SYNTHESIS AND PROPERTIES OF DELTA-N-(PHOSPHONACETYL)-L-ORNITHINE - TRANSITION-STATE ANALOG INHIBITOR OF ORNITHINE TRANSCARBAMYLASE

Procedures are described for the synthesis of δ-N-(phosphonacetyl)-l-ornithine from α-CBZ-l-ornithine and from copper-ornithine and phosphonacetic acid. δ-PALO is a stable analog of the transition state in the reaction catalyzed by ornithine transcarbamylase from rat liver. δ-PALO is a competitive inhibitor of ornithine transcarbamylase with respect to carbamyl-P, but not to l-ornithine, indicating that the inhibitor binds to the same form of the enzyme as carbamyl-P, whereas δ-PALO and l-ornithine combine with different forms of the enzyme. Replots of 1 v versus concentration of δ-PALO are linear with either carbamyl-P or l-ornithine as variable substrate. The inhibition constant (Ki) for δ-PALO with ornithine transcarbamylase is 2.7 × 10-7 m at 30 °C and pH 7.4 in 50 mm Tris-HCl. Thus δ-PALO binds some 250 times more tightly than its competing substrate, carbamyl-P. Although δ-PALO is a powerful inhibitor of ornithine transcarbamylase, it did not significantly inhibit citrulline biosynthesis in isolated rat liver mitochondria or urea biosynthesis in intact rat hepatocytes. This failure to inhibit was shown to be due to the permeability barrier imposed by the mitochondrial membranes, since citrulline biosynthesis by mitochondria modified with digitonin or Triton X-100 was substantially inhibited by low concentrations of δ-PALO. © 1978.