Published online by Cambridge University Press: 24 October 2008
The first part of this review is concerned with the balance between N input and output as urinary urea. I start with some observations on classical biochemical studies of the operation of the urea cycle. According to Krebs, the cycle is instantaneous and automatic, as a result of the irreversibility of the first enzyme, carbamoyl-phosphate synthetase 1 (EC 22.214.171.124; CPS-I), and it should be able to handle many times the normal input to the cycle. It is now generally agreed that acetyl glutamate is a necessary co-factor for CPS-1, but not a regulator. There is abundant evidence that changes in dietary protein supply induce coordinated changes in the amounts of all five urea-cycle enzymes. How this coordination is achieved, and why it should be necessary in view of the properties of the cycle mentioned above, is unknown. At the physiological level it is not clear how a change in protein intake is translated into a change of urea cycle activity. It is very unlikely that the signal is an alteration in the plasma concentration either of total amino-N or of any single amino acid. The immediate substrates of the urea cycle are NH3 and aspartate, but there have been no measurements of their concentration in the liver in relation to urea production. Measurements of urea kinetics have shown that in many cases urea production exceeds N intake, and it is only through transfer of some of the urea produced to the colon, where it is hydrolysed to NH3, that it is possible to achieve N balance. It is beginning to look as if this process is regulated, possibly through the operation of recently discovered urea transporters in the kidney and colon. The second part of the review deals with the synthesis and breakdown of protein. The evidence on whole-body protein turnover under a variety of conditions strongly suggests that the components of turnover, including amino acid oxidation, are influenced and perhaps regulated by amino acid supply or amino acid concentration, with insulin playing an important but secondary role. Molecular biology has provided a great deal of information about the complex processes of protein synthesis and breakdown, but so far has nothing to say about how they are coordinated so that in the steady state they are equal. A simple hypothesis is proposed to fill this gap, based on the self-evident fact that for two processes to be coordinated they must have some factor in common. This common factor is the amino acid pool, which provides the substrates for synthesis and represents the products of breakdown. The review concludes that although the achievement and maintenance of N balance is a fact of life that we tend to take for granted, there are many features of it that are not understood, principally the control of urea production and excretion to match the intake, and the coordination of protein synthesis and breakdown to maintain a relatively constant lean body mass.