A Role of ApoA-1 in LCAT Reaction

Abstract

Two kinds of experiments have been performed to study a role of apoA-1 in the LCAT reaction of HDL. Antiserum against apoA-1 was prepared in a rabbit, and the globulin fraction was obtained by ammonium sulfate precipitation after ultracentrifugation at D = 1.25 g/ml. Antibody fraction revealed a precipitation line against HDL as well as against apoA-1. Anti-apoA-1 significantly inhibited the esterification of cholesterol in the medium containing lecithin:cholesterol emulsions, apoA-1 and partially purified LCAT On the other hand, when anti-apoA-1 was added to the incubation mixture containing LCAT and apoHDL instead of apoA-1, the inhibitory effect of anti-apoA-1 disappeared; rather the LCAT reaction was slightly stimulated. This slight stimulatory effect of anti-apoA-1 was also demonstrated when heated HDL, or heated plasma was used as a substrate. The normal globulin fraction of rabbit serum when added in the media containing artificial substrate and apoA-1 or apoHDL and in the media with heated HDL, non-specifically slightly inhibited the LCAT reaction. LCAT could bind to HDL-Sepharose. and if the gels were pretreated with anti-apoA-1, the binding of this enzyme was slightly inhibited. The LCAT binding to apo-HDL-Sepharose was extremely low compared with its binding to HDL-Sepharose. These findings are compatible with the concept that, although apoA-1 stimulates the LCAT reaction in the system where only this apolipoprotein is present as a cofactor, it masks other activator(s), when HDL or artificial substrate with apoHDL are incubated with LCAT.

Key Words:

• anti-apoA-1
• apoA-1
• apoHDL
• apoHDL-Sepharose
• HDL
• HDL-Sepharose
• LCAT
• lecithin:cholesterol emulsions