Osteocalcin is the major non-collagenous protein of bone and is regarded as a specific index of bone formation. The aim of this study was to examine the rate of bone formation measured by osteocalcin in 38 patients with ankylosing spondylitis (AS) and its dependence on various parameters of calcium and phosphate metabolism.
Serum osteocalcin, alkaline phosphatase, parathyroid hormone, and 1,25-dihydroxyvitamin D were measured in 38 patients with ankylosing spondylitis and in 52 controls.
Mean serum osteocalcin was significantly reduced in patients with AS (men 1.7 (1.1) ng/ml; women 1.2 (1.1) ng/ml) compared with the corresponding control groups (men 3.2 (1.3) ng/ml; women 4.1 (1.7) ng/ml). In contrast, alkaline phosphatase was only slightly but significantly higher (135 (44) U/l) in patients with AS than in the corresponding controls (114 (35) U/l). Serum parathyroid hormone (AS 3.1 (0.7) v 2.7 (0.6) mE/ml) and 1,25-dihydroxyvitamin D (AS 64.0 (34.5) v 52.4 (6.7) pg/ml) were slightly but not significantly higher in patients with AS. Consequently, as both hormones are known to stimulate osteocalcin synthesis, they are not responsible for low osteocalcin levels in patients with AS. No significant correlation between alkaline phosphatase and osteocalcin was found. Low serum levels of osteocalcin in patients with AS reflect lower osteoblastic activity in AS.
Bone turnover in patients with AS is characterised by low bone formation in the presence of normal levels of calcium regulating hormones.