Vitamin K dependence of a calcium-binding protein containing gamma-carboxyglutamic acid in chicken bone

Abstract

Bone contains a small protein, osteocalcin, rich in the amino acid gamma-carboxyglutamate (gammaCGlu), which is dependent on vitamin K for its synthesis. The vitamin K dependence of osteocalcin biosynthesis has been studied both in the developing chicken embryo and in the skeletons of young chicks. Osteocalcin formation, assessed by quantitation of gammaCGlu in whole bone, begins in the 8-day embryonic skeleton and increases dramatically until hatching. Biosynthesis of gammaCGlu is warfarinsensitive; time- and dose-related inhibition of up to 75% is observed. Vitamin K deficiency reduces bone gammaCGlu concentrations by 50% in 3-week-old chicks, and this is totally corrected by including vitamin K1 in the diet. Feeding the vitamin K antagonist dicumarol decreases the bone gammaCGlu level 80% after 6 weeks, but no corresponding alterations are observed in the calcium and phosphorous content of the bone ash. Osteocalcin purified from bones of dicumoral-fed chicks exhibits several electrophoretically variant undercarboxylated protein species containing 2 and 3 gammaCGlu/57 amino acid residues rather than the normal complement of 4 gammaCGlu/57 residues.