A Study of Polyphenols and Polyphenol Oxidase in Selected Tropical Tubers
Polyphenolic constituents of the corms of colocasia esculenta var. esculenta and xanthosma sagittifolium as well as tubers of Icacina trichanta, Mirabilis jalapa, Cyanastrum cordifolium, Anchomanes difformis and Dioscorea preussil were extracted immediately after harvest, fractionated, hydrolysed and analysed by chromatographic techniques, absorption spectroscopy and mass spectral procedure. P-Hydroxybenzoic acid was conclusively identified in the extracts of I. trichanta and c. cordifolium by gas chromatographic-mass spectral (GC-MS) procedure. Quantitative analysis showed that the highest level of phenolics obtained immediately after harvest was found in M. jalapa. Xanthosoma sagittifolium was found to be more susceptibles to browning than c. esculenta. The level of phenolic content in the corms increased during storage at lower relative humidity, while no significant weight loss compared with high relative humidity. Sprouting was more pronounced at higher relative humidities. Exposure in intact corms to varying light period effected increased phenolic content was reversed when the corms were transferred to darkness. Crude polyphenol from dried acetone extracts obtained from the corms of X. sagittifolium and tubers of M. jalapa were purified while that from C. esculenta was partially purified and characterized. The greatest enzyme activity was found in M. jalapa while the activity of X. sagittifolium was higher than that of C. esculenta. The molecular weights determined by gel filtration on sephacryl 5-200 column were as follows: X. sagittifolium 150,000; M. jalapa 165,958. The submit molecular weight for X. sagittifolium by sodium dodecyl sulphate gel electrophoresis was 13,551. The pH profiles indicate that X. sagittifolium and C.esculenta both have pH optima of 6.5 while M.jalapa has a pH optimum of 6.0. The effect of temperature on the activities of the enzyme shows that the highest activities X. sagittifolium and M.jalapa were obtained at 50oc while that of C. esculenta was 400c. The km and vmax values obtained respectively were X. sagittifolium 9.2 and 5.83-2; M.jalapa 12.7 and 5.72-2 and C.esculenta 12.1 and 2.12-2. Of all the substrates tested the enzyme from the three sources showed activity towards catechol, DL 3, 4-dihydroxyphenyl alanine (DL-DOPA), pyrogallol and chlorogenic acid. The following organic chemicals were observed to inhibit the activity of polyphemol oxidase: 2-mercaptoethanol, L-cysteine, sodium sulphite, sodium metabisulphite and dithiothreitol. The absorption spectrum of each of the purified enzyme had a peak at 215nm with a broad shoulder at 280nm. The physiological and biochemical implications of these findings were discussed.