Is serum tryptophan a good indicator of CNS serotonin metabolism in chronic alcoholics with hepatopathy?

Due to the fact that tryptophan hydroxylase is not saturated under normal conditions, the concentration of brain tryptophan greatly influences the rate of serotonin synthesis in rat brain [1]. It is also widely admitted that, at least in rats, tryptophan is transported from the blood to the brain through a saturable uptake mechanism; large neutral amino-acids (principally phenylalanine, thyrosin, leucine, isoleucine and valine) compete with tryptophan for this transport. Therefore it makes sense that significant correlations between brain tryptophan and various serum variables (free tryptophan, total tryptophan, free tryptophan/competitors and total tryptophan/competitors) were reported [2]. Chronically intoxicated baboons and rats also showed a concomitant decrease in brain TRP and 5 HT concentrations and in the serum TRP/competitors ratio [3]. In humans the studies have been limited to the measurement of CSF tryptophan concentrations which reflect those of the brain [4]. In 1975 S.N. Young [4] seemed to confirm the findings previously done on animals by showing a positive relationship between total serum tryptophan and CSF tryptophan in control subjects. However, even if he didn't find any correlation in patients with advanced hepatic cirrhosis, some authors have kept on speculating about human alcoholism physiopathology on the basis of animal experiments [3]. The aim of our study was to confirm the results of Young and moreover to determine whether abstinence has any effect on them.