Glycine is a dietary amino acid that serves as both a constitutional amino acid (used to create protein structures such as enzymes) and as a neurotransmitter/neuromodulator. It is the major inhibitory neurotransmitter in the spinal cord and most glycine in neurons is synthesized de novo from serine in a process requiring folic acid. Some evidence suggests that glycine may be a conditionally essential amino acid for humans.
L-Citrulline is used as a sports performance and cardiovascular health supplement. L-Citrulline supplementation results in reduced fatigue and improved endurance for both aerobic and anaerobic prolonged exercise. There is not enough evidence to support the claim that L-citrulline supplementation improves power output during exercise.
Supplementing L-citrulline also increases ornithine and arginine plasma content. This means L-citrulline supplementation improves the ammonia recycling process and nitric oxide metabolism. L-citrulline is also used to alleviate erectile dysfunction caused by high blood pressure.
After supplementation, L-citrulline is converted into arginine in the kidneys. Supplemental L-arginine provides a spike of L-arginine in plasma, while supplemental L-citrulline increases arginine plasma levels over a longer period of time.
L-arginine and L-ornithine are subject to reduced absorption when supplemented in doses of 10g or more, which can result in diarrhea. L-Citrulline does not have this side-effect, and since it increases plasma levels of all three amino acids, it is generally preferred as a supplement over L-arginine. Though L-citrulline doubles ornithine plasma content, L-ornithine supplementation can increase ornithine plasma content even more, by 300 – 500 percent.
Taurine maintains an intracellular concentration of 5-20 µmol/g wet weight, and enters cells through its transporter, the Taurine Transporter (TauT) that belongs to the class of sodium-chloride dependent transporters (similar to creatine) and is named SLC6a6; this transporter is expressed in most, if not all, mammalian tissue. Pathological problems are observed when cellular taurine is depleted, and this is observed in as little as 48 hours when the transporter is either blocked by a competitive inhibitor or outright depleted. The reasons for this depletion is that intracellular (within the cell) synthesis of taurine is limited, and cells appear to be dependent on taurine uptake from the blood, and the average serum concentration appears to be 20-100uM (up to 100 fold less than cells), which is reason for the active transport via TauT as taurine uptake works againt a concentration gradient.