THE RELATIVE RATES OF PHOTOSYNTHETIC ASSIMILATION OF ISOTOPIC FORMS OF CARBON DIOXIDE

Abstract

Introduction The dictum that isotopes of the same element are chemically the same is, of course, only an approximation. There are many examples of significantly different chemical behavior by different isotopes. Chemical reaction velocities often are quite unequal for different isotopes, particularly with the lighter (and biologically interesting) elements; and these differences may result in some degree of isotope separation. Certain chemical equi-libria have been employed for the enrichment of some of the stable isotopes; isotope effects on reaction rate constants thus have been the basis for the production of tracers on a commercial scale. Therefore it ispatently true that the use of tracers in biochemical studies may also involve chemical reactions at which some isotope separation occurs. In the case of a series of such reactions, it iseasy to visualize an appropriate kinetic scheme whereby rather considerable isotope fractionation could result. As long as such fractionation is very small, it is of little direct concern to the tracer biochemist; if it should become very large, it could complicate and even invalidate his interpretation of tracer experiments. KAMEN (5) has called attention to this sort of complication in relation to tracer studies using the hydrogen isotope, H3, or tritium. He pointed out that discrimination by chemical reactions between ordinary hydrogen, H' (protium), and tritium could, in theory, be so large as to completely invalidate the particular experiments considered. More recently BIGELEISEN (2) has called attention to the possibility that isotopic discrimination may complicate tracer studies even with elements heavier than hydrogen. His calculation of the minimal ratio of reaction velocity constants to be expected, for reactions involving C14 and C12 for example, was 0.67. Such a discrimination factor would be of real concern in tracer methodology; but it isactually a hypothetically extreme value arrived at by compounding second and higher order effects, all assumed to be maximally effective in the direction of enhancing rather than minimizing the discrimination. It seems very unlikely that isotope fractionation occurs to this maximal extent in a single actual reaction. It may be recalled, nevertheless, that the overall biological processes studied by physiologists or biochemists are rarely single reactions. Metabolic transformations in