The trail of chromium(III) in vivo from the blood to the urine: the roles of transferrin and chromodulin

The chromium-binding oligopeptide chromodulin (also known as low-molecular- weight chromium-binding substance) has been shown to activate the tyrosine kinase activity of the insulin receptor in response to insulin and has been proposed to be part of a novel autoamplification mechanism for insulin sig naling. The model requires that Cr3+ be moved from the blood to insulin-sen sitive tissues in response to insulin and subsequently be lost in the urine as chromodulin; however, the model has not been tested by in vivo studies. In vivo studies with rats have shown that the iron transport protein trans ferrin serves as the major chromic ion transport agent and that this transp ort is stimulated by insulin. The ion is transported to a variety of tissue s, while liver and kidneys are the major target. In hepatocytes, chromoduli n occurs in appreciable levels in the cytosol and in the nucleus. Apochromo dulin levels appear to be maintained under homeostatic control, although th e only detectable form of urinary chromium is probably chromodulin. Increas es in urinary chromium loss in response to insulin are reflected by increas es in chromodulin, establishing a direct link between carbohydrate metaboli sm and the oligopeptide.