Catabolism of Adenine Derivatives in Leaves: Study of the Role of Light on the in Vivo Activity of Xanthine Dehydrogenase

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RESUMO

The in vivo activity of xanthine dehydrogenase (E.C. 1.2.1.37) was followed in leaf discs excised from illuminated or darkened plants. In cotyledons of Pharbitis nil, 24 hours of darkness enhanced the in vivo activity of xanthine dehydrogenase which increased between 2 to 5-fold depending on the concentration of hypoxanthine of the solution where cotyledon discs were incubated. The same effect occurred in leaves of several other species, in plants with both high and low ureide content. However, the effect of light was not observed in leaves of Zea mays, Pennisetum americanum and Atriplex spongiosa, whereas, it appeared very clearly in other C4 plants such as Sorghum sudanense and Portulaca oleracea. This enzymic activity in chlorophyll-deficient tobacco leaves was the same both for illuminated and darkened plants. In addition, the in vivo activity of xanthine dehydrogenase in roots of Pharbitis nil was not dependent upon the light conditions applied to leaves. In cotyledons of Pharbitis nil, the level of the in vivo activity of xanthine dehydrogenase was influenced by the energy of light and the duration of illumination. The supply of carbohydrates to darkened cotyledons had the same effect as light on the in vivo activity of xanthine dehydrogenase. It is proposed that the effect of light on the in vivo activity of xanthine dehydrogenase in leaves is mainly due to the production of photosynthates which changes the osmotic state of leaf tissue and thus modifies the level of the in vivo activity of xanthine dehydrogenase.

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