Urinaryγ-Carboxyglutamic Acid and Serum Osteocalcin as Bone Markers: Studies in Osteoporosis and Paget’s Disease*

Abstract

Osteocalcin is a major bone matrix protein with high affinity for hydroxyapatite. This property is conferred by several residues of the calcium-binding amino acid γ-carboxyglutamate (Gla), which requires vitamin K for its biosynthesis. Because this protein may play a role in the local control of calcium deposition or removal in mineralized tissue, we measured circulating osteocalcin levels and urinary excretion of its breakdown product, Gla, in patients with osteoporosis and Paget’s disease. Studies were conducted either on a metabolic ward or in ambulatory patients. Diagnoses were established by clinical and laboratory findings, and were confirmed by histological examination in 19 of 26 patients with osteoporosis. Mean urinary Gla excretion was increased (P < 0.001) in patients with osteoporosis by 50% above the normal mean; serum osteocalcin, however, was not significantly different from normal. In Paget’s disease patients, this pattern was reversed; serum osteocalcin levels were increased 3-fold (P < 0.001), while urinary Gla excretion was consistently normal, regardless of the extent or activity of the disease. These data demonstrate that measurements of urinary Gla and serum osteocalcin may provide important insights into the metabolic derangements in these and other bone disorders.