Malnutrition retinol-binding protein

Transferrin an iron-transport protein in the plasma, has a half-life of about 8 days thyroxine-binding protein, 2 days and retinol-binding protein, 0,4 days-Because of the relatively long half-life of albumin, the serum albumin level is not a particularly sensitive indicator of dietary protein status. Therefore, retinol-binding protein and thyroxine-binding protein have been used to assess malnutrition in poorly nourished populatiorrs. Note that these two proteins occur together as a complex in the bloodstream. 

There are also changes in the binding proteins in plasma as a result of the disease process. Since serum albumin falls in association with any acute iUness, this inevitably leads to a fall in plasma zinc concentration. Similarly a reduction in retinol-binding-protein concentration as part of the APR or protein malnutrition also leads to a fall in serum retinol levels, whatever the amount of retinol stores within the liver. 

Transthyretin amyloidosis (also called familial amyloid polyneuropathy) is an autosomal dominant syndrome characterized by peripheral neuropathy. This disease results from one of five mutations identified thus far in the gene for transthyretin. Transthyretin is also called prealbumin (although it has no structural relationship to albumin) because it migrates ahead of albumin in standard electrophoresis at pH 8.6. Transthyretin is synthesized in the liver and is a normal plasma protein with a concentration of 20-40 mg/dL. It transports thyroxine and retinol binding protein (Chapter 38). The concentration of transthyretin is significantly decreased in malnutrition and plasma levels are diagnostic of disorders of malnutrition (Chapter 17). 

Measurements of the levels of semm proteins such as albumin, transthyretin (also known as prealbumin), transferrin and retinol-binding protein are used as biochemical parameters in the assessment of protein energy malnutrition (Table 17-1). An ideal protein marker should have rapid turnover and present in sufficiently high concentrations in semm to be measured accurately. Transthyretin has these properties it is a sensitive indicator of protein deficiency and is effective in assessing improvement with refeeding. 

Prealbumin and retinol-binding protein, traditionally sensitive markers of protein-energy malnutrition, may not be as reliable in patients with hepatic failure. Liver failure can cause decreased concentrations of both, independent of nutritional status. Indeed, many of the commonly used markers of nutritional status correlate poorly with body cell mass in those with end-stage liver disease. Midarm muscle circumference and handgrip strength have been found to be the best... 

Retinol is nearly always present in the food in the form of esters which are hydrolysed in the lumen of the intestine. The retinol released is quite readily absorbed into the mucosal cells where it is re-esterified, chiefly with palmitic acid. The retinyl esters are then transported via the lymphatic system into the portal circulation from which they are removed and stored in the liver. Release of the vitamin from the liver depends on the production by the liver of a special retinolbinding protein (RBP). Production of the retinol-binding protein may be disturbed in diseases of the liver or kidneys or in protein/energy malnutrition. In such circumstances retinol cannot be mobilized from the stores and a secondary deficiency may result. Thus it can be seen that the level of retinol in the general circulation is normally highly regulated and is more or less independent of the body s reserves. 

Smith FR, Goodman DS, Arroyave G, Viteri F (1973) Serum vitamin A, retinol-binding protein, and prealbumin concentrations in protein-calorie malnutrition. II. Treatment including supplemental vitamin A. Amer J Clin Nutr 26 982-987... 

Ingenbleek Y, Van Den Schrieck HG, De Nayer P, De Visscher M (1975) The role of retinol-binding protein in protein-calorie malnutrition. Metabolism 24 633-641... 

In malnutrition, total serum protein, albumin, and p-glob-uhn concentrations are reduced. The increased concentration of Y-globuIin does not fully compensate for the decrease in other proteins. The concentrations of complement C3, retinol-binding globulin, transferrin, and prealbumin decrease rapidly with the onset of malnutrition and are measured to define the severity of the condition. The plasma concentrations of lipoproteins are reduced, and serum cholesterol and triglycerides may be only 50% of the concentrations in healthy individuals. In spite of severe malnutrition, glucose concentration is maintained close to that in healthy... 

---