Smooth microsomes. a trap for cholesteryl ester formed in hepatic microsomes

Area of Science:

• Biochemistry
• Cell Biology
• Membrane Biology

Background:

• Acyl-CoA:cholesterol acyltransferase (ACAT) is crucial for cellular cholesterol homeostasis.
• Microsomes are key sites for lipid metabolism, with distinct subfractions like RNA-rich and smooth microsomes.

Purpose of the Study:

• To investigate the distribution and potential interaction of Acyl-CoA:cholesterol acyltransferase (ACAT) between different microsomal subfractions.
• To elucidate the mechanism behind the higher concentration of esterified cholesterol in smooth microsomes.

Main Methods:

• Subfractionation of microsomes into RNA-rich, RNA-poor, and smooth components.
• Incubation of mixed microsomes with [1-14C]palmitoyl-CoA to trace cholesteryl ester formation.
• Measurement of radioactive cholesteryl ester distribution across subfractions.

Main Results:

• Acyl-CoA:cholesterol acyltransferase (ACAT) was primarily located in RNA-rich microsomes (85%).
• Smooth microsomes exhibited a 2-fold higher concentration of esterified cholesterol compared to RNA-rich microsomes.
• Despite lower ACAT activity, smooth microsomes contained a disproportionately high amount (54%) of radioactive cholesteryl esters, exceeding passive transfer predictions.

Conclusions:

• The distribution of esterified cholesterol suggests an interaction between RNA-rich and smooth microsomes.
• ACAT activity on the cytoplasmic surface of RNA-rich microsomes may esterify cholesterol within adjacent smooth microsomes.
• This implies a potential trans-esterification mechanism across microsomal membranes.