Bone markers in osteoporosis and malignancy.

Abstract

Normal bone turnover is characterized by a balance (coupling) between the formation of new bone by osteoblasts and the resorption of old bone by osteoclasts. This bone remodeling takes place at discrete sites in the skeleton called bone remodeling units. Initially, the cells that line bone are replaced by osteoclasts that bring about resorption of the surface of bone. Next, osteoclasts are replaced by osteoblasts that gradually refill this space. This balance between bone formation and bone resorption is altered in most metabolic bone diseases (uncoupling). Over the past two decades many investigators have attempted to develop both sensitive and specific assays for the noninvasive assessment of bone turnover. Biochemical markers of bone formation include total and bone-specific alkaline phosphatase, serum osteocalcin (bone gla protein), and more recently procollagen I extension peptides. During the extracellular processing of type I collagen, there is a cleavage of the aminoterminal (p-coll-I-N) and carboxyterminal (p-coll-I-C) extension peptides prior to fibril formation. These peptides circulate in blood and could be useful markers of bone formation.