Role of the glycine-cleavage system in glycine and serine metabolism in various organs.

stimulates glutamine release at the expense of tissue glutamate while decreasing or having little effect on alanine release (Ruderman & Lund, 1972; Chang & Goldberg, 1978). In the perfused rat hindlimb total glutamine production by resting muscle is over 4-fold greater than ammonia production, but the addition of methionine sulphoximine (a glutamine synthetase inhibitor) reverses this pattern (Goodman & Lowenstein, 1977). However, during muscular contraction, when ammonia production increases markedly, there is no change in muscle glutamine production, either during contraction or the recovery phase (Goodman & Lowenstein, 1977; Meyer & Terjung, 1979). In resting muscle the purine nucleotide cycle may turn over at a low rate so as to effect amino acid deamination, and ammonia so-generated is largely removed by glutamine formation. During contraction, the fall in intracellular ATP concentration (and in that of glutamate due to transamination to form alanine) will inhibit glutamine synthetase and prevent glutamine production.