Respiration of pea leaf mitochondria and redox transfer between the mitochondrial and extramitochondrial compartment

Abstract

In this report, properties of isolated mitochondria from pea leaves have been studied in view to their function in photosynthesis metabolism of a leaf cell. (1) The rates of respiration with various substrates and with a combination of these substrates have been measured with isolated mitochondria. The highest respiration rate was found with NADH, followed by NADPH, malate and glycine. For the oxidation of NADH, NADPH, malate, glycine and 2-oxoglutarate the apparent K m values were determined. The oxidation of malate and glycine occurred independently of each other as long as electron transport was not limiting. (2) The maximal capacity of mitochondrial ATP synthesis in a leaf was estimated as about 25% of the rate of noncyclic photophosphorylation at maximal rate of photosynthesis. (3) From measurements of NADH/NAD ratios in isolated mitochondria and from previous determinations of the NADH/NAD ratio in the cytosol of spinach leaves it is discussed that in a leaf cell the NADH/NAD ratio in the mitochondrial matrix is higher than in the cytosol. (4) A comparison of the apparent K m values obtained for the oxidation of external NADH and NADPH with the corresponding concentrations found in the cytosol of spinach leaves suggests that in a leaf cell NADPH is oxidized by mitochondria at a much higher rate than NADH. (5) From the measurement of mitochondrial respiration with glycine and malate as substrates in the presence of a defined malate / oxaloacetate ratio the function of a malate-oxaloacetate shuttle is demonstrated. It is furthermore shown that a malate-aspartate shuttle does not play any significant role in redox transfer under physiological conditions.