FRUIT PHOTOSYNTHESIS AND POME FRUIT QUALITY
The activity of MDH extracted from apple fruit was ca. 80fold in excess of that of PEPC, leaving the regulation largely at the PEPC level. Regulation of malic acid synthesis is by glucose-6-phosphate stimulation and end product inhibition of PEPC by malic acid in a strongly pH-dependent manner. PEPC had a pH optimum at pH 7.8 and was salt-sensitive at concentrations above 160 mM salt. PEPC exhibited a Ki of between 10 μM at pH 6.8 and 46 mM L-malate at pH 8. This mechanism also provides fine regulation of cytoplasmic pH in pome fruit.