serum concentration reaches toxic levels following the ingestion of large amount of vitamin D. Its measurement in the serum is widely used to assess a person’s vitamin D status [17].
25(OH)D is further hydroxylated in the proximal renal tubular epithelial cells by the CYP27B1 1α-hydroxylase to form 1,25(OH)2D (calcitriol), the active vitamin D metabolite (Fig. 1). The 1,25(OH)2D produced in the kidney is involved in the control of calcium and bone homeostasis. Unlike the hepatic CYP2R1, the renal CYP27B1 is tightly regulated mainly by PTH, the phosphaturic hormone fibroblast growth factor 23 (FGF23), and by 1,25(OH)2D itself (Fig. 1). PTH stimulates and FGF23 and 1,25(OH)2D inhibit the CYP27B1 enzyme. Moreover, increased serum calcium and phosphate inhibit the CYP27B1 activity by suppressing PTH and FGF23 respectively. The precise mechanisms by which PTH stimulates and FGF23 inhibits the CYP27B1 enzyme are still unclear. 1,25(OH)2D limits the CYP27B1 activity by inhibiting and stimulating the production of PTH and FGF23, respectively, and also by decreasing its release into the circulation by different mechanisms (