High cholesterol level is a leading cause of the obesity epidemic throughout the United States, with approximately 30% of American children calling in at overweight or obese. An increased level of cholesterol in the body is indicative of higher levels of fat in the bloodstream being carried by lipoproteins, which can build up in arteries, block blood flow to the heart, and ultimately increase the risk for heart attacks and coronary-related diseases such as coronary artery disease.
A recently-discovered enzyme's structure has been determined to be a key factor in reducing excess cholesterol to the liver, and the physical stabilization of that structure could be a new hallmark in the development of statins (drugs to combat high cholesterol levels) in order to stem this rising tide of obesity.
Lead author Kelly Manthei, a postdoctoral fellow at the University of Michigan, has conducted research to reveal how a drug-like chemical stimulates the action of the lecithin: cholesterol acyltransferase (LCAT) enzyme to help high-density lipoprotein (HDL, AKA, "good cholesterol") remove cholesterol from the blood via conversion of the lipid into a more mobile form to transport. Various mutations in LCAT have led to both partial loss of activity (known as the fish-eye disease) and full loss (known as FLD), characterized by fluid buildup in the eyes as a result of excessive cholesterol buildup.
Utilizing X-ray crystallography, Manthei's team was able to isolate the LCAT enzyme and ascertain how LCAT activators bind specifically to the enzyme to promote cholesterol transport. They utilized two different chemicals — the activator molecule and a second compound that mimics a substrate bound to the enzyme (agonist).
When viewing the results of the scan, Manthei's team was able to deduce that the two chemicals had more of an effect on the protein when presented in tandem rather than separately, suggesting that the two molecules had unique binding sites on the enzyme. Further analysis revealed that the activator molecule binds to a region close to where the HDL attaches.
However, the activator's role was not to help LCAT bind HDL more effectively, but rather to assist in the transfer of cholesterol and lipids into the catalytic center of the enzyme in order to convert it for transport in HDL.
Having understood the purpose of the activator, Manthei's team developed a modified version of the enzyme with a mutation normally seen in FLD patients and then tested that variation's ability to bind HDL and convert cholesterol in both the presence and absence of the activator molecule. They found that the activator could partly reverse the loss of activity in the mutated enzymes, resulting in similar transport of cholesterol to that of the normally functioning enzyme.
These results show a great deal of pharmacological promise to help develop new compounds specifically needed to combat high cholesterol levels, and ultimately subdue the increasing obesity rate across the United States.
