Light dependence of catalase synthesis and degradation in leaves and the influence of interfering stress conditions

The enzyme catalase (EC 1.11.1.6) is light sensitive and subject to a rapid turnover in light, similar to the D1 reaction center protein of photosystem II. After 3 h of preadaptation to darkness or to different light intensities (90 and 520 μmol m⁻² s⁻¹ photosynthetic photon flux density), sections of rye leaves (Secale cereale L.) were labeled for 4 h with l-[³⁵S]methionine. From leaf extracts, catalase was immunoprecipitated with an antiserum prepared against the purified enzyme from rye leaves. Both incorporation into catalase and degradation of the enzyme polypeptide during a subsequent 16-h chase period increased with light intensity. At a photon flux density of 520 μmol m⁻² s⁻¹, the apparent half-time of catalase in rye leaves was 3 to 4 h, whereas that of the D1 protein was much shorter, about 1.5 h. Exposure to stress conditions, such as 0.6 m NaCl or a heat-shock temperature of 40°C, greatly suppressed both total protein synthesis and incorporation of the label into catalase and into the D1 protein. Immunoblotting assays indicated that in light, but not in darkness, steady-state levels of catalase and of the D1 protein strongly declined during treatments with salt, heat shock, or translation inhibitors that block repair synthesis. Because of the common property of rapid photodegradation and the resulting dependence on continuous repair, declines in catalase as well as of the D1 protein represent specific and sensitive indicators for stress conditions that suppress the translational activities of leaves.