Temperature dependence of palmitoyl-CoA synthetase of chicken liver microsomes: Effect of Triton X-100 and lysophosphatidylcholine

Palmitoyl-CoA synthetase (EC 6.2.1.3) of chicken liver microsomes showed a break in the Arrhenius plot of activity in the region $22-26^o$. This was abolished by Triton X-100 or lysophosphatidylcholine (LPC), individually or together, NMR spectra showed that the break in the Arrhenius plot of activity can be related to the phase transitions in the phospholipid bilayer structure of the microsomes. Even though Triton X-100 and(or) LPC lower the energy of activation (Ea), there was no correlation between the lowering of Ea and the extent of stimulation of the enzyme activity. The enzyme prepn. solubilized from the microsomes by Triton X-100 after removal of the detergent, did not exhibit any break in the Arrhenius plot of activity. However, the enzyme activity of this prepn. was also stimulated by Triton X-100 and(or) LPC but without substantial change in Ea. LPC had no effect on the Km for palmitate, whereas Triton X-100 increased it by 2-fold, but the Vmax of the reaction was increased by either compd. Triton X-100 or LPC may stimulate the enzyme activity by effecting conformational change(s) that do not involve an increase in the affinity of the enzyme for the substrat