KINETICS AND PHYSIOLOGICAL SIGNIFICANCE OF PHOTOSYNTHETIC PHOSPHOENOLPYRUVATE CARBOXYLASE IN AVOCADO FRUIT

Phosphoenolpyruvate carboxylase (PEPC) was extracted from the pericarp of pre-climacteric avocado (Persea americana Mill.) fruit. The enzyme had a molecular weight of 400 kDa and was immunologically recognised by anti-PEPC prepared from Sorghum. At the optimal pH of 7.8 and a salt concentration of 50 mM, the apparent Km PEP was 80 µM and apparent Km HCO3 at pH 7.0 was 14 µM. Magnesium ions activated the enzyme with an apparent Km Mg of 22 µM, and initial rate curves indicated biphasic kinetics at saturating PEP concentrations. Chloride and orthophosphate ions inhibited PEPC in a concentration dependent manner, significantly increasing apparent Km PEP to 212 µM at 130 mM chloride and 1.2 mM at 125 mM Pi, while V. was decreased by 14 and 46 %, respectively. The enzyme was not inhibited at concentrations of up to 125 mM HCO3 at a constant total salt concentration of 150 mM at pH 7.0 and 27 °C. Inhibition of PEPC activity by malate was pH sensitive, being greatly increased at a lower pH, to an extent where degree of malate ionisation into the unionised, mono- and divalent form at the two pHs was not a factor for consideration. Concentrations required for 50 % inhibition were 150 µM at pH 7.0 and 46 mM at pH 8.0. The properties of avocado fruit PEPC resemble those found for apple fruit and appear to be intermediate between photosynthetic types. © 1991, Gustav Fischer Verlag, Stuttgart. All rights reserved.