Assays of catecholamines and their metabolites : pharmacological applications

Amongst the methods of detection coupled to H.P.L.C. electrochemical detection is currently one of the most sensitive methods. The technical progress of the last few years means that nowadays high per-formance detectors are available, notably in so far as stability and reproducibility are concerned. These de-vices measure charge transfers at the level of the chromatographic eluate following the oxidation or the reduction of the molecule being measured. Thus, all structures susceptible to reduction or oxidation can be analysed by electrochemistry. This technique con-cerns in particular, all components which contain amine functions or catechol, phenol rings, etc..

For this reason, this technique has established itself over the last ten years as the method of choice for the assay of catecholamines and their metabolites. In mammals, three catecholamines are involved in neuromediator and hormonal functions : norepinephrine, epinephrine and dopamine (see figure). The metabolism of these three amines is controlled by two en-zymes, catechol o-methyltransferase (C.O.M.T.) and mono-amine-oxidase (M.A.O.). The assay of these molecules in biological fluids requires methods which are both sensitive in order to detect concentrations close to pg/ml, and specific in order to detect those products in media which are close to molecules having the same polarity.

Using this tool, certain tumours (phaeochromocy-tomas, neuroblastomas, etc...) are revealed by analysing catecholamines and certain of their metabolites in plasma and urine. Their assay can also be shown to be particularly important in following the pharma-cological effect of a drug during clinical studies. This is the case for anti-depressants.

It is currently agreed that a state of depression is ac-companied by a deficit of biogenous amines at the level of the synapse. Antidepressants correct this de-ficiency either by inhibition of re-uptake (tricyclic anti-depressants) or by inhibition of M.A.O. For these in-hibitors of M.A.O. (I.M.A.O.), this action results in an increase in catecholamines and certain of their ami-nated metabolites (3-MT, NMN or MT, see figure), but also leads to a reduction in certain of their de-aminated metabolites in biological media (HVA, MHPG, AVM, see figure). Taking into account the ubiquitous character of the M.A.O. in the organism, the total concentration (free forms and conjugated forms either in plasma or urine) of these components, does not allow the distinction to be made between peripheral or central I.M.A.O. activity. It is currently agreed that the assay of the sulfated form of MHPG enables this ambiguity to be avoided since the conju-gate does not form except at the central nervous sys-tem. J.R.D.

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