Strategies for developing human osteocalcin standards: A critical evaluation

Abstract

Osteocalcin is a small protein uniquely produced by osteoblasts and odontoblasts. Since about 30% of the de novo synthesized osteocalcin is set free in the blood stream, it is widely used as a marker for bone formation. However, circulating immunoreactive osteocalcin (irOC) consists of several fractions, which may differ from each other with respect to size and calcium binding properties. Whereas it is generally assumed that the fraction with high affinity for hydroxyapatite reflects bone formation, we have found a high correlation between bone mass and irOC with low affinity for hydroxyapatite. Since the antibodies used in various commercial test kits for osteocalcin have different affinities for the various irOC fractions, well-defined homogenous standards have to be prepared which may be used for standardization of the detection techniques used in various laboratories. In this paper we give a critical evaluation of the strategies which may be followed.

Key Words:

• bone formation
• bone marker
• γ-carboxyglutamate
• vitamin K