Phosphoenolpyruvate Carboxykinase in Plants Exhibiting Crassulacean Acid Metabolism

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Abstract
Phosphoenolpyruvate carboxykinase has been found in significant activities in a number of plants exhibiting Crassulacean acid metabolism. Thirty-five species were surveyed for phosphoenolpyruvate carboxykinase, phosphoenolpyruvate carboxylase, ribulose diphosphate carboxylase, malic enzyme, and malate dehydrogenase (NAD). Plants which showed high activities of malic enzyme contained no detectable phosphoenolpyruvate carboxykinase, while plants with high activities of the latter enzyme contained little malic enzyme. It is proposed that phosphoenolpyruvate carboxykinase acts as a decarboxylase during the light period, furnishing CO(2) for the pentose cycle and phosphoenolpyruvate for gluconeogenesis.Some properties of phosphoenolpyruvate carboxykinase in crude extracts of pineapple leaves were investigated. The enzyme required Mn(2+), Mg(2+), and ATP for maximum activity. About 60% of the activity could be pelleted, along with chloroplasts and mitochondria, in extracts from leaves kept in the dark overnight.