Biochemical studies of tick embryogenesis. V. Purification and partial characterization of isocitrate lyase from eggs of the tick Hyalomma dromedarii.

1. Isocitrate lyase (Ds-isocitrate glyoxylate-lyase, EC 4.1.3.1) was purified approximately 93-fold from developing embryos of the tick Hyalomma dromedarii. 2. The enzyme requires Mg2+ (Km 2.1 mM) and sulfhydryl compounds for maximal activity and has a pH optimum of 7.4 in phosphate buffer. The Km of the enzyme for isocitrate is 2.4 mM. 3. Data obtained from the pH effect on Km implicate the presence of at least three dissociable groups with pK's of 5.8, 6.8 and 7.4 involved in the enzyme catalysis. 4. At the optimal pH the enzyme is competitively inhibited by oxaloacetate (Ki 0.7 mM) and pyruvate (Ki 0.63 M), noncompetitively inhibited by acetyl-CoA (Ki 1.6 mM) and succinate (Ki 1.35). 5. Inhibition by phosphoenolpyruvate in pH-dependent. The enzyme is noncompetitively inhibited by phosphoenolpyruvate at pH 4.4 (Ki 1.33 mM), 5.6 (Ki 1.7 mM) and pH 8 (Ki 1.36 mM), and competitively inhibited at pH 6.5 (Ki 1.58) and 6.8 (Ki 3.0 mM). 6. The results suggest the regulation of H. dromedarii isocitrate lyase activity during embryonic development by variations in the differential rate of enzyme synthesis an in the intracellular levels of certain metabolites.