Arsenolysis and phosphorolysis of citrulline in mammalian liver

Abstract

Citrulline, while stable in suspensions of fresh liver or extracts of acetone-dried liver, is readily split into ornithine, CO2 and ammonia if arsenate is added. This reaction is analogous to the arsenolysis of citrulline in Streptococcus faecalis. A slow fission of citrulline occurs in the presence of high concentrations (0.05M and above) or orthophosphate. Some properties of the enzyme system responsible for the arsenolysis were studied. The pH optimum of the arsenolysis is at 6.7. The Michaelis constant varies with experimental conditions between 0.01 and 0.05. Phosphate inhibits competitively the action of arsenate. The arsenolytic ability of liver preparations is not reduced by prolonged dialysis. Ornithine specifically inhibits arsenolysis and phosphorolysis. The inhibition is competitive. Phosphorolysis is greatly accelerated by the addition of washed suspensions of Escherichia coli, which contain ornithine decarboxylase. Hydroxyl-amine, which inhibits ornithine decarboxylase does not fully abolish the accelerating effect of E. coli. It is concluded that a factor in addition to ornithine decarboxylase promotes phosphoro-lysis. Its nature is unknown. Carbamyl and acetyl derivatives of amino acids do not affect the rate of arsenolysis but some compounds of this group (carbamylglutamate, carbamylglutamine, carbamylalanine and acetyl-glutamate) accelerate the rate of phosphorolysis in the presence of ornithine decarboxylase or the factor of E. coll. Among animal tissues liver was the only one in which arsenolysis of citrulline was found. Avian liver was inactive. It is assumed that the enzymes responsible for arseno-lysis are components of the system which synthesizes citrulline in the living cell. A mechanism of arsenolysis and phosphorolysis is proposed. It involves the formation of a phosphorylated or arsenolated enzyme.