Inhibiting beta-alanine transamination : a way to promote muscle histidine-containing dipeptide loading in mice?

Abstract

Purpose. Of the orally supplied beta-alanine (BA), only 2-3% is excreted in the urine and only 1-6% is converted to muscle carnosine during a long-term supplementation protocol1. Moreover, it was already shown in the ‘50s that more than 90% of the injected C14-labeled BA in rats was recovered in the expired CO2 in 5 hours². We now hypothesize that BA can be transaminated into malonate semi-aldehyde by the enzymes ‘beta-alanine - 2-oxoglutarate transaminase’ (ABAT) and ‘beta-alanine-pyruvate transaminase’ (AGXT2). Vigabatrin (VG) is a specific inhibitor of ABAT, while aminooxyacetic acid (AOA) inhibits all pyridoxal-5’-phosphate (PLP) - dependent enzymes, including ABAT and AGXT2. The aim of the present study was to elucidate the effect of specific (VG) and non-specific (AOA) BA transaminase inhibitors on BA-induced muscle HCD loading in mice. Methods. A total of 66 male C57BL/6 mice (10 weeks old) were used in this study. Mice were randomly divided in 6 groups and underwent different treatments, which differed in the amount of BA in the drinking water (0%, 0.1%, 0.6%, 1.2%) and in the subcutaneous injection of VG, AOA or saline (every 72h or 24h). After 2 weeks, muscles and organs were dissected and blood was collected. Serum BA levels were analysed by means of HPLC. Brain ABAT activity was determined by the fluorometric quantification of NADH formation following addition of GABA to a brain homogenate. Muscle and organ HCD levels were analysed by means of LC-MS/MS and qPCR analysis was used to measure the expression of carnosine-related enzymes and transporters. Results. Taurine transporter and carnosine synthesis enzyme are mainly expressed in muscles and brain. In contrast, the BA transaminating enzymes show highest expression in liver and kidney and very low expression in muscles. Brain ABAT activity is significantly decreased when VG or AOA is administered (-82.2% and -87.8%, respectively; p < 0.001). Serum BA levels are not affected by the supplementation of 0.1% BA alone or in combination with VG administration, but significant increases are found when this low amount of BA is combined with AOA (p = 0.007 vs 0.1%BA – SAL for serum). Subsequently, this leads to significantly higher HCD loading in the different muscles and highest effects are found in soleus and heart (p < 0.001 for 0.1%BA – AOA vs 0.1%BA – SAL). The more glycolytic muscles also show higher HCD loading with AOA administration (p < 0.001 and p = 0.024 vs 0%BA – SAL for tibialis anterior and gastrocnemius, respectively), but effects are absent in kidney and brain. Furthermore, no effects of specific inhibition (VG) were found in any of these tissues. Conclusion. Non-specific inhibition of BA transamination has positive effects on BA-induced HCD loading in both muscles and organs, suggesting that muscle HCD synthesis is dependent on the amount of beta-alanine transamination in mice. References: [1] Stegen S., Blancquaert L., Everaert I., et al. Med Sci Sports Exc. 2013, 45 (8): 1478-85. [2] Pihl and Fritzson. J Biol Chem. 1955, 215: 345-51