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The role of Vitamin D Binding Protein and its polymorphisms on circulating 25-hydroxyvitamin D fractions and their metabolic implications in patients with Primary Hyperparathyroidism.

Abstract

The Vitamin D system has very peculiar characteristics. Its metabolites circulate bound with high affinity to a carrier protein, Vitamin D Binding Protein (DBP), which in turn has a genetic polymorphism able to interfere in its affinity for the ligand. During its transport by the organism, it undergoes transformations by the addition of hydroxyl groups, catalyzed by enzymes from Cytochrome P450 family until it becomes its biologically active form: 1,25 (OH) 2D or calcitriol. The production of active vitamin D is strictly controlled in renal tubular cells, being more significantly stimulated by parathyroid hormone (PTH) and inhibited by Fibroblast Growth Factor 23 (FGF-23) and by calcitriol itself. Tissue action is done through specific intranuclear receptors (VDRs) that function as transcription factors in the target genes. VDR polymorphisms determine different phenotypes, which may alter their binding ability. Recently, attention has been focused on the free and bioavailable 25 (OH) D fractions, which may vary in the different polymorphisms of DBP. Therefore, there are multiple variables capable of interfering in the final effect of these hormones on their target organs, making it difficult to define parameters of normality for the vitamin D intake.We chose to evaluate these parameters in a normal population to compare with individuals with primary hyperparathyroidism, a condition quite frequent in the population, where excess PTH disrupts this mineral homeostasis, and that may bring us valuable information about this physiology. (AU)