Pectin-methylesterase PM

Pectin methylesterase (PM) is the enzyme which catalyzes the hydrolysis of the methyl ester groups in pectinic acids. In contrast to PG and protopectinase, a great deal of progress has been made during the past decade in our knowledge of PM. The older term pectase, 8 invented before the chemical action of the enzyme was understood, is now used by a decreasing number of workers while the name pectin- 

PM seems to occur in small amounts in the vegetative tissues of all higher and lower plants investigated. It is very abundant in the tomato fruit,60 orange flavedo and albedo,21 the tobacco plant,61 eggplant62 and alfalfa it usually occurs in the commercial fungal pectinase preparations that are manufactured for the clarification of fruit juices.25 In the natural products in which the enzyme is found, the major portion of it is usually strongly adsorbed on the water-insoluble cellular components of the tissue macerates.60 62 68 

Lineweaver and Ballou49 have proposed a pectinesterase unit ( PE. u. ) for expressing PM activity. One such unit is equivalent to 1/930 PMU under the same experimental conditions or the quantity of enzyme that, at 30° and optimum pH, will catalyze the hydrolysis of pectin at an initial rate of one milliequivalent ester bonds per minute in a standard substrate (0.5% citrus pectin containing 8-11% methoxyl) and 0.15 M sodium chloride. The use of the latter unit is unfortunate since the values obtained for the activity in ordinary plant materials are obtained in the third decimal place and because the experimental conditions are so 

Holden84 used a method of measurement in which the methanol liberated by PM in a buffered solution of pectinic acid in ten minutes has been estimated by the Zeisel method. This procedure is much more complicated than that described above and must be regarded as unreliable on account of the rapid change in the pH of such reaction mixtures, even if buffered. 

A recent patent issued to Leo and Taylor66 describes a method for the preparation of a colorless solution of PM from alfalfa. The ground leaves are pressed out and the juice is clarified with aluminum hydroxide. The latter may be produced in the press juice by the addition of calcium carbonate, and aluminum chloride hexahydrate. The precipitate carries 

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