The Gluconeogenic Metabolism of Pyruvate During Deacidification in Plants With Crassulacean Acid Metabolism

Abstract

Pyruvate, P_I dikinase (EC 2.7.9.1) was present in crassulacean acid metabolism (CAM) plants that lack phosphoenolpyruvate (PEP) carboxykinase (EC 4.1.1.32) but was not detected in plants that contain PEP carboxykinase or in C₃ plants. It is suggested that, during deacidification in CAM plants that contain NAD and NADP malic enzymes (EC 1.1.1.38 and EC 1.1.1.40) but not PEP carboxykinase, pyruvate, P*i dikinase reverses the glycolytic reaction catalysed by pyruvate kinase (EC 2.7.1.40) and converts pyruvate to PEP as the first step in the gluconeogenic conservation of pyruvate as storage carbohydrate. The enzyme is not required by CAM plants that contain PEP carboxykinase and produce mainly PEP during decarboxylation.

Leaf slices from Kalanchoe daigremontiana and CAM Mesembryanthemum crystallinum, two species that possess pyruvate, P_I dikinase, transfer label from exogenous [3-¹⁴C]pyruvate to carbohydrates more rapidly than either Stapelia gigantea, a PEP carboxykinase CAM plant, or C₃ Mesembryanthemum crystallinum, which lack the dikinase. Label from [2-¹⁴C]- and [3-¹⁴C]pyruvate is converted to carbohydrate at the same rate in K. daigremontiana while in S. gigantea label from [2-¹⁴C]pyruvate accumulates in carbohydrates twice as rapidly as label from [3-¹⁴C]pyruvate. The patterns observed for K. daigremontiana and for CAM M. crystallinum are consistent with the gluconeogenic anabolism of pyruvate whereas the patterns observed for S. gigantea and for C_.3 M. crystallinum suggest pyruvate is oxidized possibly via the tricarboxylic acid cycle in these species.

Deacidification in Aloe arborescens, a PEP carboxykinase CAM plant that also possesses NAD and NADP malic enzyme activity, was inhibited 80% by 0.1 mM 3-mercaptopicolinic acid (3-MPA), an inhibitor of PEP carboxykinase. It is thus likely that, in this species and probably also in other CAM plants with high PEP carboxykinase activities, a small proportion of the malic acid may be decarboxylated by malic enzymes. However, as 0.5 mM 3-MPA inhibited deacidification in K. daigremontiana by 40%, the inhibitor is probably only specific at low concentrations.

¹⁴CO₂ fixation in the light by mesophyll cells isolated from K. daigremontiana was stimulated by 20-50% in the presence of 10 mM pyruvate, but there was no increase in ¹⁴CO₂ fixation by mesophyll cells isolated from S. gigantea.