ABSTRACT: The rigidity of our skeleton is due to reinforcement with nano-sized minerals of biological apatite. Theories of skeletal apatite nucleation include initiation by the organic matrix (collagen and non-collagenous proteins), and increasing the phosphate concentration by hydrolysis of phosphoric acid monoesters. An enzyme that has been attributed to skeletal mineralization - tissue-nonspecific alkaline phosphatase (TNAP) - is known to increase inorganic phosphate concentration by catalyzing phosphoric acid monoester hydrolysis. However, the substrate for TNAP in skeletal mineralization has not been identified. TNAP also catalyzes the hydrolysis of inorganic phosphate from phosphate polymers (polyphosphates). Polyphosphates are produced by some microorganisms to store bioavailable phosphorus.
Polyphosphates and TNAP have been identified in the actively mineralizing regions of the elasmobranch (shark, stingray) skeleton. Elasmobranchs are a relevant model organism for bone mineralization because their skeletons are also reinforced with biological apatite, and the cartilaginous core of their skeletal elements can be cryo-sectioned. Bone sectioning usually requires demineralization or other sample preparation methods that may alter unstable bone mineral precursors. With cryo-sectioning, minimal sample processing is required for the identification of possible unstable mineral precursors.
The recent identification of polyphosphate and TNAP in the elasmobranch skeleton supports a new theory for skeletal mineral nucleation. The biochemical control of inorganic phosphate ion activity, and consequently, biological apatite saturation with polyphosphate intermediates will be described.
Biosketch: Sidney Omelon is a chemical engineer (Ing.) who has worked with crystallization, materials science, and biomedical projects in academia and industry. She is an assistant professor in Chemical and Biological Engineering at the University of Ottawa, where her major projects include the chemistry of bone mineralization, temporary bone replacement materials, cortical bone toughness, capturing phosphorus from municipal wastewater with mineralization, and oil-well scale formation.