Describe the process of chylomicron assembly by the enterocyte [80%]. How would this process be affected in a patient with cystic fibrosis [20%]? .
Chylomicrons are fundamental molecules which ensure mobilization of exogenous lipids, these molecules mediate the ingestion of fat. Their structures are assembled within the intestinal mucosa in enterocytes. In the smooth endoplasmic reticulum re-esterification of triglycerides from monoglycerides (85-92%) occurs and a relatively smaller proportion of cholesterol (1-3%) also undergoes this process, phospholipids (6-10%) are resynthesized and apolipoproteins responsible for lipoprotein transport also bind. Constituent elements amalgamate and travel into blood circulation to reach muscle and adipose tissue.
Upon entering the enterocyte, cholesterol and fatty acids are both led to the smooth endoplasmic reticulum. The former binds to two sterol carrier proteins SCP-1 and SCP-2 and the latter binds to I-FABP which increases solubility and provides protection against detergent effects to the cell. Apolipoprotein B-48 enters via translocation from the rough endoplasmic reticulum and Lysolecithin by means of diffusion. In the cytosolic surface of the smooth endoplasmic reticulum the processing of molecules takes place. Free fatty acids combine with 2-monoacylglycerols to form diacylglycerol, these diacylglycerols go on to bind with other free fatty acids and the product of the reaction is triacylglycerol. Similarly cholesterol also reacts with free fatty acids to form cholesteryl ester. .
In this secretion pool Lysolecithin binds to free fatty acids and re-forms lecithin, a common phospholipid. Once these products are assembled they all bind to form the chylomicron structure. Apolipoprotein B-48 is also added, this protein has a 'lipid-poor' nature and as such requires the assistance of a microsomal triglyceride transfer protein.