A simple kinetic model for the enzymatic activity of surface-active proteins against mixed micelles has been developed. This model uses the Langmuir adsorption isotherm, the classic equation for the binding of gas molecules to metal surfaces, to characterize enzyme adsorption to micelles. The number of available enzyme binding sites is equated with the number of substrate and inhibitor molecules attached to micelies; enzyme molecules are attracted to the micelle due to the affinity ofthe enzyme active site for the molecules in the micelle. Phospholipase C (Bacillus cereus) kinetics in a wide variety of dimyristoyl phosphatidylcholine/detergent micelles are readily explained by this model and the assumption of competitive binding of the detergent at the enzyme active site. Binding of phospholipase C to pure detergent micelles is demonstrated by gel filtration chromatography. Phospholipase action toward phospholipid molecules in a surface is much greater than that toward monomeric substrates ["interfacial activation" (1)]. Enzyme-specific activity also depends on the matrix used to form the surface-i.e., detergent mixed micelles (8), short-chain lecithin micelles (9, 10), bilayers (11), monolayers (12). A variety of kinetic models have been applied to these phenomena. The simplest model, applied to snake venom phospholipase A2 action toward short-chain lecithins, proposes normal Michaelis-Menten kinetics and different Vm and Km values for monomeric and micellar lipid with the monomer as a competitive inhibitor of micellar lecithin (13). Another model, proposed for pancreatic phospholipase A2, accounts for interfacial activation by proposing a second site on the enzyme that "anchors" or "recognizes" surfaces (14). Different surface-active molecules can interact differentially with the two sites and hence modulate the activity. These models have not been extended in a systematic fashion to binary or more complex surfaces except in cases in which the added surface molecule is a substrate analogue. The only detailed binary component kinetic model is that ofDennis and co-workers (15). This "surface as cofactor" model was developed for phospholipase A2 and phospholipase C kinetics using Triton X-100/lecithin micelles as substrates. The model is quite complex, requiring estimation of the surface area/head-group ratio and several assumptions (16) to fit observed activities. It is based on surface association of the enzyme followed by substrate binding in the active site to form the Michaelis complex; i.e., two distinct binding steps are involved.To generalize a kinetic model for surface-active enzymes such as the phospholipases, we have examined the action of phospholipase C (Bacillus cereus) toward dimyristoyl phosphatidylcholine (Myr2PtdCho) in mixed micelles with four different detergents: Triton X-100 (nonionic), Zwittergent 3-14 (zwitterionic), deoxycholate (anionic), and trimethylcetylammonium bromide (Me3CetNBr; cationic). The data are interpreted by using a simple model based on the Langmuir adsorption is...