Gamma-Carboxylation and Fragmentation of Osteocalcin in Human Serum Defined by Mass Spectrometry*
Year: 2015
Type: article
Abstract: Serum osteocalcin (Oc) concentration is a highly specific measure of bone turnover, but its circulating proteoform(s) have not been well defined. Based on immunological methods, the major forms are thought to be the intact polypeptide and a large N-terminal-mid molecule fragment for which there is no consensus on the precise sequence. Vitamin K-dependent gamma (γ)-carboxylated variants of Oc are also found in circulation but there have been no methods that can define how many of the three potential γ-carboxyglutamic acid (Gla) residues are γ-carboxylated or provide their relative abundances. Recent reports that uncarboxylated and partially γ-carboxylated Oc forms have hormonal function underscore the need for precise evaluation of Oc at all three potential γ-carboxylation sites. Herein, mass spectrometric immunoassay (MSIA) was used to provide qualitative and semiquantitative (relative percent abundance) information on Oc molecular variants as they exist in individual plasma and serum samples. Following verification that observable Oc proteoforms were accurately assigned and not simply ex vivo artifacts, MALDI-MSIA and ESI-MSIA were used to assess the relative abundance of Oc truncation and γ-carboxylation, respectively, in plasma from 130 patients enrolled in vitamin K supplementation trials. Human Oc was found to circulate in over a dozen truncated forms with each of these displaying anywhere from 0-3 Gla residues. The relative abundance of truncated forms was consistent and unaffected by vitamin K supplementation. In contrast, when compared with placebo, vitamin K supplementation dramatically increased the fractional abundance of Oc with three Gla residues, corresponding to a decrease in the fractional abundance of Oc with zero Gla residues. These findings unequivocally document that increased vitamin K intake reduces the uncarboxylated form of Oc. Several reports of a positive effect of vitamin K intake on insulin sensitivity in humans have shown that un- or undercarboxylation of Oc, unlike in mice, is not associated with insulin resistance. Analyses similar to those described here will be useful to understand the functional significance of Oc γ-carboxylation in human health and disease.
Source: Molecular & Cellular Proteomics
Language: English
Topic: Vitamin K Research Studies
Subfield: Nutrition and Dietetics
Field: Nursing
Domain: Health Sciences
Open Access status: hybrid
Estimated APC paid: $3,430
Awards: Osteocalcin Synthesis and Catabolism, Dietary & non-dietary components of vitamin K metabolism, Population-Based Proteomic Investigation of Type 2 Diabetes Mellitus, G3572054984, NUTRITION RESEARCH FOR HEALTH AGING AND PREVENTION OF CHRONIC DISEASE, G6783744065, Team Approach to Translate Novel Biomarkers for Diabetes, Effect of Vitamin K on Age-Related Bone Loss, G820871158, VITAMIN K INSUFFICIENCY--A RISK FACTOR FOR OSTEOPOROSIS
OpenAlex ID: W2073193811