Introduction Despite branched chain amino acids (BCAA) being used for over 30 years, its efficacy in treating hyperammonemia remains controversial. Recent studies suggest that the use of BCAA in cirrhosis is associated with increased glutamine levels, with a paradoxical increase in arterial ammonia. The mechanism of this remains uncertain. The administration of phenylacetate during increased glutamine levels reduces hyperammonemia through excretion of the ammoniagenic glutamine as phenylacetylgultamine (PAGN). We hypothesised that administration of phenylacetate with BCAA would reduce ammonia concentrations by stimulating ammonia capture by glutamine synthetase (GS) with subsequently phenylacetate removing the increased glutamine as PAGN, excreted into urine.

Despite BCAA being used for over 30 years, its efficacy in treating hyperammonemia remains controversial. Recent studies suggest that the use of BCAA in cirrhosis is associated with increased glutamine levels, with a paradoxical increase in arterial ammonia. The mechanism of this remains uncertain. The administration of phenylacetate during increased glutamine levels reduces hyperammonemia through excretion of the ammoniagenic glutamine as PAGN. We hypothesised that administration of phenylacetate with BCAA would reduce ammonia concentrations by stimulating ammonia capture by GS with subsequently phenylacetate removing the increased glutamine as PAGN, excreted into urine.

Despite BCAA being used for over 30 years, its efficacy in treating hyperammonemia remains controversial. Recent studies suggest that the use of BCAA in cirrhosis is associated with increased glutamine levels, with a paradoxical increase in arterial ammonia. The mechanism of this remains uncertain. The administration of phenylacetate during increased glutamine levels reduces hyperammonemia through excretion of the ammoniagenic glutamine as PAGN. We hypothesised that administration of phenylacetate with BCAA would reduce ammonia concentrations by stimulating ammonia capture by GS with subsequently phenylacetate removing the increased glutamine as PAGN, excreted into urine.

Method Sprague–Dawley (n=22) bile duct ligated (BDL) or sham operated rats were studied 4 weeks post surgery (262+11.51 g). Groups: Sham (n=6); BDL+saline (n=6); BDL+BCAA (0.3 g/kg/day; n=6); BDL+BCAA dose+phenylacetate (BCAA+P, 0.3+0.3 g/kg/day; n=4). Treatment was for 3 days prior to sacrifice. Arterial ammonia (COBAS), GS and Glutaminase (GA) enzyme activity was measured (using enzymatic methods) in the muscle, gut, and the liver and, the brain water was measured using the dry weight technique.

Results Plasma ammonia was significantly higher in the BCAA group compared with controls (BCAA=118±20.8; vs sham43±8.3 μM p<0.01) and remained elevated in the BCAA+P animals (78.5±31 μM) which was associated with significantly higher brain water (BCAA=79.96±0.3%; BCAA+P=79.78±1.1%; sham=75.9±0.1% p<0.02 & p<0.05 respectively). GS activity in muscle increased in all BDL groups (p<0.05 in each case), and appeared to further increase following BCAA administration (BDL=7.13±1.7 IU/mg; BCAA=9.63±2.4 IU/mg, p=0.1). Hepatic GS activity was significantly reduced in all BDL groups (p<0.05 for each vs sham), with no apparent effect of BCAA. Duodenal GA activity was elevated with BCAA treatment, and significantly higher in the BCAA+P group (BCAA+P=1.66±0.4 IU/mg vs sham 0.43±0.07 IU/mg, p<0.01).

Conclusion The administration of BCAA resulted in an unexpected worsening of hyperammonemia and brain water which was further exacerbated by addition of phenylacetate. The deleterious effect of BCAA may result from increased muscle glutamine generation, which can cycle to generate ammonia from intestinal GA. The lack of beneficial effect of additional phenylacetate may be due to further increased GA activity. The interplay between GS and GA is pivotal in regulating ammonia levels in cirrhosis, and effectiveness of new therapies aimed at ammonia must address how they alter the function of these enzymes.