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

The incorporation of radioactivity from precursors of methylester and galacturonosyl residues into pectin was investigatedusing tissue slices cut from ripening pear fruits. Incorporationfrom ¹⁴CH₃ methionine into methyl ester of water soluble pectinincreased 10 fold in 4 d at 18 °C and declined in laterstages of ripening. Activity from ³H]inositol could not bedetected in gaJacturonic acid released enzymically from solublepolysaccharides. When l³H]glucose was used as a precursor, activitycould be detected in galacturonic acid released from both thesoluble and insoluble polysaccharide fractions. Methionine wasa more efficient precursor of methyl ester groups than S-adenosylmethionine or S-methyl methionine; incorporation from all threeprecursors was inhibited under nitrogen. Radioactively labelledmethyl 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 this4 h incubation.