Activation of lecithin: cholesterol acyltransferase by human apolipoprotein A-IV.

A Steinmetz,G Utermann

doi:10.1016/s0021-9258(18)89547-3

Abstract

Human plasma apoproteins (apo) A-I and A-IV both activate the enzyme lecithin:cholesterol acyltransferase (EC 2.3.1.43). Lecithin:cholesterol acyltransferase activity was measured by the conversion of [4-14C] cholesterol to [4-14C]cholesteryl ester using artificial phospholipid/cholesterol/[4-14C]cholesterol/apoprotein substrates. The substrate was prepared by the addition of apoprotein to a sonicated aqueous dispersion of phospholipid/cholesterol/[4-14C]cholesterol. The activation of lecithin:cholesterol acyltransferase by apo-A-I and -A-IV differed, depending upon the nature of the hydrocarbon chains of the sn-L-alpha-phosphatidylcholine acyl donor. Apo-A-I was a more potent activator than apo-A-IV with egg yolk lecithin, L-alpha-dioleoylphosphatidylcholine, and L-alpha-phosphatidylcholine substituted with one saturated and one unsaturated fatty acid regardless of the substitution position. When L-alpha-phosphatidylcholine esterified with two saturated fatty acids was used as acyl donor, apo-A-IV was more active than apo-A-I in stimulating the lecithin:cholesterol acyltransferase reaction. Complexes of phosphatidylcholines substituted with two saturated fatty acids served as substrate for lecithin:cholesterol acyltransferase even in the absence of any activator protein. Essentially the same results were obtained when substrate complexes (phospholipid-cholesterol-[4-14C]cholesterol-apoprotein) were prepared by a detergent dialysis procedure. Apo-A-IV-L-alpha-dimyristoylphosphatidylcholine complexes thus prepared were shown to be homogeneous particles by column chromatography and density gradient ultracentrifugation. It is concluded that apo-A-IV is able to facilitate the lecithin:cholesterol acyltransferase reaction in vitro.

Highlights

Than apo-A-I in stimulatingthe lecithinxholesterol This enzyme catalyzes the transacylation from the /3-posiacyltransferase reaction
Apolipoprotein A-IV is a M,46,000 glycoprotein that was first demonstrated in rats (1)but was later observed in other species, including humans (2-6)
The apoprotein is synthesized as a preprotein (7, 8) primarily in mucosalcells of the small intestine and is assembled into thin:cholesterol acyltransferase-catalyzed transacylation reaction have beendefined in comparison to apo-A-I

Summary

Introduction

Than apo-A-I in stimulatingthe lecithinxholesterol This enzyme catalyzes the transacylation from the /3-posiacyltransferase reaction. Complexes of phosphatidyl- tion of phosphatidylcholine (PC) to the hydroxylgroup of cholines substitutedwith two saturatedfatty acids cholesterol, resulting in the formation of lysolecithin and served as substrate for 1ecithin:cholesterol acyltrans- cholesteryl ester (15, 16). This reaction seems to be a prereqferase even in the absence of any activator protein. Cluded that apo-A-IV is able to facilitatethe leci- Most investigations on the specificity and regulation of the thinxholesterol acyltransferasereaction in vitro. This reaction requires a protein cofactor first identified as apo-A-I (21). The apoprotein is synthesized as a preprotein (7, 8) primarily in mucosalcells of the small intestine and is assembled into thin:cholesterol acyltransferase-catalyzed transacylation reaction have beendefined in comparison to apo-A-I

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Results

Conclusion