The proteolytic degradation in vitro of the NADPH-protochlorophyllide oxidoreductase of barley (Hordeum vulgare L.)

A cell-free membrane system has been developed from isolated barley etioplasts which displays a highly selective decrease of the NADPH-protochlorophyllide oxidoreductase in vitro which is indistinguishable from that observed previously in the intact plant. The rapid breakdown of the enzyme protein in vitro is caused by a membrane-bound proteolytic activity. The protease is essentially independent of pH in the physiological pH range of 6 to 8.5. The optimum temperature for the reaction is approximately 40 degrees C. In the presence of excessive protochlorophyllide the enzyme is no longer degraded or inactivated during illumination of dark-grown plants. In the isolated membrane fraction protochlorophyllide also enhances the stability of the enzyme, a similar effect is exerted by NADPH but not by NADH. The results suggest that the inactivation of the NADPH-protochlorophyllide oxidoreductase is influenced by the interaction of the enzyme with protochlorophyllide and NADPH. In the absence of these two components the enzyme becomes susceptible to proteolytic degradation.