"Basolateral" and mitochondrial membrane transport defect in the hepatocytes in lysinuric protein intolerance.

Abstract

In lysinuric protein intolerance, diamino acid transport is defective at the basolateral membrane of the jejunal and renotubular epithelia. The plasma has depressed concentrations of ornithine and arginine but, in contrast, supranormal levels of another urea cycle intermediate, citrulline. The patients have "ornithinopenic" postprandial hyperammonemia. The concentrations of the diamino acids and citrulline in the patients' liver were normal or elevated rather than depressed. Their extrarenal plasma clearance of citrulline and its conversion to arginine and ornithine were retarded. A hypothesis reconciles these apparently conflicting findings: in the hepatocytes the transport defect is located in the "basolateral" cell membrane and in the mitochondrial membranes. The diamino acids accumulate in the cytoplasm, because exit from the cells and transport into the mitochondria are impaired, but these acids are depleted in the mitochondria, where ornithine is needed in the urea cycle. As a result, the urea cycle is reversed at this point, producing citrulline.