USING STABLE ISOTOPES TO STUDY BIOCONVERS ION OF POLYUNSATURATED FATTY-ACIDS IN MAN

The remarkable tolerance of living organisms for stable isotopes makes of polyunsaturated fatty acids (PUFA) labelled with such isotopes exc ellent tools for the study of essential fatty acid metabolism in human s, specially in situations when the actual capacity of bioconversion i s questionable : prematurity, diabetes, old populations. Isotope ratio mass spectrometry (IRMS) and gas-chromatography IRMS (GC-IRMS) are wi dely used for the study of oxidative metabolism of fatty acids C-13 la belled (with (CO2)-C-13 measurement), whereas gas-chromatography-mass spectrometry (GCMS) appears specially recommended for the study of the bioconversion of deuterium labelled AGPI. GCMS is the only method abl e to compare simultaneously the metabolism of geometric isomers labell ed with different numbers of deuterium atoms (H-2(2), H-2(2) -). Using tetradeuterated dihomogamma linolenic acid and GCMS we studied in nor mal sublets and in diabetics Delta 5 desaturase activity. We could dem onstrate in vivo, in man by this direct method the actual conversion o f 20 : 3 n-6 into 20 : 4 n-6 but we characterized also the slowness of this conversion its insulinodependency and its insufficiency in diabe tics. Very recent attempts to apply GC-IRMS to the study of metabolic fluxes with C-13 labelled AGPI with low enrichment are promissing. How ever, at the present day, GCMS and IRMS or GC-IRMS appear rather compl ementary than competitive : the first is mainly devoted to metabolic s tudies using deuterium labelled AGPI, the others being mostly applied to the study of oxidative metabolism and distribution with C-13 labell ed tracers.