Effects of pH, NADH, succinate and malate on the oxidation of glycine in spinach leaf mitochondria

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Abstract
The effect of external pH on several reactions catalyzed by glycine decarboxylase in spinach leaf mitochondria was investigated. Glycine‐dependent oxygen consumption showed a pH optimum at 7.6, whereas the release of CO2 and NH3 from glycine in the presence of oxaloacetate both showed pH maxima at 8.1. Glycine‐dependent reduction of 2,6‐dichlorophenolindophenol. on the other hand showed a pH optimum at 8.4. It is concluded that these three reactions have different rate‐limiting steps. The rate of the glycine‐bicarbonate exchange reaction catalyzed by glycine decarboxylase showed no optimum in the pH range investigated, pH 7–9, but increased with decreasing pH. This suggests that CO2 may be the true substrate in this reaction.The oxidation of glycine inhibited the oxidation of both malate, succinate and external NADH since the addition of malate, succinate or NADH to mitochondria oxidizing glycine in state 3 resulted in a rate of oxygen consumption which was lower than the sum of the rates when the substrates were oxidized individually. The addition of malate, succinate or NADH did not, however, decrease the rate of CO2 or NH, release from glycine. It is suggested that the preferred oxidation of glycine by‐spinach leaf mitochondria may constitute an important regulatory mechanism for the function of leaf mitochondria during photosynthesis.