Lysinuric protein intolerance (LPI) is a rare autosomal recessive defect of the dibasic amino acid transporter, which is caused by mutations in the SLC7A7 gene and results in L-arginine deficiency.¹ L-arginine is the only known substrate for nitric oxide (NO) production,² and thus the availability of L-arginine is crucial for regulation of NO production.³ NO is an endothelium derived vascular relaxation factor that plays an important role in vascular endothelial function.^4–,⁶ However, the effects of L-arginine deficiency on portal circulation in humans have not been studied.

Portal circulation was evaluated in seven Japanese LPI patients (six males). Seven healthy subjects (six males) without liver disease served as controls. The study was approved by the ethics committee of our university. Mean age of the LPI patients was 18.9 (10.7) years (range 5–32) and that of the controls …