The relationship 01 the distribution of pectic substances to tissue softening was exanined in ripening mangos at four stages of ripeness. Water-soluble and alkali-soluble pectin declined and ammonium oxalate soluble pectin increased as the mango lost its firmness and became soft. Polygalacturonase and cellulase activities of cell wall preparations ir creased markedly during ripening. The decline in alkali-soluble peci in and the increase in polygalacturonase activity correlated well wit11 the loss of firmness. Alkali-soluble pectin declined slowly in rip : mangos stored at 4°C. This decline correlated with loss of fnmne:s of the stored mangos. The cellulase activity of cell wall preparatio 1s from ripe mangos increased during 4°C storage and the increase correlated with the decrease in firmness.
Representative samples of fresh orange juice (OJ) with various flavor scores were assayed for peroxidase activity. In thk assays, pphenylenediamiue (PPDA), the hydrogen donor, was oxidized by H, 0, after a short time lag (about 1 min) caused by interaction of ascorbic acid in the juice with a PPDA-oxidation intermediate. A search was made for compounds that are native substrates of OJ peroxidase. Thus, the peroxidase activity of a protein fraction precipitated from neutralized OJ with ammonium sulfate and dialyzed free of ascorbic acid was tested with H,O, and a number of hydrogen donor compounds that are normal juice constituents. Ascorbic acid was very reactive, as were the phenolic acids (caffeic, gentisic and coumaric). The flavonoids, criodictyol, hesperidin and naringin were unreactive. Reduced nicotinamide adenine dinucleotide (NADti) was also reactive in the presence of hydroquinone and other compounds that mediate electron transfer through intermediate states. The dialyzed protein fraction also catalyzed the oxidation of pyridoxal-PO,. indoleacetic acid. dihvdroxvmaleic acid, and NADH-; pcresol l& 0, plus Mn++. Although dJ appears to contain many compounds that are reactive with peroxidase, their reactivities in OJ are apparently very slow due to the level of H, 0,. Concentrations of ascorbic and caffeic acids did not change in OJ incubated at 30°C for 4 hr. Processing conditions that increased peroxidase activity and pulp content in OJ decreased quality of the juice.
An alcohol: NAD oxidoreductase [EC #1.1.1.1] was extracted from Valencia oranges and examined for substrate specificity with aliphatic alcohols and aldehydes reported in citrus. The oxidoreductase oxidizes alcohols to the corresponding aldehydes in the homologous series from ethanol to octanol.The enzyme reduces aldehydes to the corresponding alcohol in the series from acetaldehyde to decanal. The maximum rates of reduction of the aldehydes are higher than the oxidation rates for the alcohols. The unsaturated alcohols, 2-propen-l-ol, 2-buten-lol, 4-penten-l-ol, and 2-hexen-l-ol are oxidized faster than the saturated homologs. The enzyme has a higher affinity for the aldehydes than for the saturated alcohols.
SUMMARY: Extracts of orange juice vesicles oxidized p‐hydroquinone (HQ) and o‐dihydroxyphenylalanine (DOPA) with O2. Most of the oxidase activity was associated with a particulate fraction that sedimented at 100,000 × g. Sonic disruption of the particulates followed by chromatography on DEAE‐cellulose increased specific activities with both substrates. The partially purified enzyme oxidized numerous naturally occurring o‐diphenols and o‐methoxyphenols. Acidic media below pH 4 and heating above 70°C destroyed most of the oxidase activity. KCN, diethyl‐dithiocarbamate and 8‐hydroxyquinoline but not EDTA inhibited the oxidase. The partially purified enzyme reduced benzoquinone and oxidized the reduced form of nicotinamide‐dinucleotide (NADH). Ubiquinone, a benzoquinone derivative, but not menadione or vitamin K1, naphthoquinone derivatives, could replace benzoquinone in the oxidation of NADH. Ubiquinone, benzoquinone and similar p‐quinones may function in the orange as the oxidation‐reduction couple between NADH‐quinone reductase and diphenol oxidase.
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