The long-term objective of these studies is to better elucidate the pathogenesis of calcium pyrophosphate dihydrate (CPPD) crystal deposition disease, a common form of arthritis, so that logical approaches to treatment and/or prophylaxis may be formulated. Disordered inorganic pyrophosphate (PPi) metabolism has been strongly implicated as a factor in contributing to CPPD crystal formation in vivo. Recent reports support this hypothesis by demonstrating elevated intracellular PPi in cultured cells of a single kindred with familial CPPD deposition; and elevated nucleoside triphosphate pyrophosphohydrolase (NTPPPH) activity, which generates PPi, in CPPD-containing cartilage. Subsequent and preliminary studies suggest that: elevated intracellular PPi is present in skin-derived fibroblasts from patients with both sporadic and familial CPPD deposition disease; NTPPPH activity is elevated in joint fluids and on the surface of cultured cells for patients with CPPD deposition; NTPPPH on chondrocytes in an ectoenzyme capable of generating PPi at the extracellular site where CPPD crystals form in cartilage. This proposal will specifically aim at determining: (1) if elevated intracellular PPi is a valid disease "marker"; (2) the metabolic source of intracellular PPi; (3) whether elevated ecto-NTPPPH is also a disease "marker"; (4) whether extracellular substrate is available in cartilage to interact with ecto-NTPPPH, thereby generating extracellular PPi; (5) which factors modulate NTPPPH activity; (6) what physical and biochemical factors affect ppi elaboration by cartilage. The knowledge of chondrocyte ectoenzymes which we glean will be used to devise an in vivo cartilage organ culture model of CPPD deposition which would facilitate studies of therapeutic interventions and of the biochemical effects of crystals in cartilage.