Cytosolic modulators of activities of microsomal enzymes of cholesterol biosynthesis. Purification and characterization of a non-specific lipid-transfer protein.
Rat liver cytosol contains proteins which in the presence of low-molecular-weight metabolites modulate activities of membrane-bound enzymes of cholesterol biosynthesis. In the preceding paper, we identified Z-protein as a mediator in fatty acyl-CoA modulation of microsomal cholesterol synthetic and metabolizing enzymes. In this communication, we describe a second cytosolic protein which displays cholesterol-exchange activity. Purification of the protein to over 10000-fold and homogeneity has been achieved by gel permeation HPLC on an analytical Spherogel TSK-2000 SW column. Elution of both a single peak of active protein and one SDS-polyacrylamide gel electrophoresis species upon HPLC-purification suggests that homogeneous protein aggregates, with loss of exchange activity. In addition to stimulating microsomal enzymes of sterol synthesis, incubations of microsomes with cholesterol-containing liposomes and the protein consistently yields a 2-3-fold stimulation of microsomal acyl CoA: cholesterol acyltransferase activity. Under similar incubation conditions the protein enhances only slightly the extent of inhibition of microsomal hydroxymethylglutaryl-CoA reductase by liposomal cholesterol. The protein also catalyzes net transfer of cholesterol between membranes of different cholesterol content. The lipid-transfer protein and another cytosolic protein, also implicated in the regulation of sterol synthetic enzymes, appear identical. Regulation of activities of several membrane-bound enzymes of cholesterol metabolism in which the lipid-transfer protein and cytosolic Z-protein modulate uptake of lower-molecular-weight water-insoluble and water-soluble effectors, respectively, is discussed.
D000818:Animals; D001665:Binding Sites; D002352:Carrier Proteins / Q000302:isolation & purification* / Q000378:metabolism; D002784:Cholesterol / Q000096:biosynthesis*; D002788:Cholesterol Esters / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism*; D005260:Female; D006903:Hydroxymethylglutaryl CoA Reductases / Q000378:metabolism; D007425:Intracellular Membranes / Q000378:metabolism; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D051381:Rats; D002785:Sterol O-Acyltransferase / Q000378:metabolism
D002352:Carrier Proteins; D002788:Cholesterol Esters; C030363:lipid transfer protein; D002784:Cholesterol; D006903:Hydroxymethylglutaryl CoA Reductases; D002785:Sterol O-Acyltransferase