PECTIN METHYL ESTERASE FROM ORANGE FRUIT - CHARACTERIZATION AND LOCALIZATION BY IN-SITU HYBRIDIZATION AND IMMUNOHISTOCHEMISTRY

Pectin methyl esterase (PME) from orange (Citrus sinensis L.) fruit pe els has been purified by ammonium sulphate precipitation, and ion-exch ange and gel-filtration chromatography. Characterization of the enzyme revealed a 36-kDa protein with an isoelectric point > 9, a pH optimum at 7 and temperature optimum at 50 degrees C. The substrate specifici ty and kinetic experiments showed that the affinity of PME for pectin was highly dependent on the degree of esterification (DE) of the pecti n, with K-m values of 0.7 mg ml(-1) for pectin with a DE of 70% and 17 mg ml(-1) for pectin with;a DE of 25%. The sequences of the NH2-termi nal end of digested peptides from the mature protein were obtained. A DNA fragment of 501 bp was cloned by polymerase chain reaction amplifi cation using degenerate primers and was further used for screening of a cDNA library. Two cDNA clones were isolated encoding PMEs of 584 ami no acids and 362 amino acids, respectively, including a putative signa l peptide. The deduced amino acid sequence showed full identity to the sequenced peptides. Polyclonal antibodies raised against orange peel PME were used for immunohistochemistry. The main localization of PMEs was in the outer cell layers of the juice vesicles, in the outer cell layers of the lamellae between the segments and in the inner cell laye rs of the albedo in the peel. In-situ hybridization showed that the mR NA is very abundant in the fruit and was found in the same cell layers as the native enzyme. A very intensive staining for PME mRNA was also seen in the core and in the flavedo close to the oil glands.