Activity or mass concentration of bone-specific alkaline phosphatase as a marker of bone formation

Background: Recently published data identified bone-specific alkaline phosphatase (BALP) as a good marker of bone formation in different bone diseases and osteoporosis. Two methods are available for BALP determination: one measures enzyme activity, the other its mass concentration. We compared results for BALP activity and its mass concentration in a group of 88 healthy pre- and postmenopausal women to identify which is a more useful marker for detecting early menopausal bone remodelling changes. Methods: We measured BALP activity and BALP mass concentration in relation to femoral neck (FN) and lumbar spine (LS) bone mineral density (BMD) and some other widely used bone markers: osteocalcin (OC), procollagen type I N-terminal propepticle (PINP) and serum C-terminal telopepticle cross-links of type I collagen (CTx) in serum samples from 50 premenopausal (age 45.9 +/- 4.6 years) and 38 postmenopausal (age 54.4 +/- 4.5 years) women. Results: Healthy postmenopausal women exhibited 34.2% (p < 0.01) and 27.3% (p=0.000) higher levels of BALP activity and mass concentration than premenopausal women, respectively. At the same time, FN and LS BMD were not significantly different between the groups. CTx values were significantly higher in postmenopausal women (p=0.018), while OC and PINP were not. We observed significant correlation between BALP activity and mass concentration (r=0.85, p < 0.01). The correlation between BALP activity and FN BMD or LS BMD was insignificant. BALP mass correlated significantly with LS BMD (r=-0.370, p=0.033) but not with FN BMD. As expected, we proved a significant positive correlation for both BALP methods with the other bone markers measured in our study. Conclusions: Postmenopausal women have slightly higher bone turnover. Since LS and FN BMD were not significantly lower in our group of healthy post-menopausal women, but BALP and CTx were markedly higher, we suggest that measurements of BALP and CTx might be useful as early markers of higher bone turnover. Finally, our results did not show any differences between the clinical utility of BALP activity and BALP mass concentration measurements.