Reaction of lecithin: cholesterol acyltransferase with a water soluble substrate: effects of surfactants.

The reaction of fatty acid esters of p-nitrophenol, including the butyrate ester (PNPB) with lecithin: cholesterol acyltransferase (LCAT) has already been described (Bonelli, F.S. and Jonas, A. (1989) J. Biol. Chem. 264, 14723-14728) as a means to investigate the events at the active site of LCAT in the absence of interfacial interactions. Since various surfactants at low concentrations are known to affect the reaction of LCAT with particulate substrates, we set out to analyze their effects on the enzyme in solution using the PNPB substrate to monitor enzyme activity. All the surfactants studied by us (sodium dodecyl sulfate (SDS), dodecyltrimethyl ammonium bromide (DTAB), sodium laurate, sodium cholate, Triton X-100, and BIGCHAP) were able to interact with LCAT below their critical micellar concentrations. The ionic detergents caused inhibition of LCAT at concentrations ranging from 10(-4) to 10(-3) M, whereas the two nonionic detergents actually activated the enzyme in a similar concentration range. From the kinetic constants, the patterns of inhibition, and the well documented effects of the detergents on other proteins, we propose that SDS binds cooperatively to LCAT and elicits inhibitory structural changes; laurate and cholate bind to specific sites either in the active site cavity or in negative effector sites elsewhere; and the nonionic detergents may produce a slight interfacial activation of the phospholipase reaction near their critical micellar concentrations.