Relationships between lower plasma L-tryptophan levels and immune-inflammatory variables in depression

Michael Maes*, Herbert Y. Meltzer, Simon Scharpè, Eugéne Bosmans, Eduard Suy, Ingrid De Meester, Joseph Calabrese, Paul Cosyns

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

138 Scopus citations


Despite much research, the pathophysiology underlying lower L-tryptophan (L-TRP) availability in major depression has remained elusive. The present study investigates whether lower L-TRP availability in major depression is related to immune activation which may occur in that illness and is known to modulate L-TRP metabolism. Toward this end, the authors have measured the following in depressed patients and normal control subjects: plasma levels of L-TRP, and the competing amino acids (CAA) valine, leucine, isoleucine, tyrosine, and phenylalanine, together with indices of immune function such as haptoglobin (Hp) and transferrin (Tf) plasma levels, dipeptidyl peptidase IV (DPP IV) serum activity, and mitogen-induced culture supernatant interleukin-6 (Il-6) production. Both plasma levels of L-TRP and the L-TRP/CAA ratio were significantly lower in major depressed subjects as compared with healthy control subjects. There were significant correlations between plasma L-TRP levels, on the one hand, and Tf plasma levels, DPP IV activity (both positive), Il-6 production, and Hp plasma levels (both negative), on the other. Up to 63.7% of the variance in L-TRP plasma concentrations could be explained by DPP IV, Hp, Il-6 values, and gender. Up to 50% of the variance in the L-TRP/CAA ratio could be explained by Hp values (negative correlation) and gender. It is hypothesized that lower plasma L-TRP availability in major depression may be related to the immune response in that illness.

Original languageEnglish (US)
Pages (from-to)151-165
Number of pages15
JournalPsychiatry Research
Issue number2
StatePublished - Nov 1993


  • Affective disorder
  • dipeptidyl peptidase IV
  • haptoglobin
  • inflammation
  • interleukin-6
  • serotonin
  • systemic immune activation
  • transferrin

ASJC Scopus subject areas

  • Psychiatry and Mental health
  • Biological Psychiatry
  • Psychology(all)

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