Phosphoenolpyruvate Carboxylase from the Crassulacean PlantBryophyllum fedtschenkoiHametet Perrier

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Abstract
The total activity (capacity) of phosphoenolpyruvate carboxylase in crude extracts of leaves of Bryophyllum fedtschenkoi, measured on a fresh weight basis, varies considerably with leaf position and with photoperiodic treatment. No diurnal variation in capacity was detected under long – day or short – day conditions. The enzyme in freshly prepared crude extracts is about 50 times more sensitive to inhibition by malate at pH 7 than that in aged extracts, or in the fully purified state. Desensitization in extracts proceeds rapidly unless protective measures are taken, and appears to be irreversible. A pH-dependence study shows that the effect of desensitization on the kinetic parameters of malate inhibition is identical to that of increasing the pH by 2.0 units over a wide range, but the maximum velocity at pH 7.8 is virtually unaffected. The significance of the results for both the experimental determination of cyclic changes in enzyme capacity, and the theories concerning the mechanism of a circadian rhythm of phosphoenolpyruvate carboxylase activity in vivo are discussed.