Occurrence and biosynthesis of catalase at different stages of seed maturation

Abstract

It was to be shown whether during the biogenesis of microbodies some of their components were already present in the cell prior to the organelle's assembly. To this end, the occurrence and properties of catalase in soluble and particular fractions of ripening cucumber seeds were examined. Homogenates of seeds from ripening fruits were fractionated by isopycnic density gradient centrifugation, and thus catalase was found in three different fractions: as a soluble enzyme in the gradient supernatant, as a membrane fraction at density d=1.18 kg l^-1, and in association with microbodies. In the early steps of seed formation, catalase was detected at density d=1.18 kg l^-1 and in the gradient supernatant. At a later stage of seed maturation, however, catalase was primarily associated with microbodies which exhibited an equilibrium density of d=1.23 kg l^-1. M _r as well as subunit M _r of catalase were determined, and their close immunological relationship to leaf peroxisomal catalase and glyoxysomal catalase was demonstrated. Biosynthesis of catalase at different stages of seed maturation was investigated by in vivo labeling with l-[³⁵S]methionine, l-[¹⁴C]leucine and δ-[³H]aminolaevulinic acid. Electrophoretic analysis of de novo synthesized catalase subunits revealed the occurrence of a heavy form (M _r 57,500) in the soluble fraction; this form was preferentially labeled. A light form, M _r 53,500, was detected in microbodies and also in the soluble fraction. The findings lend support to the hypothesis that the rate of catalase synthesis is highest in an early stage of seed formation, when globulins have already been formed, but before de novo synthesis of malate synthase has commenced. Prior to microbody assembling, a cytoplasmic pool of catalase was labeled.