CYCLOHEXIMIDE SENSITIVITY OF OROTIC-ACID BIOSYNTHESIS INDUCED BY AMMONIA AND GLYCINE ADMINISTRATION

Administration of either ammonia ol glycine to both rats and mice resu lts in an increased synthesis in the liver and urinary excretion of er otic acid. The two most relevant observations obtained are that carbam oyl phosphate synthesized inside the mitochondria is involved in the i ncreased synthesis of erotic acid and that this latter process is almo st completely abolished by cycloheximide and actinomycin D, inhibitors of protein and RNA synthesis. Orotic acid synthesis could be controll ed by an induction-suppression mechanism. Inhibition of synthesis of e xcess erotic acid brought about by N-(phosphonacetyl)-L-aspartic acid but not by acivicin, suggests :hat glutamine-dependent cytosolic synth esis of carbamoyl phosphate, is not involved. Administration of ornith ine together with glycine completely suppressed the synthesis of eroti c acid, but promoted a twofold increase of urea excretion. The concent ration of ornithine rather than that of carbamoyl phosphate or the act ivity of the enzymes involved, may represent a limiting factor control ling both the flux of ammonia in the urea cycle and the availability o f mitochondrial carbamoyl phosphate for erotic acid synthesis. Two enz ymes have been found to be induced by glycine: ornithine decarboxylase and aspartate transcarbamoylase (aspartate carbamoyltransferase). Bot h enzymes may contribute to the increase in erotic acid synthesis, asp artate transcarbamoylase more directly and ornithine decarboxylase by lowering the ornithine. concentration. Ornithine decarboxylase activit y was completely suppressed but that of aspartate transcarbamoylase wa s further increased by cycloheximide treatment. Inhibition of erotic a cid biosynthesis by cycloheximide appears to be the result of a decrea sed availability in the cytosol of carbamoyl phosphate synthesized ins ide the mitochondria.