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The present study has demonstrated the presence of circulating reverse tri-iodothyronine (rT3) levels and its main generating pathway in rainbow trout. A specific rT3, RIA using thyronine-stripped sera in the standard curve was standardised, allowing precise and accurate quantification of radioimmunoassayable rT3. We also demonstrated that trout skin is an important source of rT3 production. T3 or thyroxine trout skin inner-ring deiodination (IRD) activity was assessed by using rT3 RIA and/or paper chromatography. The kinetic characterisation of this deiodinative pathway disclosed a typical deiodinase type III (DIII) enzyme, except for its conspicuous thermodependency which attained its maximal catalytic efficiency at 15 degrees C. This finding suggested the expression of enzymatic variants, which is a common functional array in teleosts. Both circulating rT3 and DIII activity were present in juvenile and adult individuals and were inversely correlated with age, weight and length. In conclusion, this study demonstrated that (1) skin IRD activity and its product rT3 are present throughout the development of rainbow trout, and (2) trout skin DIII activity attains its higher catalytic efficiency in the physiological range of temperature for this species, thus suggesting the expression of enzymatic variants.
