Dialyzed wheat leaf extracts, catalase, and horse-radish peroxidase catalyze the decarboxylation and oxidation of indoleacetic acid at pH 5.0–6.0 in the presence of critical concentrations of manganese and monohydric phenols or resorcinol. The equivalent of 1 mole of carbon dioxide is liberated and 1 mole of oxygen consumed per mole of substrate. Manganic ions formed by a phenol–peroxidase–peroxide system initiate the decarboxylation and oxidation. A naturally occurring ether soluble factor from wheat leaves, and maleic hydrazide, can substitute for the active phenols. Catechol, hydroquinone, pyrogallol, seopoletin, and riboflavin, etc. competitively inhibit the oxidation. The nature of the active peroxide is discussed and a reaction sequence involving an organic peroxide or radical rather than hydrogen peroxide is submitted as being a possibility.
The kinetics of the malic dehydrogenase in cell-free extracts of wheat seedlilgs have been srudied bf ttte Warbuig technique, using methyle.ne blue. The enzyme behaves nnal6gously to the"animal en,.vqe jn .lh.at it is coenzyme I-Iinked and its oridatioi product, oxalacelic acid, is inhibitory.to.the reactron.
5-Aminolaevulinate hydro-lyase (EC 4.2.1.24), which catalyzes the formation of porphobilinogen from 5-aminolaevulinate (5-ALA), was isolated from wheat leaves and partially purified. The enzyme was specific for 5-ALA, sulfhydryl-dependent, and required divalent cations for maximum activation. Pyrophosphate, EDTA, and ATP were strongly inhibitory. Laevulinate, but not ethyl laevulinate, was also an inhibitor. The pH optima were 7.5–7.6 in Tris and 7.2–7.8 in phosphate buffer. Michaelis constants for 5-ALA, Mg++, and Mn++were 1 × 10−3 M, 3.1 × 10−4 M, and 3.7 × 10−5 M respectively. The enzyme was localized partially in the chloroplasts and also in 'remaining particles'. Inhibition studies indicated that a carbonyl group γ to an ionized carboxyl group is necessary for binding the substrate to the active site.
The tropical weed Bidens pilosa L. (Asteraceae) contains a number of polyacetylenes which are phototoxic to bacteria, fungi, and human fibroblast cells in the presence of sunlight, artificial sources of long-wave ultraviolet light, or cool-white fluorescent light. The principle photoactive compound in the leaf, phenylheptatriyne, is present in the cuticle as well as in the underlying cells. Experiments with calf thymus DNA indicate that, unlike photoactive furanocoumarins, phenylheptatriyne does not form interstrand cross linkages with DNA in ultraviolet light.
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