Hydrolysis of triphosphoinositides in developing central and peripheral nervous systems of the chicken

Abstract

Triphosphoinositide phosphatase and phosphodiesterase activities were measured in homogenates of brain and sciatic nerve from chick embryos and chicks. The characteristics of these enzymes were the same in chick brain and nerve and similar to previous reports for mammalian brain. Both activities were measured in the presence of cetyltrimethylammonium bromide to provide optimal stimulation. Triphosphoinositide phosphatase activity was dependent upon Mg 2+ . Selective inhibition by EGTA and stimulation by Ca 2+ suggests that the nervous tissue triphosphoinositide phosphodiesterases specifically require Ca 2+ . Competition by the phosphodiesterase in phosphotase assays was prevented by EGTA. Homogenates of both tissue required supplementation with Mg 2+ but non-specific phosphatase activity was absent. In triphosphoinositide phosphodiesterase assays without added Mg 2+ , competition by the phosphatase was minimal and was further suppressed by the addition of a small amount of Ca 2+ which was required for optimal phosphodiesterase activity. The increase in brain triphosphoinositide phosphatase but not phosphodiesterase was coincident with the increase in synthetic enzymes and the deposition of myelin. The specific activity of both phophohydrolase activities was lower in sciatic nerve than in brain. The developmental patterns of the enzymes were also different. The triphosphoinositide phosphatase did not change with age while the triphosphoinositide phosphodiesterase decreased steadily with no marked change during the period of most active myelination. The catabolic activities greatly exceeded the reported synthetic activities in both brain and nerve.