Plasma Vitamin D Binding Protein GcGlobulin

Cholecalciferol, calcidiol, calcitriol, and 24-hydroxycalcidiol are all transported bound to the same plasma binding protein - Gc-globulin, also known as the group-specific component or transcalciferin. There are three major forms of Gc-globulin, with differing primary structures, and a number of minor variants of each because of differences in postsynthetic glycosylation. There is considerable polymorphism among human populations; because of this, Gc-globulin has been investigated both for its interest in population genetics and also its potential value in forensic medicine. All the variants bind vitamin D and its metabolites with similar affinity. It is noteworthy that the absence of Gc-protein has never been detected, suggesting that a deletion of this protein may be fatal. Cholecalciferol is also transported in plasma lipoproteins, so that about 60% is normally bound to Gc-globulin and 40% to lipoproteins. It is only that fraction bound to lipoproteins that is taken up by the liver for 25-hydroxylation (Haddad et al., 1988). In addition to its role in the plasma transport of vitamin D, and control over tissue uptake, Gc-globulin represents the major storage site for the vitamin, mainly as calcidiol.

The plasma binding protein has a higher affinity for calcidiol and 24-hydroxycalcidiol than for calcitriol or cholecalciferol. The plasma concentration of Gc-globulin is about 6 mmol per L - considerably higher than the concentrations of other hormone binding proteins, such as thyroxine binding globulin (300 ^mol per L), cortisol binding globulin (800 ^mol per L), or sex hormone binding globulin (40 ^mol per L in males and 80 ^mol per L in females) and far in excess of circulating vitamin D. As a result of this, whereas the other hormone binding globulins are about 50% saturated under normal conditions, the vitamin D binding protein is only about 2% saturated. This means that changes in the circulating concentration of the protein are unlikely to have any significant effect on the small proportion of vitamin D metabolites that is free, rather than protein-bound. Again, unlike other hormone binding globulins, the plasma concentration of Gc-globulin is not affected by vitamin D status or other factors that affect calcium homeostasis and vitamin D metabolism (Cooke and Haddad, 1989; Haddad, 1995).

Peripheral Neuropathy Natural Treatment Options

Peripheral Neuropathy Natural Treatment Options

This guide will help millions of people understand this condition so that they can take control of their lives and make informed decisions. The ebook covers information on a vast number of different types of neuropathy. In addition, it will be a useful resource for their families, caregivers, and health care providers.

Get My Free Ebook


Post a comment