Membrane metabolism enzymes

From the above, it is clear that the gut wall represents more than just a physical barrier to oral drug absorption. In addition to the requirement to permeate the membrane of the enterocyte, the drug must avoid metabolism by the enzymes present in the gut wall cell as well as counter-absorptive efflux by transport proteins in the gut wall cell membrane. Metabolic enzymes expressed by the enterocyte include the cytochrome P450, glucuronyltransferases, sulfotransferases and esterases. The levels of expression of these enzymes in the small intestine can approach that of the liver. The most well-studied efflux transporter expressed by the enterocyte is P-gp. 

The water-soluble vitamins generally function as cofactors for metabolism enzymes such as those involved in the production of energy from carbohydrates and fats. Their members consist of vitamin C and vitamin B complex which include thiamine, riboflavin (vitamin B2), nicotinic acid, pyridoxine, pantothenic acid, folic acid, cobalamin (vitamin B12), inositol, and biotin. A number of recent publications have demonstrated that vitamin carriers can transport various types of water-soluble vitamins, but the carrier-mediated systems seem negligible for the membrane transport of fat-soluble vitamins such as vitamin A, D, E, and K. 

Passive transcellular transport across the intestinal epithelium involves three discrete steps (1) uptake across the apical membrane, (2) diffusion through the cytoplasm, and (3) efflux across the basolateral membrane. Occasionally, drug molecules without favorable physicochemical properties traverse the intestinal epithelium using endogenous membrane transporters.6-8 In addition, the intestinal mucosa, with its numerous drug-metabolizing enzymes and efflux transporters, such as P-glycoprotein (Pgp), functions as a biochemical barrier.9... 

The endoplasmic reticulum (ER) is an extensive closed membrane system consisting of tubular and saccular structures. In the area of the nucleus, the ER turns into the external nuclear membrane. Morphologically, a distinction is made between the rough ER (rER) and the smooth ER (sER). Large numbers of ribosomes are found on the membranes of the rER, which are lacking on the sER. On the other hand, the sER is rich in membrane-bound enzymes, which catalyze partial reactions in the lipid metabolism as well as biotransformations. 

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