Lipoxidase enzyme was isolated and partly purified from pea seeds by ultracentrifugation, ammonium sulphate precipitation, Sephadex and DEAE-Cellulose column chromatography. The pH optimum of the enzyme was 7.2 and the Michaelis-Menton constant 2.3 x M. Disc gel electrophoresis revealed the presence of 3 to 4 isoenzymes, while the molecular weight determination in the presence of sodium dodecyl sulphate gave a value of 7.4 x lo4. The presence of lipoxidase in pea mitochondria and in the peroxisome-like bodies was demonstrated. A low enzyme activity was found with the purified chloroplasts but amuch higher activity was found in the plastids andin thecytoplasm of theetiolated tissue. The enzyme was found not to be compartmentalised in any particulate fraction of the pea seedlings investigated.

Some in-vivo and in-vitro characteristics of pectin methylesterase (PME) from cell wall and whole potato tuber were determined and its role was elucidated in the precooking and cooling steps of commercial dehydrated mashed potato production. The isolated cell walls contained 16.3% anhydrogalacturonic acid with a degree of esterification (DE) of 56, and 25 % of tuber total PME. Cell wall pre-cooking in model systems resulted in a DE of 53.2. Native cell wall pectin solubilisation was much less than that of H-cell walls devoid of metal cations. Calcium but not Mg, supplied as Ca(Mg)-starches, retarded pectin solubilisation, while Ca-uptake by retained pectin was high when cooling followed pre-cooking. Texture measurements of potato slices pre-cooked, cooled and cooked suggested a firming effect which could be attributed to Ca-release from gelatinised starch during pre-cooking, and stabilisation of Cabridges with free COO-groups of cell wall pectin duringcooling. The role ascribed to PME in causing firming is de-emphasised.

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