Lipoprotein lipase: some effects of activator proteins

This paper considers how apolipoprotein CII from human plasma lipoproteins and T1 and T2 proteins from egg yolk lipoproteins stimulate the activity of lipoprotein lipase. These activator proteins stabilized the enzyme much more effectively than a thousandfold higher concentration of albumin did, indicating a direct interaction with the enzyme. The effects of the activators were seen also at 1 M NaCl. Thus, forces other than electrostatic are implicated. Centrifugation experiments showed that 125I-labeled lipase bound equally well to the emulsion droplets in the absence of activator protein as in its presence. This was true even under conditions when the activator caused a severalfold increase in the rate of hydrolysis. Thus, the activator makes enzyme at the interface more effective in hydrolysis. By optimizing the conditions it was possible to obtain almost as high rates of triglyceride hydrolysis in the absence as in the presence of activator. Thus, the main effect of the activator protein is probably not on a rate-limiting chemical step. Under most conditions, the rate of hydrolysis was much below optimal and activator increased it. This was always the case with phosphatidyl-choline/triglyceride emulsions, where the activator enhanced hydrolysis of both lipids. Other experiments showed that the activator enhanced triglyceride hydrolysis in the absence of phospholipids and phospholipid hydrolysis in the absence of triglycerides. It is suggested that interaction with activator orientates the enzyme and/or the lipid substrate for effective hydrolysis at the surface of lipoproteins/model substrates.