Does glutamine act as a substrate for transamination reactions in the liver of fed and fasted sheep?

The present study investigated the relative importance of glutamine as a tr ansamination source in the ovine liver by examination of the labelling of a mino acids (AA) in the hepatic free pool, mixed liver and plasma proteins o f fed and fasted sheep, following infusion of isotopically-labelled glutami ne. in a cross-over design four sheep were either fasted for 3 d or fed to 1.2 x energy maintenance and finally euthanased. At each intake, the sheep were infused for 6 h with [2-N-15]glutamine (150 mu mol/h) and samples of t otal plasma protein isolated. Following the terminal infusion, liver tissue total proteins were prepared and hydrolysed and N-15-enrichments in sevent een AA were determined by GC-combustion-isotope-ratio mass spectrometry. Al l AA were enriched (relative to natural abundance) except lysine and threon ine, with the lowest enrichments in phenylalanine and histidine. There was no effect of the fed v. fasted state, except for leucine and isoleucine in liver protein (P < 0.05). Enrichments in liver protein were greater than in plasma protein (P < 0.01, except proline) and probably reflect the faster turnover rate of hepatic constitutive proteins compared with export protein s. Amination to methionine was greater than that to phenylalanine (P < 0.01 ), suggesting a mechanism for preferentially protecting the former. This fa ctor could be important for ruminant production, as methionine is often con sidered to be the first limiting AA for animals offered certain silages and conserved forages. Enrichments in all AA (except for glutamine, alanine an d aspartate) were less than that for glutamate (P < 0.01), and thus transam inations may have occurred with glutamine directly or via glutamate, follow ing the action of hepatic glutaminase.