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The degree of individuality measured in the plasma hormones is such that conventional population-based reference ranges may not correctly identify major alterations in these hormones in individual subjects.

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Our results show a bimodal seasonal pattern in the availability of plasma L-tryptophan that matches seasonal patterns in the prevalence of violent suicide in the local population and depression in other studies.

Previous studies have suggested that depression and suicide are related to alterations in total cholesterol serum concentrations, and that an altered distribution of haptoglobin (Hp) phenotypes in major depression indicates that variation on chromosome 16 may be associated with that illness. Lecithin:cholesterol acyl transferase (LCAT, EC 2.3.1.43), the enzyme that catalyzes the esterifying reaction of cholesterol in serum, is located close to the Hp gene. This study examined the serum concentrations of total and free cholesterol and the esterified cholesterol ratio in 26 healthy controls, 47 unipolar depressed subjects (16 minor, 14 simple major and 17 melancholic depressed subjects) and 12 relatives of melancholic subjects. Depressed subjects (regardless of subtype) and relatives of depressed subjects had a significantly lower esterified cholesterol ratio than normal controls. No significant differences in total or free cholesterol concentrations were found between the above study groups. In depressed subjects, there were no significant relationships between the esterified cholesterol ratio, total or free cholesterol and postdexamethasone adrenocorticotropic or cortisol values, Hp phenotypes, severity of illness or suicidal symptoms. It is hypothesized that lower esterification in serum cholesterol may constitute a vulnerability factor for depression through alterations in cell membrane microviscosity.