The pathway of glutamine synthesis in germinating castor bean endosperm was investigated by feeding experiments with (2,3-14C)succinate and by determining enzyme activities related to pyruvate formation and utilization.14C of (2,3-14C)succinate was rapidly and sequentially incorporated into amino acids in the following order: aspartate or alanine, glutamate and glutamine.14CO2was slowly released, especially during the early hours of incubation. Fluorocitrate inhibited14CO2release while aminooxyacetate stimulated it slightly. Fluorocitrate inhibited the incorporation of14C into glutamate and glutamine. Aminooxyacetate inhibited14C incorporation into aspartate, alanine, glutamate and glutamine. Glutamine synthetase activity was detected in a soluble fraction. NAD-malic enzyme activity was detected in mitochondria by sucrose density gradient centrifugation. Activities of pyruvate decarboxylase and aldehyde dehydrogenasewere detected. Aldehyde dehydrogenase was partially purified about 60-fold by ammonium sulfate fractionation and the DEAE-cellulose chromatography. TheKmvalues of the enzyme were 0.71 miu for NAD and 0.43 mM for acetaldehyde. Based on these results and properties of pyruvate kinase reported previously (9), the metabolism of pyruvate in cytosol and mitochondria was discussed in connection with glutamine synthesis in germinating castor bean endosperm.
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