Evaluation of the mixed interaction between apolipoproteins A-II and C-I by equilibrium sedimentation

The mixed interaction between human apolipoproteins C-I and A-II, each of which self-associate in aqueous solution, has been evaluated by sedimentation equilibrium measurements. In order to simplify data analysis apoC-I and apoA-II were modified by treatment with 2-nitrophenylsulfenyl chloride and tetranitromethane respectively. The molecular properties of the resulting derivatives, S-apoC-I and N-apoA-II, each of which have appreciable extinction coefficients above 350 run, were indistinguishable from the corresponding unmodified species. Sedimentation equilibrium data were obtained with mixtures of S-apoC-I and native apoA-II, N-apoA-II and native apoC-I, and native apoC-I and native apoA-II. Mixed complex formation was detected readily with all mixtures investigated. The combined results were most consistent with a single mixed oligomer containing 2 molecules of apoA-II and 4 molecules of apoC-I. The corresponding equilibrium constant was 31 248 ± 890 ($\text{XXX}\text{gm}$)⁵.