An ambulance for retinol

Abstract

During inflammation, serum amyloid A proteins transport retinol to infected tissues.

Keywords: acute phase response; crystal structure; retinol transport; serum amyloid A.

Figure 1.. Retinoid transport is different in health (blue) and disease (red).

Carrots contain molecules that are converted into retinol (‘vitamin A’) in the upper small intestine. Retinyl esters are converted to retinol either inside the intestine or at the intestinal epithelial cell ‘brush border’ on the surface of the cells. Carotenoids are ultimately reduced to retinol after entering the absorptive cells lining the intestine. When an animal is healthy, retinol molecules in the intestinal epithelial cells go through a process called re-esterification, and are packed into particles called chylomicrons. These move out of the epithelial cells and into the lymph and blood. Retinol then binds to a serum retinol binding protein (RBP), which moves retinol to the liver, where retinol is stored until it is needed. To leave the liver, retinol again binds to a retinol binding protein, and may be transported to a range of locations, including the lymph node (LN) and the spleen. During inflammation, levels of the retinol binding protein drop; and Derebe, Zlatkov et al. have found that retinol is instead transported out of the liver to where it is needed by serum amyloid A (SAA) proteins.