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First order rate constants for the degradation (degradation constants) of catalase in the cotyledons of sunflower (Helianthus annuus L.) were determined by measuring the loss of catalase containing (14)C-labeled heme. During greening of the cotyledons, a period when peroxisomes change from glyoxysomal to leaf peroxisomal function, the degradation of glyoxysomal catalase is significantly (P = 0.05) slower than during all other stages of cotyledon development in light or darkness. The degradation constant during the transition stage of peroxisome function amounts to 0.205 day(-1) in contrast to the constants ranging from 0.304 day(-1) to 0.515 day(-1) during the other developmental stages. Density labeling experiments comprising labeling of catalase with (2)H(2)O and its isopycnic centrifugation on CsCl gradients demonstrated that the determinations of the degradation constants were not substantially affected by reutilization of (14)C-labeled compounds for catalase synthesis. The degradation constants for both glyoxysomal catalase and catalase synthesized during the transition of peroxisome function do not differ. This was shown by labeling the catalases with different isotopes and measuring the isotope ratio during the development of the cotyledons. The results are inconsistent with the concept that an accelerated and selective degradation of glyoxysomes underlies the change in peroxisome function. The data suggest that catalase degradation is at least partially due to an individual turnover of catalase and does not only result from a turnover of the whole peroxisomes.

Catalase Degradation in Sunflower Cotyledons during Peroxisome Transition from Glyoxysomal to Leaf Peroxisomal Function