Avian mitochondrial glutamine metabolism.

Intact avian liver mitochondria were shown to synthesize glutamine from glutamate in the absence of exogenous ATP and ammonia. With L-[U-14C]glutamate as the substrate, there was an approximate 1:1 stoichiometry between glutamate deaminated (as measured by the release of 14CO2 due to alpha-keto-[14C]glutarate oxidation) and glutamate amidated. With L-[15N]glutamate as the substrate, the isolated glutamine was shown by low and high resolution mass spectrometry of its phenylisothiocyanate derivative to contain 15N in both the alpha-amino and amide groups. Thus, for each mole of glutamate taken up, approximately 0.5 mol is deaminated and the other 0.5 mol serves as a substrate for glutamine synthetase previously localized in these mitochondria (Vorhaben, J. E., and Campbell, J. W. (1972) J. Biol. Chem. 247,2763). The permeability of L-glutamine to intact avian liver mitochondria was studied by a rapid centrifugation technique. Efflux as well as influx of L-glutamine were both rapid and appeared to occur by a passive, energy-independent process. These results indicate that the mitochondrial glutamine synthetase present in uricotelic species represents the primary ammonia detoxication reaction in that ammonia released intramitochondrially during amino acid catabolism is converted to glutamine for efflux to the cytosol where it may serve as a substrate for purine (uric acid) biosynthesis.