Kinetic dependence of phospholipase A 2 activity on the detergent Triton X-100

A kinetic analysis is presented for the dependence of one form of phospholipase A(2) from cobra (Naja naja) venom on the presence of the nonionic detergent Triton X-100 for its activity towards egg phosphatidylcholine and synthetic dipalmitoyl glycerophosphorylcholine as substrates. An automatic recording pH-stat apparatus was employed in order to continuously monitor enzyme activity. The results obtained in this study are interpreted in terms of a change in the physical state of the phospholipid when Triton X-100 micelles convert phospholipid bilayers into mixed Triton X-100-phospholipid micelles; this is consistent with the requirement of this enzyme for substrates which are in micellar form rather than either monomers or bilayers. An apparent inhibition of phospholipase A(2) activity at high concentrations of Triton X-100 is described and discussed in terms of the micellar nature of the substrate.