Levels of carboxylated and undercarboxylated osteocalcin in patients with type 2 diabetes

Introduction:  Osteocalcin (OC) is a bone-derived protein that undergoes vitamin K-dependent carboxylation. The undercarboxylated form of the protein (ucOC) is released in the circulation during the process of bone resorption. Experimental studies on mice and rats have revealed that ucOC is involved in the regulation of energy homeostasis, linking in this way the bone, pancreas, and adipose tissue metabolism. Experimental studies suggest no hormonal role for the carboxylated form (cOC) of the protein. Aim:  In the current study we aimed to examine the levels of OC in its carboxylated and undercarboxylated form in patients with type 2 diabetes and control subjects, and to compare the vitamin K status between the two groups.  Materials and Methods:  The present cross-sectional study involved a sample of 46 adults type 2 diabetes patients and a control group of 19 individuals. The carboxylated and undercarboxylated forms of OC were measured in serum by using highly sensitive sandwich-type enzyme immunoassay kits. Vitamin K status was evaluated by the ratio ucOC/cOC. Student’s two-tailed unpaired t-test was used to compare the groups. Results:  UcOC and cOC serum levels were significantly lower in patients with type 2 diabetes compared to controls. We found no difference in the vitamin K status between the groups. Conclusion:  Our results show that OC might be involved in the regulation of carbohydrate metabolism. In humans, it appears that the carboxylation state might not be essential for the hormonal role of the protein as in mice and rats.