On the Biosynthesis of Guanidinosuccinic Acid in Rat Liver Effect of Urea, Ornithine and D, L-Norvaline

Abstract

The biosynthesis of guanidinosuccinic acid (GSA), implicated as a uremic toxin, has been reported to be closely linked with the urea cycle. Our previous report of increased GSA synthesis by urea in isolated rat hepatocytes prompted us to study in vivoo To investigate the relation between urea synthesis and GSA synthesis, ornithine, a stimulator and D, L-norvaline an inhibitor of urea synthesis were administrated to normal rats and hepatic GSA levels were determined At the same time, the hepatic concentrations of arginine and aspartate which are the members of the urea cycle and were also proposed to be the precursors of GSA were determined to clarify the metabolic pathway of GSA synthesis. GSA was determined fluorometrically using HPLC with a cation exchange regin column (Hitachi 2610, 4X125mm). Amino acids were determined by an amino acid analyzer (JLC6AH). Three hours after the intraperitoneal injection of urea (1mg N/g body weight), hepatic GSA level increased to 48 nmol/g wet liver which is about 7 times that of the control rats fasted for 15 hours. By the intraperitoneal injection of ornithine (20μmol/g body weight), the increase of hepatic GSA level by urea was inhibited by 51% and by D, L-norvaline (7.5μmol/g body weight), the increase of hepatic GSA level by urea was also inhibited by 71%. By the injection of urea, the hepatic concentration of arginine also remained at the undetectable level and that of aspartate decreased from 2.1 to 1.4μmol/g wet liver, By the injections of ornithine plus urea and D, L-norvaline plus urea, the hepatic concentration of arginine increased to 0.013 and 0.04μmol/g wet liver, respectively, and that of aspartate did not change by the former and decreased to. 0.6μmol/g wet liver by the latter. These results indicate that the change of the concentration of arginine and aspartate did not correspond to the change of hepatic GSA level and also they suggest that GSA synthesis is competitive to urea synthesis and some steps of GSA synthesis are catalizad by the urea cycle enzymes inhibited by D, L-norvaline.