RESUMEN
PURPOSE: Chronic ß-alanine supplementation leads to increased levels of muscle histidine-containing dipeptides. However, the majority of ingested ß-alanine is, most likely, degraded by two transaminases: GABA-T and AGXT2. In contrast to GABA-T, the in vivo role of AGXT2 with respect to ß-alanine metabolism is unknown. The purpose of the present work is to investigate if AGXT2 is functionally involved in ß-alanine homeostasis. METHODS: Muscle histidine-containing dipeptides levels were determined in AGXT2 overexpressing or knock-out mice and in human subjects with different rs37369 genotypes which is known to affect AGXT2 activity. Further, plasma ß-alanine kinetic was measured and urine was obtained from subjects with different rs37369 genotypes following ingestion of 1400 mg ß-alanine. RESULT: Overexpression of AGXT2 decreased circulating and muscle histidine-containing dipeptides (> 70% decrease; p < 0.05), while AGXT2 KO did not result in altered histidine-containing dipeptides levels. In both models, ß-alanine remained unaffected in the circulation and in muscle (p > 0.05). In humans, the results support the evidence that decreased AGXT2 activity is not associated with altered histidine-containing dipeptides levels (p > 0.05). Additionally, following an acute dose of ß-alanine, no differences in pharmacokinetic response were measured between subjects with different rs37369 genotypes (p > 0.05). Interestingly, urinary ß-alanine excretion was 103% higher in subjects associated with lower AGXT2 activity, compared to subjects associated with normal AGXT2 activity (p < 0.05). CONCLUSION: The data suggest that in vivo, ß-alanine is a substrate of AGXT2; however, its importance in the metabolism of ß-alanine and histidine-containing dipeptides seems small.