FINE STRUCTURAL LOCALIZATION OF ACETYL COENZYME A CARBOXYLASE IN RAT HEPATOCYTES

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Abstract
Acetyl coenzyme A (CoA) carboxylase, the enzyme catalyzing the first step in fatty acid biosynthesis, has been localized in the hepatocytes of glutaraldehyde-fixed rat livers. This reaction in which the hydrolysis of adenosine triphosphate (ATP) is coupled to the synthesis of malonyl-CoA utilizes biotin, bicarbonate, acetyl-CoA, ATP and manganese or magnesium. ATP hydrolysis yields adenosine diphosphate and phosphate, the latter of which is subsequently converted to a lead phosphate precipitate. In tissues incubated in a complete medium, lead phosphate was seen to be closely associated with the outer surfaces of the membranes of the granular endoplasmic reticulum. When ATP was omitted from the medium no intracellular lead deposit was noted. A small amount of reaction product was seen when bicarbonate, biotin or acetyl-CoA was omitted from the medium. Reaction product was not seen when the biotin inhibitor avidin was added to the incubation medium or when tissue sections were boiled in buffer prior to incubation. The possibility of using a similar technique in studying other coupled enzyme systems is considered.

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