Phosphoenol-pyruvate-carboxylase activity in cotton and Sorghum seeds and its relation to seedling development

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Abstract
Cotton (Gossypium hirsutum) seeds and Sorghum vulgare caryopses are able to incorporate CO2 through a PEP-carboxylating enzyme (EC 4.1.1.38). The enzyme activity is optimal at pH 8.2 and is unaffected by ATP, GDP or acetyl CoA. The partially purified cotton enzyme is stimulated by inorganic phosphate with an apparent Km of 0.3 mM. The enzymes from both cultivars are inhibited by pyrophosphate, malate, and aspartate but not by succinate. Kinetic studies for Sorghum and cotton seed enzymes show apparent Km values for carbonate of 5 mM and 1.2 mM and for PEP of 36 μM and 5 mM, respectively. The Vmax values are 90 and 3.3 nmol min-1 mg protein-1, respectively. A two-fold increase in the enzyme activity from cotton seeds occurs after 2 h under laboratory germination conditions after which the activity drops sharply to 1/3 of the original activity after 5 h imbibition. No such change was observed in Sorghum caryopses enzyme. A correlation between PEP-carboxylase activity and seed vigor in both cultivars was demonstrated.