Isotope exchange reactions exchangeable pools

Equation (21) is an excellent approximation to Equation (20) for moderate to high kj/kj ratios ( 10 and higher) for processes that occur by first-order kinetics. It is important to note, however, that a specific rate law does not appear anywhere in Equations (20) and (21), and they are equally valid for any reaction process where Xpejm) is small. Equation (21) illustrates that oxidation of Fe(II)a, to Fe(III)a, produces a markedly different isotopic mass balance than that associated with DIR. In cases where the product of DIR is Fe(II)aq, the concentration of this component is continually increasing, changing the relative mass balance among the exchangeable pools of Fe over time. 

N2O. Isotope dilution is a well-known principle used to determine intermediates in metabolic pathways. The suspected intermediate, which is imlabeled, is added to cells that are metabolizing a labeled substrate. If the compound is an intermediate, the subsequent reaction is saturated, causing the labeled material to pool with the added compound. The radioactivity of this pool should increase and that of the terminal product should decrease as the quantity of nonlabeled compound is increased. This method assumes that the intermediate is readily exchanged across the cytoplasmic membrane. 

If a high value of the ratio isotope of substrate/ Isotope of intermediate) 1. e. ( h/ C) Is found In the product, a coupling of the two reactions in question is highly probable. If the value of this ratio of the product is higher than the respective value of the relsolated Intermediate we can be sure that the bound form of Intermediate does not freely exchange with the externally added pool of intermediate. That proves a "coupling" of two reactions. That, I believe, is the essential point posed by the concept described. 

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