Turnover of catalase heme and apoprotein moieties in cotyledons of sunflower seedlings.

The turnover of catalase apoprotein and catalase heme was studied in cotyledons of sunflower (Helianthus annuus L.) seedlings by density labeling of apoprotein and radioactive labeling of heme moieties. The heavy isotope (50% (2)H(2)O) and the radioactive isotope ([(14)C]5-aminolevulinic acid) were applied either during growth in the dark (day 0-2.5) or in the light (day 2.5 and 5). Following isopycnic centrifugation of catalase purified from cotyledons of 5-day-old seedlings, superimposition curve fitting was used to determine the amounts of radioactive heme moieties in native and density-labeled catalase. Data from these determinations indicated that turnover of catalase heme and apoprotein essentially was coordinate. Only small amounts of heme groups were recycled into newly synthesized apoprotein during growth in the light, and no evidence was found for an exchange of heme groups in apoprotein moieties. It followed from these observations that degradation of catalase apoprotein was slightly faster than that of catalase heme. A degradation constant for catalase apoprotein of 0.263 per day was determined from the data on heme recycling and the degradation constant of catalase heme determined previously to be 0.205 per day (R Eising, B Gerhardt [1987] Plant Physiol 84: 225-232).