Oxidation of formate by peroxisomes and mitochondria from spinach leaves

1. Spinach (Spinacia oleracea L.) leaf extracts catalyse the oxidation of formate to CO2. 2. Two enzymic systems are responsible for this oxidation, the peroxidatic action of catalase (EC 1.11.1.6) and NAD-dependent formate dehydrogenase (EC 1.2.1.2). 3. Formate dehydrogenase is mainly, if not exclusively, located in the mitochondria. This enzyme has a pH optimum of 6–6.5 and a Km for formate of 1.7mm in the presence of 1 mm-NAD+. 4. Peroxidatic action of catalase is presumed to take place in peroxisomes, since these seem to be the subcellular site of catalase. Formate oxidation at pH5 by chloroplast and mitochondrial fractions is due to their ability to generate H2O2 and the presence of contaminating catalase. 5. During photorespiration, peroxidatic oxidation of formate by catalase can occur over a wide range of pH values, but the rate of this reaction is probably controlled by the concentration of formate present, to an extent dependent on the pH.