In the present paper the catecholase and cresolase activities of eggplant polyphenol oxidase (PPO) are described. To preserve the latter activity, a partially purified enzyme was used. Peroxidase was removed from the preparation to avoid its interference with PPO during phenol oxidation. The partially purified eggplant PPO was fully active. The catecholase/cresolase ratio of 41.1 indicated that, in a pH close to the physiological, diphenol oxidation predominates over monophenol oxidation. The characteristic lag phase of the cresolase activity is modulated by the pH, the monophenol and diphenol concentrations, and the enzyme's concentration. The effect of several inhibitors was also tested, and the K(i) values of the two most effective (tropolone and 4-hexylresorcinol) were determined.

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Fatty acid hydroperoxide lyase (HPO-lyase) was puri®ed 300-fold from tomatoes. The enzymatic activity appeared to be very unstable, but addition of Triton X 100 and b-mercaptoethanol to the buer yielded an active enzyme that could be stored for several months at À808C. The enzyme was inhibited by desferoxamine mesylate (desferal), 2-methyl-1,2-di-3-pyridyl-1-propanone (metyrapone), nordihydroguaiaretic acid (NDGA), n-propyl gallate and butylated hydroxyanisole, suggesting the involvement of free radicals in the reaction mechanism and the existence of a prosthetic group in the active center. However, no heme group could be demonstrated with the methods commonly used to identify heme groups in proteins. Only 13-hydroperoxides from linoleic acid (13-HPOD) and a-linolenic acid (a-13-HPOT) were cleaved by the tomato enzyme, with a clear preference for the latter substrate. The pH-optimum was 6.5, and for concentrations lower than 300 mM a typical Michaelis±Menten curve was found with a K m of 77 mM. At higher a-13-HPOT concentrations inhibition of the enzyme was observed, which could (at least in part) be attributed to 2E-hexenal. A curve of the substrate conversion as a function of the enzyme concentration revealed that 1 nkat of enzyme activity converts 0.7 mmol a-13-HPOT before inactivation. Headspace analysis showed that tomato HPO-lyase formed hexanal from 13-HPOD and 3Z-hexenal from a-13-HPOT. A trace of the latter compound was isomerized to 2E-hexenal. In addition to the aldehydes, 12-oxo-9Z-dodecenoic acid was found by GC/MS analysis. To a small extent, isomerization to 12-oxo-10E-dodecenoic acid occurred. #

Soybean lipoxygenase-1 is able to oxidize dilinoleoyl phosphatidylcholine at pH 7.5 and 10. The reaction could be followed spectrophotometrically from the increase of the absorbance at 234 nm. An intermediate product and a final product were detected. In the intermediate product only one of the linoleoyl chains (either sn1 or sn2) was oxidized. In the final product, both linoleic acid units were converted into hydroperoxides. Apparently, oxidation of one of the linoleoyl chains leads to a disruption of the structure of the mixed bilayer disk, making the remaining fatty acid unit more accessible to the action of the enzyme. The specificity of lipoxygenase-1 when acting on phospholipids is not affected by pH. The exclusive production of 13-hydroperoxyoctadecadienoic acid derivatives of dilinoleoyl phosphatidylcholine at pH 7.5 and 10 may result from the blockage of the carboxylic end of the fatty acid.