Diurnal Variation in Transaminase Activity in the Cactus Plant

Abstract

THE rhythmic acidification and deacidification occurring in cactus and other succulent plants may be expected to be associated with rhythmic alteration in the activity of enzymes involved in the formation and transformation of the organic acids. A change in the activity of an enzyme may be envisaged as being brought about by altering the total amount of catalytic site available, that is, by the synthesis or destruction of enzyme protein, or by the elaboration or removal of activator or inhibitor molecules from the site of enzyme action, or by the accumulation of products of reaction. It has been reported from this laboratory¹ that cactus plants growing in garden soil in the open showed a marked diurnal variation in aldolase activity and it was suggested that the peak of its activity, which was reached in the dark, was linked with the breakdown of carbohydrate to the 3-carbon atom acceptor of carbon dioxide. Continuing the above investigations, we have found that the phylloclades of Nopalea dejecta grown in the open under natural conditions of light and darkness show a remarkable diurnal variation in glutamic oxaloacetic transaminase activity.