Changes in bone morphogenic enzymes and lipid composition of equine osteochondrotic subchondral bone

Osteochondrosis (OC) is a disturbance in the process of endochondral ossification, a process in which cartilage is mineralised and transformed into bone. In this process different biochemical events occur, of which the cartilage component has been studied so far almost exclusively. In this study we concentrated on the biochemical characterisation of normal and osteochondrotic subchondral bone, by analysis of enzyme activities, DNA content and phospholipids (PL). In subchondral bone, lysyl oxidase and both total and bone alkaline phosphatase activity were significantly increased in all degrees of OC. DNA content was increased only in the most established grade of OC investigated (grade 4). Furthermore, lactate dehydrogenase activity was significantly lower in grades 2 and 3 OC, but was normal in grade 4 OC, indicating that severe cell damage is not probable. Nonbuffer extractable PL content was substantially higher in osteochondrotic subchondral bone. The phosphatidylethanolamine (PE) to phosphatidylcholine (PC) ratio in both normal and OC subchondral bone was very low (typically 0.21 w/w, PE/PC), which indicates that these PLs were not from cellular origin and could be important in the maturation process of mineralised cartilage into bone and hence in the pathogenesis of OC.