SERINE, D- (D-serine) is a non-essential amino acid occurring in natural form as the L-isomer. It is synthesized from glycine or threonine. It is involved in the biosynthesis of purines, pyrimidines and other amino acids. A considerable level of D-serine was discovered, surprisingly, in the mammalian brain in the early 1990s. Since then, D-serine has been considered to be a co-agonist of glutamate at the glycine site of NMDA receptors. D-serine plays an important role in the central nervous system as an endogenous ligand for the glycine site of glutamate N-Methyl-D-Aspartate (NMDA) receptors. D-serine is synthetized by racemization of L-serine in most neural and non-neural cells, and modulates a variety of physiological functions in mammals. D-Serine synthesis is attributed to Serine Racemase (SR), which catalyses the synthesis of D-serine from L-serine. D-serine may play a role in the pathophysiology of neuropsychiatric disorders, such as schizophrenia, which may be linked to NMDA receptor hypo-function. Studies in genetic and pharmacological animal models with decreased D-serine levels have shown that these animals displayed behavioural abnormalities similar to those seen in schizophrenia. Moreover, exogenous administration of D-serine and related compounds improved several phenotypes relevant to schizophrenia, which could have positive clinical implications in humans. The results of a clinical trial in Taiwanese schizophrenic patients who received D-serine as adjuvant treatment indicated that those patients who received D-serine treatment, improved positive, negative and cognitive symptoms seen in schizophrenia. In addition, this clinical trial showed that D-serine did not worsen side effects from other antipsychotics, which may be due to its selective action at the NMDA-glycine site. Therefore, D-serine could be considered as a therapeutic approach for schizophrenia, which is different from the dopaminergic approach. It has also been shown that exogenous d-serine administration can suppress appetite and alter food preference. Thus NMDA receptor and its co-agonist d-seine participate in the control of appetite and food preference, which can be used to suppress obesity. D-serine has been shown to have cognitive-enhancing properties in different brain disorders and in age-related cognitive decline. From a clinical perspective, it is important to highlight that in a recent double-blind placebo-controlled cross-over study our group observed that an acute oral administration of 30 mg/kg of d-serine improved spatial learning and problem solving. D-serine may be especially useful for depression because of its acute and chronic antidepressant effects,

Creatinine is a product of metabolism of creatine phosphate, a molecule that serves as a rapidly mobilizable reserve of a brain and skeletal muscle. Creatinine is excreted by kidneys with little or no reabsorption. Serum creatinine is the most commonly used indicator of renal function.