The average δ 13 C-value of glycerol from plant origin is 4-5‰ more negative than that of carbohydrates from the same source. This depletion is exclusively dealing with position C-1 of the molecule. This is also observed for glycerol from other natural sources, although to a smaller extent, while synthetic glycerol shows a statistical 13 C-pattern. On the basis of these data an illegal addition of glycerol to wine should be detectable. In fermentations of glucose with yeast the extent of the depletion in position C-1 of glycerol proved to be reciprocal to its yield. Furthermore a total carbon and isotope balance showed that the depletion of glycerol is a compensation for small 13 C-enrichments in the corresponding positions of main products. An isotope effect on the aldolase reaction and a noncomplete equilibration of the triose phosphates are discussed as main reasons for these findings.

Inter- and intra-molecular non-statistical isotope distributions do not only require the existence of a kinetic isotope effect on a defined enzyme catalyzed reaction, but also the prerequisite that this reaction is located at a metabolic branching point. Furthermore a metabolic and isotopic balance demand that the extent of the isotopic shift is reciprocal to the products' yields. On this base the (13)C-enrichment of L-ascorbic acid in position C-1 and the depletion of glycerol in C-1 are interpreted. The (13)C-pattern of natural malic acid is discussed as a consequence of isotope effects on the carboxylation of pyruvate and PEP and on the pyruvate dehydrogenase reaction. The patterns of natural products synthezised by transfer of "active acetaldehyde" is proposed to be due to an isotope effect on the thiamine pyrophosphate containing lyase reaction. An isotope effect on the reduction of "active formaldehyde" to "active methyl" and the existence of corresponding pools is responsible for (13)C-enrichments and depletions of natural products in positions bearing these intermediates. Finally a model for the main nitrogen pools and for isotope discriminations between α-amino, ω-amino-N and amide pools in plants is proposed.

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