Vitamin K is essential for the carboxylation of
glutamic acid residues, such as
osteocalcin. Recent studies have reported that
vitamin K reduces vertebral and
hip fractures without increasing bone mass in patients with
osteoporosis, suggesting that
vitamin K could affect bone quality. However, the mechanism is unknown. To investigate the involvement of the carboxylation of
osteocalcin in bone quality, the present preliminary study examined serum bone markers and ultrasound velocity, a possible
indicator of bone quality, in 14 healthy prepubertal children (eight boys and six girls) aged between 7 and 12 years. Venous blood was collected between 0800 and 0900 h after an overnight fast, and serum levels of intact, carboxylated and undercarboxylated
osteocalcin, bone-specific
alkaline phosphatase and
type I procollagen carboxyl extension
peptide were measured. Speed of sound in the right tibia was measured using a SoundScan 2000 Compact (Myriad Ultrasound System, Rehovot, Israel). As a result, there was no significant correlation between the serum bone markers and the Z score for the speed of sound. In contrast, the ratio of serum carboxylated
osteocalcin to serum intact
osteocalcin was positively correlated with the Z score for the speed of sound (r = 0.621, P = 0.016). These findings suggest, for the first time, that carboxylation of
osteocalcin is related to bone quality. Further studies are needed to clarify the role of carboxylation of
osteocalcin in bone, and this will provide a new insight into the mechanism of
vitamin K treatment in
osteoporosis.