Decreased serum levels of D-serine in patients with schizophrenia: evidence in support of the N-methyl-D-aspartate receptor hypofunction hypothesis of schizophrenia.

The hypofunction of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptors has been implicated in the pathophysiology of schizophrenia. Several lines of evidence suggest that D-serine may function as an endogenous agonist of the glycine site of the NMDA receptor. The aim of this study was to examine whether serum levels of D- and L-serine in patients with schizophrenia are different from those of healthy controls. Forty-two patients with schizophrenia and 42 age- and sex-matched healthy controls were enrolled in this study. Symptoms were assessed using the Brief Psychiatric Rating Scale. Serum levels of total serine and D- and L-serine were measured by high-performance liquid chromatography. Serum levels of D-serine in the patients with schizophrenia were significantly (z = -3.30, P =.001) lower than those of healthy controls. In contrast, serum levels of total (D and L) serine (z = -2.40, P =.02) and L-serine (z = -2.49, P =.01) in the schizophrenic patients were significantly higher than those of controls. In addition, the percentage of D-serine in the total serine in the schizophrenic patients was significantly (z = -4.78, P<.001) lower than that of controls, suggesting that the activity of serine racemase, an enzyme catalyzing the formation of D-serine from L-serine, may have been reduced in the schizophrenic patients. Reduced levels of D-serine may play a role in the pathophysiology of schizophrenia, and serum D- and L-serine levels might provide a measurable biological marker for schizophrenia.