Fruit Softening II. Precursor Incorporation into Pectin by Pear Tissue 6

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Abstract
The incorporation of radioactivity from precursors of methyl ester and galacturonosyl residues into pectin was investigated using tissue slices cut from ripening pear fruits. Incorporation from 14CH3 methionine into methyl ester of water soluble pectin increased 10 fold in 4 d at 18 °C and declined in later stages of ripening. Activity from [3H]inositol could not be detected in gaJacturonic acid released enzymically from soluble polysaccharides. When l3H]glucose was used as a precursor, activity could be detected in galacturonic acid released from both the soluble and insoluble polysaccharide fractions. Methionine was a more efficient precursor of methyl ester groups than S-adenosyl methionine or S-methyl methionine; incorporation from all three precursors was inhibited under nitrogen. Radioactively labelled methyl ester did not decline during a 225 min ‘chase’ following a 15 min ‘pulse’ of [14CH3]methionine; the total pectin content of slices increased by 20% during this 4 h incubation.

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