Cytosine is a pyrimidine nucleobase, one of the five main bases of nucleic acids. In DNA and RNA cytosine is paired with guanine. Only small amounts of cytosine administered with food are incorporated in DNA. The majority of cytosine is synthesized de-novo starting from carbamoyl phosphate.

Citrulline (name derived from citrullus, the Latin word for watermelon, from which it was first isolated) is an amino acid. It is made from ornithine and carbamoyl phosphate in one of the central reactions in the urea cycle. It is also produced from arginine as a by-product of the reaction catalyzed by nitric oxide synthase (NOS) family. Citrulline supplements have been claimed to promote energy levels, stimulate the immune system and help detoxify ammonia (a cell toxin). Citrulline is not involved in protein synthesis. Several pharmacokinetic studies have confirmed that citrulline is efficiently absorbed when administered orally. Oral citrulline could be used to deliver arginine to the systemic circulation or as a protein anabolic agent in specific clinical situations (for example in case of malnourishment), because recent data have suggested that citrulline, although not a component of proteins, stimulates protein synthesis in skeletal muscle through the mammalian target of rapamycin signaling pathway. Citrulline is converted to L-arginine by argininosuccinate synthase. L-arginine is in turn responsible for citrulline's therapeutic effects. Many of L-arginine's activities, including its possible anti-atherogenic actions, may be accounted for by its role as the precursor to nitric oxide (NO).

Methylenetetrahydrofolate reductase (MTHFR) is an enzyme required for the formation of 5-methyltetrahydrofolate (5-MTHF), a form of folate able to cross the blood-brain barrier and which is necessary as a substrate for the remethylation of homocysteine to methionine by methionine synthase. Patients with severe MTHFR deficiency cannot make 5-MTHF and have extremely low levels in the CSF. Only treatment with oral 5-MTHF given as calcium mefolinate resulted in an increase in CSF 5-MTHF.

Ornithine is an amino acid produced in the urea cycle by the splitting off of urea from arginine. It is a central part of the urea cycle, which allows for the disposal of excess nitrogen. Ornithine is also a precursor of citrulline and arginine. Arginine stimulates the pituitary release of growth hormone. Burns or other injuries affect the state of arginine in tissues throughout the body. As de novo synthesis of arginine during these conditions is usually not sufficient for normal immune function, nor for normal protein synthesis, ornithine may have immunomodulatory and wound-healing activities under these conditions (by virtue of its metabolism to arginine).

Guanosine is an endogenous guanine nucleoside. Guanosine was shown to be protective in several in vitro and/or in vivo experimental models of central nervous system (CNS) diseases including ischemic stroke, Alzheimer's disease, Parkinson's disease, spinal cord injury, nociception, and depression. The mechanisms underlying the neurobiological properties of guanosine seem to involve the activation of several intracellular signaling pathways and a close interaction with the adenosinergic system, with a consequent stimulation of neuroprotective and regenerative processes in the CNS. Several guanosine analogues, i.e. acyclovir (and its oral prodrug valaciclovir), penciclovir (in its oral prodrug form, famciclovir) and ganciclovir, are widely used for the treatment of herpesvirus (i.e. HSV-1, HSV-2, VZV and HCMV) infections.

L-carnitine (L-beta-hydroxy-gamma-N,N,N-trimethylaminobutyric acid) is conditionally necessary for mitochondrial transport and metabolism of long-chain fatty acids, and thus for myocardial energetic metabolism. D-carnitine is not biologically active and might interfere with proper utilization of the L isomer, and so there are claims that the racemic mixture (DL-carnitine) should be avoided. The pharmacological effects of carnitine are stereospecific: L-carnitine was effective in various animal and clinical studies, while D- and DL-carnitine was found to be ineffective or even toxic to some cells and tissues, such as muscle cells and the myocardium. DL-carnitine caused symptoms of myasthenia and cardiac arrhythmias, which disappeared after L-carnitine administration.

Carnosine is a dipeptide composed of the amino acids beta-histidine and l-alanine. It is widely present in muscle and brain tissues. It possesses strong and specific antioxidant properties, protects against radiation damage, and promotes wound healing. The antioxidant mechanism of carnosine is attributed to its chelating effect against metal ions, superoxide dismutase (SOD)-like activity, ROS and free radicals scavenging ability. In addition, was shown, that carnosine significantly protects against TCA-induced liver carcinogenesis in rats, through its antioxidant, antinutritive, and anti-inflammatory effects, and induction of apoptosis. It also may be a therapeutic agent against Parkinson's disease. Experiments on animal have shown the sepsis healing therapeutic potential of carnosine.