The in-vitro influence of serum amyloid A isoforms on enzymes that regulate the balance between esterified and un-esterified cholesterol

The intracellular balance between un-esterified and esterrfied cholesterol is regulated by two enzyme activities, cholesterol ester hydrolases, which drive the balance in favor of un-esterrfied cholesterol, and acyl-CoA: cholesterol acyl transferase (ACAT) which acts in the opposite direction. During acute inflammation upo-serum amyloid A (apoSAA) isoforms 1.1 and 2.1 become major constituents of high density lipoprotein and this complex is internalized by macrophages. Mixtures of the two isoforms have been shown to enhance cholesterol esterase activity. Using a purlfed form of the pancreatic enzyme we have explored the mechanism by which apoSAA may accomplish this stimulation. The pancreatic esterase cleaves cholesteryl-oleate with a Km of 0.255 mM, releasing both cholesterol and ole-ate. Cholesterol exhibits a product inhibition which is relieved by isoform 2.1 but not 1.1 nor apolipoprotein A-I. The NH2-terminal 16 residues of isoform 2.1 had no effect on the esteruse, but the 80 residue peptide consti...