IN-VIVO INHIBITION OF GALACTONO-GAMMA-LACTONE CONVERSION TO ASCORBATEBY LYCORINE

Maize embryos are endowed with the ascorbate biosynthetic system and t he last enzyme of the pathway, galactono oxidase (or dehydrogenase), i s very active; L-galactono-gamma-lactone addition gives rise to a 3-fo ld increase in cellular ascorbate content. Lycorine, an alkaloid extra cted from members of the Amaryllidaceae, strongly inhibits the in vivo conversion of L-galactono-gamma-lactone to ascorbic acid. Data report ed here seem to suggest that lycorine forms a relatively stable associ ation with galactono oxidase; incubation with 50 mu M lycorine shows a marked inhibitory effect that persists when the alkaloid is removed f rom the incubation medium. The inhibitory effect of lycorine is signif icantly higher in onion roots and pea embryos in comparison with maize embryos. This different sensitivity to the alkaloid can be explained by the inability of onion and pea to overcome the decrease in ascorbat e biosynthesis by means of dehydroascorbate reductase, which has a sig nificantly lower activity in these two species than that in maize embr yos. Galactono oxidase also efficiently utilizes L-gulono-gamma-lacton e, the physiological substrate of the animal enzyme. Considering that lycorine induces scurvy-like symptoms in ascorbic acid-synthesizing an imals, it is reasonable to suppose that lycorine inhibits ascorbate bi osynthesis in both plants and animals by acting on the last step in th e biosynthetic pathway leading from sugar to ascorbate.