A method is presented for the preparation of pure phthalonic acid (PTA) in high yields. This PTA was used to determine the capacity of the malate/aspartate shuttle in pea (Pisum sativum) leaf mitochondria. The inhibition of glycine-dependent O2 uptake in the combined presence of 5 mM-aspartate and 5 mM-2-oxoglutarate (2-OG) was decreased by 55 +/- 22% (n = 13) in washed and 50 +/- 2% (n = 11) in purified mitochondria by 0.23 mM-PTA. This concentration of PTA had no effect on the oxidation of 5 mM-2-OG, suggesting that part of the observed inhibition of O2 uptake in the presence of aspartate and 2-OG was due to the production of oxaloacetate (OAA) by aspartate aminotransferase external to the mitochondrial inner membrane. Levels of external aspartate aminotransferase were estimated to be 24 +/- 1% (n = 4) and 13 +/- 1% (n = 4) of the total mitochondrial activity in washed and purified mitochondria respectively. Malate/aspartate-shuttle activity was estimated directly by measuring rates of malate efflux from isolated mitochondria and was found to match estimates of shuttle activity based on the PTA-insensitive inhibition of O2 uptake. Comparisons of malate/aspartate- and malate/OAA-shuttle activities indicated potentially similar rates of NADH export from pea leaf mitochondria under conditions in vivo. These extrapolated to whole-tissue rates of 5-11 mumol of NADH.h-1.mg of chlorophyll-1. The potential role of the malate/aspartate shuttle in the support of photorespiratory glycine oxidation in leaf tissue is discussed.
The photorespiratory hydrogen shuttle. Synthesis of phthalonic acid and its use in the characterization of the malate/aspartate shuttle in pea (Pisum sativum) leaf mitochondria
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I B Dry, E Dimitriadis, A D Ward, J T Wiskich; The photorespiratory hydrogen shuttle. Synthesis of phthalonic acid and its use in the characterization of the malate/aspartate shuttle in pea (Pisum sativum) leaf mitochondria. Biochem J 1 August 1987; 245 (3): 669–675. doi: https://doi.org/10.1042/bj2450669
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