A new spectrophotometric assay for citrate synthase and its use to assess the inhibitory effects of palmitoyl thioesters.

We have demonstrated that citrate synthase may be assayed by a simple, discontinuous, spectrophotometric procedure based on the measurement of oxaloacetate utilization with 2,4-dinitrophenylhydrazine. The assay is applicable both to the purified enzyme and to cell extracts, and has the advantage that it can be used in the presence of high concentrations of thiols and thioesters. We have used this new assay in part of our investigations into the inhibitory effects of palmitoyl thioesters on diverse citrate synthases. Both palmitoyl-CoA and palmitoyl thioglycollate inhibit citrate synthases from pig heart, Bacillus megaterium and Escherichia coli, the E. coli enzyme showing the greatest sensitivity to these effectors. With palmitoyl-CoA the extent of inhibition is time-dependent, but the enzymes can be protected from the effect by the substrates oxaloacetate and acetyl-CoA. Using the dinitrophenylhydrazine assay, we have shown that the thioester bond is essential for inhibition; that is, if the palmitoyl thioesters are cleaved to give a mixture of palmitate and a thiol compound, the inhibitions of pig heart and B. megaterium citrate synthases are eliminated and that of the E. coli enzyme is markedly decreased.