In Vivo Properties of Phosphoenolpyruvate Carboxylase in Crassulacean Acid Metabolism Plants. Is Pineapple CAM Not Regulated by PEPC Phosphorylation?

Abstract

The in vivo sensitivity to inhibition by L-malate of phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) in rapidly prepared crude extracts (within 2 min) was investigated for two CAM plants Kalanchoe pinnata and pineapple (Ananas comosus (L.) Merr. cv. Smooth-cayenne N67-10) . The phosphorylation process may not occur in pineapple PEPC. Variations were also observed in apparent kinetic properties and molecular mass as well as subunit composition of PEPC from these plants. Two different subunits (major polypeptide 112 kDa and minor polypeptide 119 kDa) were separated from day and night forms of PEPC from K. pinnata but only one identical subunit (107 kDa) from pineapple. The integrity of both forms of in vivo PEPC from these plants was demonstrated by Western blot analyses using anti-N-terminus antibody. K. pinnata lost malate sensitivity about 87% whereas pineapple PEPC lost only 21% sensitivity at night in the presence of 2 mM L-malate. The apparent Ki (L-malate) was 6.4-fold higher in phase I (5.8 mM) than that in phase III (0.9 mM) from K. pinnata but it was only 1.0 and 0.5 mM for day and night PEPC respectively from pineapple. These findings suggest phosphorylation process may not be the major regulatory mechanism in pineapple PEPC.