Citrate synthase from Crithidia fasciculata: inhibition by adenine nucleotides and suramin

Citrate synthase (EC 4.1.3.7) was purified to electrophoretic homogeneity from Crithidia fasciculata ATCC 11745. 2. The purified enzyme had an optimal pH of 8.0-8.5, apparent Km values for acetyl-CoA and oxaloacetate of 5.5 and 3.5 microM, respectively, and was not activated by NH4Cl or KCl, nor inhibited by NADH or alpha-oxoglutarate. 3. Adenine nucleotides inhibited the enzyme, ATP being the most effective. The inhibition was strictly competitive towards acetyl-CoA and of the mixed type with respect to oxaloacetate. 4. The trypanocidal drug suramin inhibited both the C. fasciculata and the pig liver citrate synthases, being strictly competitive with respect to oxaloacetate, and non-competitive towards acetyl-CoA. The competitive inhibition with respect to the divalent anion oxaloacetate might be due to the strongly anionic nature of suramin, which has six sulfonic groups in its molecule.