Insensitivity of Carbamoyl-Phosphate Synthetase Towards Inhibition by Carbamoyl Phosphate Makes it Unlikely that Mitochondrial Metabolite Transport Controls Ornithine Cycle Flux

Abstract

The first two enzymes involved in the synthesis of urea in the mammalian liver, —carbamoyl phosphate synthetase (ammonia) (CPS) and ornithine carbamoyl transferase (OCT), are located in the mitochondria whereas the other 3 enzymes (argininosuccinate synthetase, argininosuccinate lyase and arginase) are cytosolic. Because of this dual localization continuous transport of metabolites across the mitochondrial inner membrane (e.g. of ornithine, citrulline, glutamate, aspartate and ATP) is an essential part of the pathway of urea synthesis. Whether these transport systems contribute to the control of urea synthesis under in vivo conditions is an open question. In principle, flux control analysis (Kacser & Burns, 1973) should enable one to answer this question. One experimental approach would be to titrate these transport systems with specific inhibitors (Groen et al., 1982b). Unfortunately, except in the case of the adenine nucleotide translocator, suitable inhibitors of these transport systems are not available. However, as will be discussed below, an alternative approach is to make use of knowledge of the in situ kinetic properties of the ornithine cycle enzymes.