Requirements for amino acid fluxes (and NH4+)
While it is clear that there are losses of essential amino acids from brain parenchyma and thus that some influx of amino acids must occur, it is difficult to obtain a quantitative estimate of the influx required. Using radiolabelled amino acids in rats, Dunlop et al. [361–363] found a turnover rate for the protein content of rat brains to be about 0.6% h−1. Using a protein content of about 100 mg for each gram of brain and the molecular weight of an average amino acid, perhaps 125 Da, that corresponds to a rate of incorporation of amino acids of about 80 nmol g−1 min−1. Similarly amino groups required for de novo synthesis of glutamate amount to about 100 nmol g−1 min−1 (see legend to Fig. 17).
Many of the amino acids needed for protein synthesis are supplied either by de novo synthesis (which, however, still requires some source of amino groups, see Fig. 16) or by recycling those released during protein breakdown, which averaged over enough time must be occurring at the same rate as synthesis. Furthermore it may be possible to reuse some of the NH4+ lost from the glutamate/glutamine cycle in the de novo synthesis of glutamate. Thus the sum of the estimates above, 180 nmol g−1 min−1, is likely to exceed the actual requirement for amino-acid input.
Because the brain parenchyma must be in N balance and there must be net inputs of essential amino acids, there must also be a route or routes for N removal. As the brain normally doesn’t produce urea as a means of disposing of NH4+ [364–366], the two main routes for exit to be considered are efflux of NH4+ and efflux of glutamine. Fluxes of NH4+ are easily demonstrated to occur in both directions across the blood–brain barrier and are almost certainly by diffusion across the membranes of NH3 combined with transport either of H+ in the same direction or, more likely, of HCO3− in the opposite direction [4, 359, 367]. Because concentrations of NH4+ in brain, 150–300 µM, and CSF, 100–300 µM, normally exceed those in arterial plasma, 50–250 µM [359], it is likely that there is some net NH4+ efflux. However, an arterio-venous difference in NH4+ concentration and thus its net transport have only been demonstrated in the brain when plasma NH4+ concentration is raised as in hepatic insufficiency [359, 368]. There is then net NH4+ entry, rapid incorporation of the NH4+ into glutamine by reaction with glutamate [359], and efflux of the resultant glutamine. Glutamine efflux is considered further in Sect. 5.5.4.
Lee et al. [360] made the interesting suggestion that much of the NH4+ that moves from brain microvascular endothelial cells to plasma is produced within the endothelial cells by glutaminase acting on glutamine. However, that taken alone would suggest that there should also be a substantial efflux of glutamate, which has not been observed. Alternatively the NH4+ effluxed may derive from metabolism of both glutamine and glutamate. This is considered further in Sect. 5.5.4.