RESUMO
Lycorine, an alkaloid extracted from Amarillidaceae, strongly inhibits the "in vivo" conversion of galactono-gamma-lactone to ascorbic acid. Lycorine seems to act as a non-competitive inhibitor on galactono-gamma-lactone oxidase, because the alkaloid rapidly forms a stable bound with the enzyme. In fact, a short incubation period with 50 microM lycorine gets a high inhibitory effect that persists when the alkaloid is removed from the incubation medium. Considering that lycorine induces scurvy-like symptoms in ascorbic acid-synthesising animals, it is reasonable to suppose that in both plants and animals lycorine inhibits the last step in the biosynthetic pathway leading from sugar to ascorbate.
Assuntos
Alcaloides de Amaryllidaceae , Ácido Ascórbico/biossíntese , Fenantridinas/farmacologia , Proteínas de Plantas/antagonistas & inibidores , Desidrogenase do Álcool de Açúcar/antagonistas & inibidores , Allium/metabolismo , Depressão Química , Fabaceae/metabolismo , Plantas MedicinaisRESUMO
The capacity of ascorbic acid biosynthesis in potato tuber tissue is closely correlated with the ascorbic acid content of the cells: the lower the endogenous content of ascorbic acid, the greater its biosynthesis. At the highest level of ascorbic acid found in the cells, the biosynthetic capacity is virtually zero. In these conditions, adding glucose (the first precursor of ascorbic acid) has no effect whatsoever, whereas adding galactono-gamma-lactone (the last precursor) induces a high rate of ascorbic acid synthesis. It is suggested that AA biosynthesis is subject to a regulatory mechanism "in vivo" which controls an initial step in the biosynthetic pathway. The last step in this pathway, catalyzed by galactone oxidase, is never blocked and, moreover, its activity is greater than that of the preceding steps.