Monoamine oxidase and free tryptophan in human plasma: normal variations and their implications for biochemical research in affective disorders.

A methodological study of healthy subjects was conducted to investigate factors influencing plasma free tryptophan and platelet monoamine oxidase (MAO) activity. The following changes were found during the menstrual cycle: free tryptophan was lower during the "ovulatory" phase than during the "premenstrual" phase, MAO activity was 20% lower during the "premenstrual" phase as compared with the "neutral" phase (days: menses-24 to -1). During pregnancy MAO activity was also lower than the values found six weeks postpartum. MAO activity in women was significantly higher than in men and exhibited greater variability. A circadian rhythm for free tryptophan was observed (minimum at midday, maximum at midnight). No diurnal variation in MAO activity could be established. Preliminary data from untreated endogenous depressive patients showed no difference in free tryptophan concentrations at 8 a.m. or 4 p.m. from control values.