Liver regulated processes

After uptake of the chylomicron retinyl esters, hydrolysis and reesterification occur in the liver. The resulting retinyl esters (predominantly retinyl palmitate) are stored in the liver and can be mobilized as needed in a highly regulated process. Vitamin A mobilization from hepatic retinyl ester stores takes place as the free alcohol retinol bound to a specific plasma transport protein retinolbinding protein (RBP). 

Retinol is mobilized from the liver, and delivered to peripheral target tissues, as the retinol-RBP complex. Retinol mobilization and delivery are highly regulated processes that are particularly controlled by processes that regulate the rates of synthesis and secretion of RBP by the liver. These processes are discussed in full in Chapter 8, this volume. 

Vitamin D3 enters the skin microcirculation after formation and is bound to a specific globulin in the serum. It and vitamin D2 which is absorbed from the gut are subsequently metabolized to the 25-hydroxy derivative (25-OH-D) in the liver by an enzyme system which may or may not be regulated. The subsequent release of 25-OH-D from the liver is not well understood. There is evidence that it is secreted into the bile and subsequently reabsorbed by the intestine. The relative importance of this "enterohepatic" process and the release of this metabolite directly into the circulation from the liver is not known (26). 

Bcl-2 B cell lymphoma protein 2 (Bcl-2) is a family of proteins that regulate apoptosis (programmed cell death). Apoptosis is a necessary process whereby aged or damaged cells are replaced by new cells. Dysfunction of the apoptosis process results in disease inhibition of apoptosis results in cancer, autoimmune disorder, and viral infection, whereas increased apoptosis gives rise to neurodegenerative disorders, myelodysplastic syndromes, ischemic injury, and toxin-induced liver disease. 

Excess nitrogen is eliminated from the body in the urine. The kidney adds small quantities of ammonium ion to the urine in part to regulate acid-base balance, but nitrogen is also eliminated in this process. Most excess nitrogen is converted to urea in the liver and goes through the blood to the kidney, where it is eliminated in urine. 

Figure 3.23 A sequence of processes explaining the role of glucokinase in the liver and fi-cells in regulation of the blood glucose concentration. The increase in the plasma insulin increases glucose uptake by muscle and decreases fatty acid mobilisation from adipose tissue which lowers the plasma fatty acid level which also increases glucose uptake (Chapter 12).

It is important to point ont that these four mechanisms are not mntuaUy exclnsive. Indeed, it is probable that, for some reactions, aU fonr mechanisms play a role in regnlation of flnx and this combination conld provide an enormons increase in sensitivity. An example is the regulation of the enzyme phosphorylase in mnscle and liver, and hence the process of glycogenolysis (Chapters 6 and 12). 

---