A Galactolipase Activity Associated with the Thylakoids of Wheat Leaves ( Triticum aestivum L.)

When chloroplasts prepared from the primary leaves of wheat are incubated at pH 6.0, a large proportion of the thylakoid galactolipids, particularly monogalactosyl diacylglycerol (MGDG), is rapidly degraded. The hydrolysis is incomplete, producing Iyso-MGDG, together with free fatty acids (predominately linolenic acid, LNA). Initial rates are extremely rapid (in excess of l/µmol LNA mgchl -1 min -1) but an endpoint is reached after 1–2 min, when as much as half the thylakoid MGDG has been partially hydrolysed. The «galactolipase» activity is associated with the thylakoid membranes, but is exposed to the stromal surface. It is inhibited by the sulfhydryl-enyzme inhibitors phenylmercuric acetate and p-chloromercuribenzoid acid. The enzyme is inactivated by exposure to temperatures > 30 °C, and is sensitive to detergent or sonication treatments that disturb the integrity of the thylakoid membranes. It has not yet been possible to isolate an active protein fraction.