Effects of Native and Modified Bovine Serum Albumins on the Lecithin-Cholesterol Acyltransferase Reaction in the Presence or Absence of Cd²⁺ Ion

Abstract

The effects of native and modified bovine serum albumins on the esterification of cholesterol in sonicated dispersions of lecithin-cholesterol mixtures by lecithincholesterol acyltransferase [EC 2. 3. 1. 43] (LCAT) in human plasma were studied in the presence or absence of Cd²⁺ ion.
As the modified albumins, 52%- and 84%-acetylated, 48%- and 82%-succinylated, and 24%- and 63%-glycine methylesterified albumins were prepared chemically. These modified albumins still contained native structure of serum albumin, as revealed by intrinsic viscosity, ultraviolet absorption and circular dichroism measurements.
The acyltransferase activity was stimulated by the addition of native albumin in vitro. This stimulatory action of native albumin was reduced by acetylation or succinylation of its amino groups. However, no inhibitory action of acetylated and succinylated albumins was observed. On the other hand, the stimulatory action of native albumin completely disappeared upon methylation¹ and glycine methylesterification of its carboxyl groups. In contrast, the esterified albumins showed a potent inhibitory action on the acyltransferase. The inhibitory action of these esterified albumins with apparently increased positive charges was greater than that of poly-L-lysine, which has many positive charges of the molecule.
On the other hand, the acyltransferase activity was decreased by the addition of 1×10^-3M Cd²⁺ ion. The decreased acyltransferase activity in the presence of Cd²⁺ ion recovered progressively on the addition of increasing amounts of native, acetylated, and succinylated albumins. In particular, the extent of recovery of the decreased acyltransferase activity produced by acetylated and succinylated albumins in the presence of Cd²⁺ ion was greater than that with native albumin. However, no recovery effect of the esterified albumins on the decreased acyltransferase activity was observed in the presence of Cd²⁺ ion. On the contrary, additive inhibitory actions of Cd²⁺ ion and esterified albumin on the acyltransferase activity were observed.