Abnormal cell membrane composition and functional cobalamin (vitamin B₁₂) deficiency was reported in schizophrenic individuals. We aimed to investigate the relationship between cobalamin state and cell membrane composition in patients with schizophrenia.

Malondialdehyde (MDA), cholesterol, and phospholipid classes in the erythrocyte membranes of 18 schizophrenic and 20 healthy individuals of the same age and sex distribution were determined. Serum vitamin B₁₂, plasma total homocysteine, serum folate, and urine methylmalonic acid (uMMA) concentrations were measured in both groups.

In the schizophrenic group, uMMA, membrane MDA, membrane cholesterol, membrane phosphatidylinositol concentrations were significantly higher and the membrane phosphatidylserine concentrations were lower than the control group values. In schizophrenic individuals, uMMA concentrations have a significant positive correlation with membrane MDA and a negative correlation with membrane cholesterol concentrations (P < 0.05). The negative correlation of uMMA with membrane cholesterol concentrations may be a biological response to the increased membrane rigidity. Also, a free radical elevation related to the elevated uMMA concentrations in the erythrocyte membrane, might reflect the role of methylmalonic acid (MMA) in membrane damage.

Our present findings suggest that, functional vitamin B₁₂ deficiency representing itself by MMA elevations in schizophrenic individuals could damage cell membrane.