Phosphate Starvation and a Glycolytic Bypass Catalyzed by Phosphoenolpyruvate Carboxylase in Suspension-Cultured Catharanthus roseus Cells

Abstract

Pathways involved in the conversion of phosphoenolpyruvate (PEP) to pyruvate, the final step in glycolysis, were investigated after transfer of stationary-phase cells from suspension cultures of Catharanthus roseus to fresh complete or phosphate (Pi)-deficient Linsmaier and Skoog medium. In addition to pyruvate kinase (PK). enzymes that can function in an alternative pathway, namely, PEP carboxylase (PEPC), NAD-malate dehydrogenase and NAD-malic enzyme, were present in significant amounts in C. roseus cells. The activity of PEPC in Pi-starved cells was more than twice that in cells in the complete medium (Pi-fed cells), while that of PK in Pi-starved cells was lower than that in Pi-fed cells. No significant differences were observed in the levels of NAD-malate dehydrogenase and NAD-malic enzyme between these two types of cell. At cytosolic pH, the Km value of PEP (45 μᴍ ) for PEPC was lower than that for PK (100 (μᴍ). The activity of PEPC was inhibited by malate, citrate, aspartate and ATP. The activity of PK was also inhibited by ATP, but to a lesser extent. The cellular levels of PEP, adenylates and malate, which are substrates and effectors of PK and PEPC, in Pi-fed and Pi-starved cells suggest that the contribution of PEPC to the metabolism of PEP increased in Pi-starved cells in vivo. Evidence for operation of a bypass from malate to pyruvate in vivo was supported by the rapid release of 14CO2 from organic compounds derived from fixed NaH14CO3 and from [4-14C]malate.