Hepatic fatty acid oxidation : activity, localization and function of some enzymes involved

Fatty acid oxidation is an important pathway for energy production in mammals and birds. In animal tissues the enzymes of fatty acid oxidation are located in the mitochondrion. Recent reports suggest that this is not the case in Castor bean endosperm. In this tissue the enzymes of B-oxidation are localized in a very fragile cell organelle, called the glyoxysomes . Fatty acids are transported in the blood complexed to albumin, or in esterified form as triglycerides and phospholipids, complexed to protein (lipoproteins). Lipoproteins are synthesized in the liverand in the intestinal epithelium (chylomicrons). Before entering the cell these triglycerides are generally hydrolyzed by lipoprotein lipase, an enzyme activated by heparin and probably present in the endothelial cells of the capillary wall. From the foregoing it is evident that fatty acid presented to the cell for further metabolism is in the form of "free" fatty acid. Fatty acids cannot participate in any reaction of intermediary metabolism, before they have been "activated" to their thioester with CoA. This reaction is necessary for triglyceride and phospholipid biosynthesis, for acyl interchange between complex lipids, for chain-elongation reactions and also for oxidative degradation of fatty acids.